Eye Anatomy

A Brief Anatomy of the Eye
Gray's Anatomy 39th

The eyeball, the peripheral organ of vision, is situated in a skeletal cavity, the orbit, the walls of which help to protect it from injury. The orbit also has a more fundamental role in the visual process itself, in providing a rigid support and direction to the eye and in forming the sites of attachment for its external muscles. This setting permits the accurate positioning of the visual axis under neuromuscular control, and determines the spatial relationship between the two eyes - essential for binocular vision and conjugate eye movements.

The eyeball is embedded in orbital fat, separated from it by a thin fascial sheath. It is composed of the segments of two spheres of different radii. The anterior segment, part of the smaller sphere, is transparent and forms c.7% of the surface of the whole globe. It is more prominent than the posterior segment, which is part of a larger sphere and opaque, and forms the remainder of the globe. The anterior segment is bounded by the cornea and the lens, and is incompletely subdivided into anterior and posterior chambers by the iris. These chambers are continuous through the pupil. The anterior chamber is slightly overlapped by the sclera peripherally. The angle between the iris and cornea therefore forms an annulus of greater diameter than the limbus, the junction between the sclera and cornea. The difference between these two varies from 1 to 2 mm, the angle being deeper above and below than at the sides of the eyeball. The posterior chamber lies between the posterior surface of the iris and the anterior aspect of the lens and its supporting ligament, the zonule, and is triangular in section. The apex of the triangle is the point where the iris touches the lens, and the base, or zonular region, extends among the collagenous bundles of the zonule, sometimes even into a retrozonular space between the zonule and the vitreous humour in the posterior segment of the eyeball. The posterior segment consists of the parts of the eye posterior to the zonule and lens.

The anterior pole is the centre of the anterior (corneal) curvature, and the posterior pole is the centre of its posterior (scleral) curvature; a line joining these two points forms the optic axis. (By the same convention, the eye has an equator, equidistant between the poles: any circumferential line joining the poles is a meridian.) The optic axes of the two eyes, in their primary position, are parallel and do not correspond with the orbital axes, which diverge anterolaterally at a marked angle to each other . The optic nerves follow the orbital axes and are therefore not parallel; each enters its eye c.3 mm medial (nasal) to the posterior pole. The ocular vertical diameter (23.5 mm) is rather less than the transverse and anteroposterior diameters (24 mm); the anteroposterior diameter at birth is c.17.5 mm and at puberty 20-21 mm; it may vary considerably in myopia (c.29 mm) and in hypermetropia (c.20 mm). In females all diameters are on average slightly less than in the male.

Figure 1 The organization of the eye, viewed from above. In this illustration the left eye and part of the lower eyelid are depicted in horizontal section and also cut away to show internal structure.

OCULAR FIBROUS TISSUE

The eye has three layers enclosing its contents. From the outer surface these are a fibrous layer, which consists of the sclera behind and the cornea in front; a vascular, pigmented layer which consists of (from behind forwards) the choroid, ciliary body and iris, collectively termed the uveal tract; and a neural layer, known as the retina.

The fibrous layer of the eyeball (Fig. 1) has an opaque posterior sclera and a transparent anterior cornea. Together these form the protective enclosing capsule of the eye, a semi-elastic structure which when made turgid by intraocular pressure, determines with great precision the optical geometry of the visual apparatus. The sclera also provides attachments for the extraocular muscles which rotate the eye, its smooth external surface rotating easily on the adjacent tissues of the orbit. The cornea admits light, refracts it towards a retinal focus, and plays an important role in the image-processing mechanism

OCULAR VASCULAR TUNIC

The vascular tunic, or uveal tract (Fig. 2), consists of the choroid, ciliary body and iris (Fig. 3), which collectively form a continuous structure. The choroid covers the internal scleral surface, and extends forwards to the ora serrata. The ciliary body continues forward from the choroid to the circumference of the iris, which is a circular diaphragm behind the cornea and in front of the lens. It presents an almost central aperture, the pupil

Figure 2 The vascular arrangements of the uveal tract. The long posterior ciliary arteries, one of which is visible (A), branch at the ora serrata (b) and feed the capillaries of the anterior part of the choroid. Short posterior ciliary arteries (C) divide rapidly to form the posterior part of the choriocapillaris. Anterior ciliary arteries (D) send recurrent branches to the choriocapillaris (e) and anterior rami to the major arterial circle (f). Branches from the circle extend into the iris (g) and to the limbus. Branches of the short posterior ciliary arteries (C) form an anastomotic circle (h) (of Zinn) round the optic disc, and twigs (i) from this join an arterial network on the optic nerve. The vorticose veins (J) are formed by the junctions (k) of suprachoroidal tributaries (l). Smaller tributaries are also shown (m, n). The veins draining the scleral venous sinus (o) join anterior ciliary veins and vorticose tributaries. (By permission from Hogan MJ, Alvarado JA, Weddell JE 1971 Histology of the Human Eye. Philadelphia: WB Saunders.)

Figure 3. Composite view of the surfaces and internal strata of the iris. In a clockwise direction from above, the pupillary (A) and ciliary (B) zones are shown in successive segments. The first (brown iris) shows the anterior border layer and the openings of crypts (c). In the second segment (blue iris), the layer is much less prominent and the trabeculae of the stroma are more visible. The third segment shows the iridial vessels, including the major arterial circle (e) and the incomplete minor arterial circle (f). The fourth segment shows the muscle stratum, including the sphincter (g) and dilator (h) of the pupil. The everted 'pupillary ruff' of the epithelium on the posterior aspect of the iris (d) appears in all segments. The final segment, folded over for pictorial purposes, depicts this aspect of the iris, showing radial folds (i and j) and the adjoining ciliary processes (k). (By permission from Hogan MJ, Alvarado JA, Weddell JE 1971 Histology of the Human Eye. Philadelphia: WB Saunders.)

RETINA

The retina is the sensory neural layer of the eyeball. It is a most complex structure and should be considered as a special area of the brain, from which it is derived by outgrowth from the diencephalon . It is dedicated to the detection and early analysis of visual information and is an integrated part of the much larger apparatus of visual analysis present in the thalamus, cortex and other areas of the central nervous system.

Layers of Retina

The retina is organized into layers or zones where distinctive components of its cells are clustered together or in register to form continuous strata. These layers extend uninterrupted throughout the photoreceptive retina except at the exit point of the optic nerve fibres at the optic disc, although certain layers are much reduced at the foveola where the photoreceptive elements predominate. The names given to the different layers reflect in part the components present within them, and also their position in the thickness of the retina. Conventionally, those structures furthest from the vitreous (i.e. towards the choroid) are designated as outer or external, and those towards the vitreous are inner or internal.

Customarily, ten retinal layers are distinguished (Fig. 4), beginning at the choroidal edge and passing towards the vitreous. These are: retinal pigment epithelium; layer of rods and cones (outer segments and inner segments); external limiting membrane; outer nuclear layer; outer plexiform layer (OPL); inner nuclear layer (INL); inner plexiform layer (IPL); ganglion cell layer; nerve fibre layer; internal limiting membrane. Some of these are subdivisible into substrata, and an innermost plexiform layer between layers 8 and 9 has also been demonstrated.


The composition of the different retinal layers is as follows:

Layer 1: Pigment epithelium

This is a simple low cuboidal epithelium which forms the back of the retina, and, therefore forms the boundary with the choroid, from which it is separated by a thick composite basal lamina.

Layer 2: Rod and cone cell processes

This contains the photoreceptive outer segments and the outer part of the inner segments of rod and cone cells.

Layer 3: External limiting membrane

This layer appears as a distinct line by light microscopy. It consists of a zone of intercellular junctions of the zonula adherens type (p. 7) between the processes of radial glial cells and photoreceptor processes.

Layer 4: Outer nuclear layer

This consists of several tiers of rod and cone cell bodies and their nuclei, the cone nuclei lying outermost. Mingled with these are the outer and inner fibres from the same cell bodies, directed outward to the bases of inner segments, and inwards towards the outer plexiform layer.

Layer 5: Outer plexiform layer

This is a region of complex synaptic arrangements between the processes of the cells whose cell bodies lie in the adjacent layers. The outer plexiform layer contains the synaptic processes of rod and cone cells, bipolar cells, horizontal cells, and some interplexiform cells (which in this account are grouped with the amacrines).

Layer 6: Inner nuclear layer

This is composed of three nuclear strata. Horizontal cell nuclei form the outermost zone, then in sequence inwards, the nuclei and cell bodies of bipolar cells, radial glial cells, and the outer set of amacrine cells, including the interplexiform cells whose dendrites cross this layer.

Layer 7: Inner plexiform layer

This is divisible into three layers depending on the types of contact occurring. The outer or 'OFF' layer contains synapses between 'OFF' bipolar cells, ganglion cells and some amacrines; a middle or 'ON' layer contains synapses between the axons of 'ON' bipolars and the dendrites of ganglion cells and displaced amacrines; and an inner 'rod' layer contains synapses between rod bipolars and displaced amacrines. (Refer to Wässle & Boycott 1991 for an explanation of the 'OFF' and 'ON' cell designations.)

Layer 8: Ganglion cell layer
This layer contains the nuclei of the displaced amacrine cells. Its inner regions consist of the cell bodies, nuclei and initial segments of retinal ganglion cells of various classes.

Layer 9: Nerve fibre layer

This contains the unmyelinated axons of retinal ganglion cells. It forms a zone of variable thickness over the inner retinal surface, and is the only component of the retina at the point where the fibres pass into the nerve at the optic disc. The inner aspect of this layer contains the nuclei and processes of astrocytes which, together with radial glial cells, ensheath the nerve fibres. Between the nerve fibre layer and the ganglion cells there is another narrow innermost plexiform layer where neuronal processes make synaptic contact with the axon hillocks and initial segments of ganglion cells.

Layer 10: Internal limiting membrane

This is a glial boundary between the retina and the vitreous body. It is formed by the end feet of radial glial cells and astrocytes, and is separated from the vitreous body by a basal lamina.

Figure 4 The layered arrangement of neuronal cell bodies in the retina and the interconnections of their processes in the intervening plexiform layers. Also shown are the two principal types of neuroglial cell in the retina; microglia are also present but not shown.

Optic disc

The optic disc is the region where retinal tissues meet the neural and glial elements of the optic nerve and the connective tissues of the sclera and meninges. It is the exit point for the optic nerve fibres, and a point of entry and exit for the retinal circulation. It is the only site where anastomoses occur with other arteries (the posterior ciliary arteries). It is visible, by ophthalmoscopy, and is a region of much clinical importance, since it is here that the central vessels can be inspected directly: the only vessels so accessible in the whole body. Oedema of the disc (papilloedema) may be the first sign of raised intracranial pressure, which is transmitted into the subarachnoid space around the optic nerve and compresses the central retinal vein where it crosses the space.

The optic disc is superomedial to the posterior pole of the eye, and so lies away from the visual axis. It is round or oval, usually c.1.6 mm in transverse diameter and 1.8 mm in vertical diameter, and its appearance is very variable (for details see Jonas et al 1988). In light-skinned subjects, the general retinal hue is a bright terracotta-red, with which the pale pink of the disc contrasts sharply; its central part is usually even paler and may be light grey. These differences are due in part to the degree of vascularization of the two regions, which is much less at the optic disc, and also to the total absence of choroidal or retinal pigment cells, since the retina is represented in the disc by little more than the internal limiting membrane. In subjects with strongly melanized skins, both retina and disc are darker . The optic disc does not project at all in many eyes, and rarely does it project sufficiently to justify the term papilla. It is usually a little elevated on its lateral side, where the papillomacular nerve fibres turn into the optic nerve. There is usually a slight depression where the retinal vessels traverse its centre.

RETINAL VASCULAR SUPPLY

The central retinal artery enters the optic nerve as a branch of the ophthalmic artery, c.1.2 cm behind the eyeball. It travels in the optic nerve to its head, where its fascicles traverse the lamina cribrosa. At this level, which is usually not visible to ophthalmoscopy, the central artery divides into two equal branches, superior and inferior. After a few millimetres, these divide into superior and inferior nasal, and superior and inferior temporal, branches. Each of these four supplies its own 'quadrant' of the retina, although each territory is much more than a quadrant, since the branches ramify as far as the ora serrata. Corresponding retinal veins unite to form the central retinal vein. However, the courses of the venous and arterial vessels do not correspond exactly, and arteries often cross veins, usually lying superficial to them. In severe hypertension the arteries may press on the veins and cause visible dilations distal to these crossings. Arterial pulsation is not visible by routine ophthalmoscopy without higher magnification.

The branching of the artery is usually dichotomous, and equal rami diverge at angles of 45-60°. Smaller branches may leave singly and at right angles. Arteries and veins ramify in the nerve fibre layer, near the internal limiting membrane, which accounts for their clarity when seen through an ophthalmoscope . Arterioles pass deeper into the retina and may penetrate to the internal nuclear lamina, from which venules return to larger superficial veins. The question of whether or not the dense capillary bed is diffusely organized or layered is unsettled. Some lamination has been identified, most noticeably at the interface between the inner nuclear and outer plexiform layers. The structure of the blood vessels resembles that of vessels elsewhere, except that the internal elastic lamina is absent from the arteries, and muscle cells may appear in their adventitia. Capillaries have a non-fenestrated endothelium.

OCULAR REFRACTIVE MEDIA

The components of the eye that transmit and refract light are the cornea, the aqueous humour, the lens and the vitreous body. Of these, only the refracting power of the lens can be varied.

Aqueous humour

To satisfy the requirements of vision the eye has its own circulatory system. Aqueous humour is secreted into the posterior chamber by the non-pigmented epithelium of the ciliary processes. It passes into the anterior chamber through the pupil and drains to the scleral venous sinus at the iridocorneal angle through the spaces of the trabecular tissue. It is responsible for maintaining the metabolism of the avascular transparent media, vitreous, lens and cornea, and it also maintains and regulates the relatively high intraocular pressure (c.17 mmHg), and hence the constancy of the ocular dimensions of the eyeball, via the balance between production and drainage. Depth of the anterior chamber may be assessed using slit-lamp biomicroscopy, and the filtration angle may be viewed directly by gonioscopy. Any interference with its drainage into the sinus increases intraocular pressure leading to the condition of glaucoma.

Lens

The lens is a transparent, encapsulated, biconvex body, which lies between the iris and the vitreous body. Posteriorly, the lens contacts the hyaloid fossa (p. 719) of the vitreous body. Anteriorly, it forms a ring of contact with the free border of the iris, but further away from the axis of the lens the gap between the two increases to form the posterior chamber of the eye (p. 708). The lens is encircled by the ciliary processes, and is attached to them by the zonular fibres which issue mainly from the pars plana of the ciliary body. Collectively, the fibres form the zonule which holds the lens in place and transmits the forces which stretch the lens (except in visual accommodation).

The lens has a characteristic shape. Its anterior convexity is less steep, and has a greater radius of curvature, than the posterior, which has a more parabolic shape. The central points of these surfaces are the anterior and posterior poles; a line connecting these is the axis of the lens. The marginal circumference of the lens is its equator. In fetuses the lens is nearly spherical, has a slight reddish tinge, and is soft, such that it breaks up on application of the slightest pressure. A hyaloid artery from the central retinal artery traverses the vitreous body to the posterior pole of the lens, whence its branches spread as a plexus. This covers the posterior surface and is continuous round the capsular circumference with the vessels of the pupillary membrane and iris.

In infants and adults the lens is avascular, colourless and transparent, but still quite soft in texture. In old age, the anterior surface becomes a little more curved, which pushes the iris forward slightly. It becomes less clear, with an amber tinge, and its nucleus is denser. In cataract, the lens gradually becomes opaque, causing blindness.

The dimensions of the lens are optically and clinically important, but they change with age as a consequence of continuous growth. Its equatorial diameter at birth is 6.5 mm, increasing rapidly at first, then more slowly to 9.0 mm at 15 years of age, and even more gradually to reach 9.5 mm in the ninth decade. Its axial dimension increases from 3.5-4.0 mm at birth to 4.75-5.0 mm at age 95. The radii of curvature reduce throughout life; the anterior surface shows the greater change as the lens thickens (Brown 1974). Average adult radii of the anterior and posterior surfaces are 10 mm and 6 mm respectively; the reduction during accommodation occurs mainly at the anterior surface.

Vitreous body

The vitreous body fills the vitreous chamber, and occupies about four-fifths of the eyeball. It is hollowed in front as a deep concavity, the hyaloid fossa, which is adapted to the lens. It is colourless, consisting of c.99% water, but not entirely structureless. At its perimeter it has a gel-like consistency (100-300μm thick) and is firmly attached to the surrounding structures of the eye; nearer the centre it has a more liquid zone in the form of long glycosaminoglycan chains, fills the whole vitreous. In addition, the peripheral gel or cortex contains a random loose network of type II collagen fibrils which are occasionally grouped into fibres. The cortex also contains scattered cells, the hyalocytes, which possess the characteristics of mononuclear phagocytes. They are responsible for the production of . Whilst they are normally in a resting state, they have the capacity to be actively phagocytic in inflammatory conditions. Hyalocytes are not present in the cortex bordering the lens. The liquid vitreous is absent at birth, appears first at 4 or 5 years, and increases to occupy half the vitreous space by the seventh decade. The cortex is most dense at the pars plana of the ciliary body adjacent to the ora serrata, where attachment is strongest, and this is often referred to as the base of the vitreous. Here the vitreous is thickened into a mass of radial (zonular) fibres which form the suspensory ligament of the lens

A narrow hyaloid canal runs from the optic nerve head to the central posterior surface of the lens. In the fetus this contains the hyaloid artery which normally disappears about 6 weeks before birth. It persists as a very delicate fibrous structure and is of no functional importance.

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Gaster Anatomy II

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

ARTERIES

The arterial supply to the stomach comes predominantly from the coeliac axis although intramural anastomoses exist with vessels of other origins at the two ends of the stomach (Figs 8, 9). The left gastric artery arises directly from the coeliac axis. The splenic artery gives origin to the short gastric arteries as well as the left gastroepiploic artery and may occasionally give origin to a posterior gastric artery. The hepatic artery gives origin to the right gastric artery and the gastroduodenal artery, which in turn gives origin to the right gastroepiploic artery.

Left gastric artery

The left gastric artery is the smallest branch of the coeliac axis. It ascends to the left of the midline and crosses the left crus of the diaphragm beneath the peritoneum of the upper posterior wall of the lesser sac. Here it lies adjacent to the left inferior phrenic artery and medial or anterior to the left suprarenal gland. It runs forwards into the superior portion of the lesser omentum adjacent to the superior end of the lesser curvature. It turns anteroinferiorly to run along the lesser curvature between the two peritoneal leaves of the lesser omentum. At the highest point of its course, it gives off an oesophageal branch. In its course along the lesser curvature, it gives off multiple branches that run onto the anterior and posterior surfaces of the stomach and anastomose with the right gastric artery in the region of the incisura angularis.

The left gastric artery may arise from the common hepatic artery or its branches. The most common variant is an origin from the left hepatic artery, when the left gastric artery passes between the peritoneal layers of the superior lesser omentum to reach the lesser curvature of the stomach. Other variants include a common origin with the common hepatic artery. An aberrant left hepatic artery can occasionally arise from the left gastric artery: identification of an aberrant origin may be of importance during surgical mobilization of the upper stomach.

Short gastric arteries

The short gastric arteries are variable in number, commonly between five and seven, and arise from the splenic artery, its divisions, or from the proximal left gastroepiploic artery. They pass between layers of the gastrosplenic ligament to supply the cardiac orifice and gastric fundus, and anastomose with branches of the left gastric and left gastroepiploic arteries. An accessory left gastric artery may arise with these vessels from the distal splenic artery.

Left gastroepiploic artery

The left gastroepiploic artery arises from the splenic artery as its largest branch near the splenic hilum. It runs anteroinferiorly between the layers of the gastrosplenic ligament and into the upper gastrocolic omentum. It lies between the layers of peritoneum close to the greater curvature, running inferiorly to anastomose with the right gastro epiploic artery. It gives off gastric branches to the fundus of the stomach through the gastrosplenic ligament and to the body of the stomach through the gastrocolic omentum. These are necessarily longer than the gastric branches of the right gastroepiploic artery and may be 8-10 cm long. Epiploic (omental) branches arise along the course of the vessel and descend between the layers of the gastrocolic omentum into the greater omentum. A particularly large epiploic branch commonly originates close to the origin of the left gastroepiploic artery, descends in the lateral portion of the greater omentum and provides a large arterial supply to the lateral half of the omentum.

Posterior gastric artery

Variant:
A distinct posterior gastric artery may occur. When present, it arises from the splenic artery in its middle section posterior to the body of the stomach. It ascends behind the peritoneum of the lesser sac towards the fundus. It reaches the posterior surface of the stomach in the gastrophrenic fold.

Right gastric artery

The right gastric artery arises from the hepatic artery as it passes forwards from the posterior wall of the lesser sac into the lower border of the lesser omentum above the first part of the duodenum. The right gastric artery then runs between the peritoneal layers of the lesser omentum just above the medial end of the lesser curvature. It passes superiorly along the lesser curvature, giving off multiple branches onto the anterior and posterior surfaces of the stomach, and anastomoses with the left gastric artery.

The origin of the right gastric artery is often variant. The most common alternative origins are from the common hepatic, left hepatic, gastroduodenal or supraduodenal arteries.

Figure 8 Arterial supply of the stomach

Gastroduodenal artery

The gastroduodenal artery arises from the common hepatic artery posterior and superior to the first part of the duodenum. It gives origin to the right gastroepiploic and superior pancreaticoduodenal arteries at the lower border of the first part of the duodenum.
Right gastroepiploic artery

The right gastroepiploic artery originates from the gastroduodenal artery behind the first part of the duodenum, anterior to the head of the pancreas. It passes inferiorly towards the midline between the layers of the gastrocolic omentum. It lies inferior to the pylorus and then runs laterally along the greater curvature. It ends by anastomosing with the left gastroepiploic artery. It is adjacent to the pylorus but, more distally, lies c.2 cm from the greater curvature of the stomach. Gastric branches ascend onto the anterior and posterior surfaces of the antrum and lower body of the stomach while epiploic branches descend into the greater omentum. It also contributes to the supply of the inferior aspect of the first part of the duodenum.

Arterial anastomoses of the stomach

There is an anastomosis between the oesophageal arteries originating from the thoracic aorta and the vessels supplying the fundus in the region of the cardiac orifice. At the pyloric orifice the extensive network of vessels supplying the duodenum allows for some anastomosis between vessels of superior mesenteric artery origin and the pyloric vessels. The major named vessels supplying the stomach form extensive arterial anastomoses both on the serosal surface and around the curvatures. The right and left gastroepiploic arteries and the left and right gastric arteries anastomose freely with each other along the greater and lesser curvatures respectively. Anastomoses also form between the short gastric and left gastric arteries in the region of the fundus, and between the right gastric and right gastroepiploic arteries in the region of the antrum. In addition to the extensive serosal anastomoses, networks form within the stomach wall at intramuscular, submucosal and mucosal levels. A true plexus of small arteries and arterioles is present within the submucosa: it supplies the mucosa and shows considerable regional variation both in the gastric wall and in the proximal duodenum. The rich arterial supply to the stomach ensures that the high mucosal blood flow required for physiological functioning is maintained even if one or more vessels become occluded. As a consequence, the stomach exhibits considerable resistance to ischaemia even when multiple arterial supplies are lost.

The pyloric arteries are rami of the right gastric and right gastroepiploic arteries and pierce the duodenum distal to the sphincter around its entire circumference. They pass through the muscular layer to the submucosa where they divide into two or three rami, which turn back into the pyloric canal beneath the mucosa and run to the end of the pyloric antrum . They supply the entire mucosa of the pyloric canal. Branches of these pyloric submucosal arteries may anastomose close to their origin with the duodenal submucosal arteries. Their terminal rami also anastomose with gastric arteries from the prepyloric antrum. The pyloric sphincter is supplied by the gastric and pyloric arteries via rami that leave their parent vessels in the subserosal and submucosal levels to penetrate the sphincter.

Dieu la Foy lesions

Abnormalities of the intramural vascularity of the stomach are a rare cause of bleeding from the upper gastrointestinal tract. So-called 'Dieu la Foy' lesions commonly occur in the proximal body or fundus. When not actively bleeding, they appear as small, raised, red dots marking the mucosal surface of the proximal body or fundus. They were originally thought to be small arteriovenous malformations of the submucosal plexus. It is now considered that such lesions are caused by a larger than normal penetrating arterial vessel running through the muscular coat of the stomach into the submucosa before branching into the submucosal plexus. Although not a pathological abnormality, the vessel has a greater than normal calibre for arteries at this level. The pulsatile flow, combined with its proximity to the overlying mucosa, may then lead to focal ulceration and rupture of the vessel following minor trauma, leading to profuse intraluminal bleeding.

VEINS

The stomach veins drain ultimately into the portal vein. A rich submucosal and intramural network of veins gives rise to veins that usually accompany the corresponding named arteries. They drain either into the splenic or superior mesenteric veins although some pass directly into the portal vein.

Short gastric veins

Four or five short gastric veins drain the gastric fundus and the upper part of the greater curvature. They drain into the splenic vein or one of its large tributaries.

Left gastroepiploic vein
The left gastroepiploic vein drains both anterior and posterior gastric surfaces and the adjacent greater omentum. It runs superolaterally along the greater curvature, between the layers of the gastrocolic omentum. It receives multiple tributaries from the anterior and posterior surfaces of the body of the stomach and the greater omentum, and drains into the splenic vein within the gastrosplenic ligament.

Right gastroepiploic vein
The right gastroepiploic vein drains the greater omentum, distal body and antrum of the stomach. It passes medially, inferior to the greater curvature, in the upper portion of the gastrocolic omentum. Just proximal to the pyloric constriction it passes posteriorly to drain into the superior mesenteric vein below the neck of the pancreas. It may receive the superior pancreaticoduodenal vein close to its entry into the superior mesenteric vein.

Left gastric vein
The left gastric vein drains the upper body and fundus of the stomach. It ascends along the lesser curvature to the oesophageal opening where it receives several lower oesophageal veins. It then curves posteriorly and medially behind the posterior peritoneal surface of the lesser sac. It drains into the portal vein directly at the level of the upper border of the first part of the duodenum.

Right gastric vein
The right gastric vein is typically small and runs along the medial end of the lesser curvature. It passes under the peritoneum as it is reflected from the posterior aspect of the pylorus and first part of the duodenum onto the posterior wall of the lesser sac. It drains directly into the portal vein at the level of the first part of the duodenum. It receives the prepyloric vein as it ascends anterior to the pylorus at the level of the pyloric opening.

Posterior gastric veins
Distinct posterior gastric veins may occur. When present, they accompany the posterior gastric artery from the middle of the posterior surface of the stomach. They drain into the splenic vein and may occur as multiple small vessels.

Gastric varices
Variceal dilatation of the submucosal veins of the stomach may occur in the presence of portal hypertension. The anastomosis between portal and systemic venous circulations occurs around the lower oesophagus and upper stomach. Submucosal veins close to the cardiac orifice may become involved in the pathological flow of blood from the stomach and other upper abdominal viscera into the oesophageal veins. Gastric varices present less commonly in clinical practice than oesophageal varices. Occasionally gastric varices exist without the presence of oesophageal varices. In these circumstances, it may be that the effective 'point of meeting' between portal and systemic venous systems is lower than usual and occurs in the upper stomach rather than the lower oesophagus.

LYMPHATIC DRAINAGE

The stomach has a rich network of lymphatics that connect with lymphatics draining the other visceral organs of the upper abdomen. At the gastro-oesophageal junction the lymphatics are continuous with those draining the lower oesophagus. In the region of the pylorus they are continuous with those draining the duodenum. In the main, they follow the course of the arteries supplying the stomach, however many separate node groups are now recognized (Fig. 11). The relationship of separate node groups to the regions of the stomach and the vascular territories supplied is of great importance during resection of the stomach, particularly for malignancy. Pancreatic and hepatic lymphatics play a considerable role in draining areas of the stomach during disease.

Figure 9 Lymph node stations of A, the stomach and B, upper abdominal viscera.

INNERVATION

The stomach is innervated by sympathetic and parasympathetic fibres. The sympathetic supply originates from the fifth to twelfth thoracic spinal segments and is mainly distributed to the stomach via the greater and lesser splanchnic nerves and the coeliac plexus. Periarterial plexuses form along the arteries and supply the stomach from the coeliac axis. Additional innervation comes from fibres of the hepatic plexus, which pass to the upper body and fundus via the upper limit of the lesser omentum. Some innervation is also provided via direct branches from the greater splanchnic nerves.

The parasympathetic supply is from the vagus nerves (Fig. 10). Usually one or two rami branch on the anterior and posterior aspects of the gastro-oesophageal junction. The anterior nerves are mostly from the left vagus and the posterior from the right vagus, both emerging from the oesophageal plexus.

The anterior nerves supply filaments to the cardiac orifice and divide near the oesophageal end of the lesser curvature into gastric, pyloric and hepatic branches. Gastric branches (between four and ten) radiate on the anterior surface of the body and fundus. The greater anterior gastric nerve is the major gastric branch and lies in the lesser omentum near the lesser curvature. Pyloric branches (generally two) originate below the cardiac orifice. The smaller of the two nerves runs between the peritoneal layers of the lesser omentum almost horizontally towards its free edge and turns down on the left side of the hepatic artery to reach the pylorus. The larger nerve usually arises from the greater anterior gastric nerve during its course over the anterior surface of the stomach and runs inferomedially to the pyloric antrum. Hepatic branches (one or two) originate from the pyloric branches and run superiorly to contribute to the hepatic plexus.

The posterior nerves produce two main groups of branches, gastric and coeliac. Gastric branches originate behind the cardiac orifice and upper body of the stomach. They radiate over the posterior surface of the body and fundus and extend to the antrum but do not reach the pyloric sphincter. The largest is termed the greater posterior gastric nerve and runs posteriorly along the lesser curvature, giving branches to the coeliac plexus. Coeliac branches are often larger than the gastric branches. They run beneath the peritoneum, deep to the posterior wall of the lesser sac, at the upper limit of the lesser omentum to reach the coeliac plexus. Hepatic branches (one or two) are often small and originate from the coeliac branches. No true plexus occurs on either the anterior or posterior gastric surfaces, but plexuses are present in the submucosa and between the layers of the muscularis externa.

The gastric sympathetic nerves are vasoconstrictor to the gastric vasculature and inhibitory to gastric musculature. The sympathetic supply to the pylorus is motor, and brings about pyloric constriction. The sympathetic supply also conducts afferent impulses that mediate sensations, including pain. The parasympathetic gastric supply is secretomotor to the gastric mucosa and motor to the gastric musculature. It is also responsible for coordinated relaxation of the pyloric sphincter during gastric emptying.

Coeliac plexus

The coeliac plexus is the largest major autonomic plexus, sited at the level of the twelfth thoracic and first lumbar vertebrae. It is a dense network uniting two large coeliac ganglia and surrounds the coeliac artery and the root of the superior mesenteric artery (Fig. 13). It is posterior to the stomach and lesser sac, anterior to the crura of the diaphragm and the commencement of the abdominal aorta, and lies between the suprarenal glands. The plexus and ganglia are joined by greater and lesser splanchnic nerves and branches from the vagus and phrenic nerves. The plexus extends as numerous secondary plexuses along adjacent arteries.

The coeliac ganglia are irregular masses on each side of the coeliac trunk adjacent to the suprarenal glands. They lie anterior to the crura of the diaphragm. The right ganglion is posterior to the inferior vena cava, the left ganglion posterior to the origin of the splenic artery. The ipsilateral greater splanchnic nerve joins the upper part of each ganglion. The lower part of each ganglion forms a distinct subdivision usually termed the aorticorenal ganglion. This receives the ipsilateral lesser splanchnic nerve and gives origin to the majority of the renal plexus. It most commonly lies anterior to the origin of the renal artery. The coeliac plexus is connected to or gives rise to the phrenic, splenic, hepatic, superior mesenteric, suprarenal, renal and gonadal plexuses.

Phrenic plexus

The phrenic plexus lies around the inferior phrenic arteries on the crura of the diaphragm. It arises as a superior extension of the coeliac ganglion and often receives one or two sensory branches from the phrenic nerve. The left phrenic plexus is usually larger than the right. On the left it supplies branches to the left suprarenal gland and the cardiac orifice of the stomach. The right phrenic plexus joins the phrenic nerve, forming a small phrenic ganglion. This distributes branches to the inferior vena cava, suprarenal gland and hepatic plexus.

Figure 10 Distribution of the vagal nerves to the stomach.

REFERRED PAIN
The majority of the sensation of pain arising from the stomach is poorly localized. In common with other structures of foregut origin, it is referred to the central epigastrium. Pain arising from the region of the gastro-oesophageal junction may involve innervation from the oesophagus and is commonly referred to the lower retrosternal and subxiphoid areas.

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Gaster Anatomy I

STOMACH ANATOMY I

Gray's Anatomy 39th

The stomach is the widest part of the alimentary tract and lies between the oesophagus and the duodenum. It is situated in the upper abdomen, extending from the left upper quadrant downwards, forwards and to the right, lying in the left hypochondriac, epigastric and umbilical areas. It occupies a recess beneath the diaphragm and anterior abdominal wall that is bounded by the upper abdominal viscera on either side. Its mean capacity increases from c.30 ml at birth, to 1000 ml at puberty, to c.1500 ml in adults. The peritoneal surface of the stomach is interrupted by the attachments of the greater and lesser omenta, which define the greater and lesser curvatures separating two surfaces

PARTS OF THE STOMACH

The stomach is divided for descriptive purposes into the fundus, body, pyloric antrum and pylorus, by arbitrary lines drawn on its external surface. The internal appearance and microstructure of these regions varies to some degree. The fundus is dome shaped and projects above and to the left of the cardiac orifice to lie in contact with the left dome of the diaphragm. It lies above a line drawn horizontally from the incisura cardiaca to the greater curvature. The body extends from the fundus to the incisura angularis, which is a constant external notch at the lower end of the lesser curvature. A line drawn from the incisura angularis to an indentation on the greater curvature defines the lower boundary of the body. The pyloric antrum extends from this line to the sulcus intermedius. At this point, the stomach narrows to become the pyloric canal, which is usually only 1-2 cm in length and terminates at the pyloric orifice.

Figure 1 The parts of the stomach.

GASTRIC RELATIONS

GASTRIC CURVATURES

Lesser curvature

The lesser curvature extends between the cardiac and pyloric orifices and forms the medial (posterior and superior) border of the stomach. It descends from the medial side of the oesophagus in front of the decussating fibres of the right crus of the diaphragm. It curves downwards and to the right and lies anterior to the superior border of the pancreas. It ends at the pylorus just to the right of the midline. In the most dependent part there is typically a notch, the incisura angularis, whose position and appearance vary with gastric distension. The lesser omentum is attached to the lesser curvature and contains the right and left gastric vessels.

Greater curvature

The greater curvature is four or five times longer than the lesser. It starts from the incisura cardiaca formed between the lateral border of the abdominal oesophagus and the fundus of the stomach. It arches upwards, posterolaterally and to the left. Its highest convexity, the apex of the fundus, is approximately level with the left fifth intercostal space just below the left nipple in males, but varies with respiration. From this level it sweeps inferiorly and anteriorly, slightly convex to the left, almost as far as the tenth costal cartilage in the supine position, where it turns medially to end at the pylorus. There is frequently a groove, termed the sulcus intermedius, in the curvature close to the pyloric constriction. The start of the greater curvature is covered by peritoneum, which continues over the anterior surface of the stomach. Laterally the greater curvature gives attachment to the gastrosplenic ligament and beyond this to the greater omentum, which contains the gastroepiploic vessels. The gastrosplenic ligament and the greater omentum, together with the gastrophrenic and splenorenal ligaments, are continuous parts of the original dorsal mesogastrium. The names merely indicate regions of the same continuous sheet of peritoneum and associated connective tissue.

Gastric volvulus

Volvulus of the stomach is much less common than volvulus of either the sigmoid colon or caecum. Two types of gastric volvulus may occur. The first, organoaxial volvulus, occurs about a line of rotation running from below the cardiac orifice to the pylorus. The antrum, body and fundus rotate upwards, with the greater curvature coming to lie above the lesser curvature as the volvulus progresses. The second, mesenteroaxial volvulus, occurs about a line drawn 'across' the body of the stomach, usually just above the incisura angularis. This type of volvulus is perpendicular to the line of organoaxial volvulus. The distal body and antrum rotate anteriorly, superiorly and laterally whilst the upper body and fundus rotate posteriorly, medially and inferiorly. Although relatively mobile within the upper abdomen, the stomach is normally tethered to the oesophagus at the gastro-oesophageal junction, to the duodenum at the pylorus, to the spleen by the gastrosplenic omentum, and to the liver by the lesser omentum. The attachment to the transverse colon via the gastrocolic omentum also restrains the stomach but is the most mobile of all. For either type of gastric volvulus to occur, it is necessary for some or all of these points of tethering to be loosened either by previous surgical division or by chronic lengthening and loosening of their connective tissue. Organoaxial volvulus is most common because the lesser omentum, gastrosplenic ligament and gastrocolic omentum are more likely to undergo chronic lengthening by traction than the other attachments of the stomach. Mesenteroaxial volvulus requires the gastro-oesophageal junction and pylorus to be sufficiently mobile as to come into close approximation. These structures are firmly tethered and consequently this form of gastric volvulus is much less common. Despite the profuse gastric arterial supply, either type of volvulus may compromise the vascularity of the stomach.

GASTRIC SURFACES

When the stomach is empty and contracted, the two surfaces tend to lie facing almost superiorly and inferiorly, but with increasing degrees of distension they come to face progressively more anteriorly and posteriorly.

Anterior (superior) surface

The lateral part of the anterior surface is posterior to the left costal margin and in contact with the diaphragm, which separates it from the left pleura, the base of the left lung, the pericardium and the left sixth to ninth ribs (Fig. 2). It lies posterior to the costal attachments of the upper fibres of transversus abdominis, which separate it from the seventh to ninth costal cartilages. The upper and left part of this surface curves posterolaterally and is in contact with the gastric surface of the spleen. The right half of the anterior surface is related to the left and quadrate lobes of the liver and the anterior abdominal wall. When the stomach is empty, the transverse colon may lie adjacent to the anterior surface. The entire anterior (superior) surface is covered by peritoneum.

Posterior (inferior) surface

Figure 2 Anterior relations of the stomach, viewed from behind

The posterior surface lies anterior to the left crus and lower fibres of the diaphragm, the left inferior phrenic vessels, the left suprarenal gland, the superior pole of the left kidney, the splenic artery, the anterior pancreatic surface, the splenic flexure of the colon and the upper layer of the transverse mesocolon (Fig. 3). Together these form the shallow stomach bed: they are separated from the stomach by the lesser sac (over which the stomach slides as it distends). The upper left part of the surface curves anterolaterally and lies in contact with the gastric surface of the spleen. The greater omentum and the transverse mesocolon separate the stomach from the duodenojejunal flexure and ileum. The posterior surface is covered by peritoneum, except near the cardiac orifice, where a small, triangular area contacts the left diaphragmaticcrus and sometimes the left suprarenal gland. The left gastric vessels reach the lesser curvature at the right extremity of this bare area in the left gastropancreatic fold. The gastrophrenic ligament passes from the lateral aspect of this bare area to the inferior surface of the diaphragm.

GASTRIC ORIFICES

CARDIAC ORIFICE AND GASTRO-OESOPHAGEAL JUNCTION

The opening from the oesophagus into the stomach is the cardiac orifice (Fig. 4). It is typically situated to the left of the midline behind the seventh costal cartilage at the level of the eleventh thoracic vertebra. It is c.10 cm from the anterior abdominal wall and 40 cm from the incisor teeth. The short abdominal part of the oesophagus curves sharply to the left as it descends and is continuous with the cardiac orifice. The right side of the oesophagus is continuous with the lesser curvature, the left side with the greater curvature. There is no specific anatomical cardiac sphincter related to the orifice.

Internally, the transition between oesophagus and stomach is difficult to define because mucosa of gastric fundal pattern extends a variable distance up into the abdominal oesophagus. It usually forms a 'zig-zag' squamo-columnar epithelial junction with the oesophageal epithelium above this Z line (p. 1152). This is often referred to as the gastro-oesophageal junction, for histological and endoscopic purposes. A sling of longitudinal gastric muscle forms a loop on the superior, left, side of the gastro-oesophageal junction between the oesophagus and the lesser curvature, and this is taken as the external boundary of this junction.

GASTRO-OESOPHAGEAL REFLUX

Figure 3 Posterior relations of the stomach.

Figure 4 The valve-like structure formed by the angle of the wall at the cardiac orifice. (Provided by Donald E Low, Department of Surgery, Virginia Mason, Seattle, USA.)

Reflux of gastric contents into the abdominal and lower thoracic oesophagus as a result of transient relaxation of the lower oesophageal sphincter occurs as a normal event in most individuals for a small percentage of their daily life. It also occurs as a result of a weak lower oesophageal sphincter, or of hiatus hernia which disrupts the normal anatomical barriers (p. 1083). Several anatomical and physiological factors normally prevent gastro-oesophageal reflux. The folds of gastric mucosa present in the gastro-oesophageal junction, the mucosal rosette, contribute to the formation of a fluid-and gas-tight seal. They also help to ensure that even low levels of tone within the lower oesophageal wall muscles may occlude the lumen of the junction against low pressures of gastric gas. The angle of the cardiac orifice may help to form a type of 'flap valve' and the length of abdominal oesophagus is buttressed externally by pads of adipose connective tissue at and below the level of the diaphragmatic hiatus. However, the major anti-reflux mechanism is the tonic contractions of the lower oesophageal musculature, which forms an effective high pressure zone (HPZ) (p. 986). The specialized smooth muscle of the wall of the lower oesophagus and the encircling fibres of the crural diaphragm exert a radial pressure that can be measured by a sensing device as it is withdrawn from the stomach into the oesophagus (Paterson 2001). If reflux is to be prevented, this pressure must always exceed the difference between the pressures on either side of the junction, i.e. the difference between intra-abdominal pressure (transferred to the stomach, and augmented by any contraction of the stomach wall itself), and intrathoracic pressure (transferred to the oesophagus).

During expiration, pressure exerted by tonic contraction of the smooth muscle of the lower oesophagus is normally sufficient to oppose the gastro-oesophageal pressure gradient. During inspiration, intra-abdominal pressure rises and intrathoracic pressure becomes more negative, increasing the risk of reflux. This tendency is opposed by additional pressure exerted by contraction of the crural fibres of the diaphragm. (Activation of the crural diaphragm slightly before the costal diaphragm would ensure that contraction of peri-oesophageal fibres preceded the increase in gastro-oesophageal pressure gradient.) The anti-reflux barrier must of course be lowered for swallowing and vomiting. Swallowing is followed immediately by expiration, which relaxes the crural fibres and allows the oesophageal contents to be transferred to the stomach by peristaltic movement. Vomiting is produced by bursts of activity involving co-contraction of the diaphragm, intercostal and abdominal muscles in a pattern distinct from that of respiration: this activity is coordinated with relaxation of the crural fibres around the oesophagus (Miller, 1990).

Barrett's oesophagus

The squamous epithelium lining the lower oesophagus may be pathologically replaced by a columnar, gastric type epithelium. This may occur as islands, strips, or circumferentially, and may extend for a variable length up the lower oesophagus. This process is most likely to be the result of the chronic reflux of gastric contents, acid or alkali, into the oesophagus with a resultant change in mucosal cell type. The abnormal columnar type epithelium present in the anatomical oesophagus is referred to as Barrett's epithelium.

PYLORIC ORIFICE

The pyloric orifice is the opening into the duodenum. The circular pyloric constriction on the surface of the stomach usually indicates the location of the pyloric sphincter and is often marked by a prepyloric vein crossing the anterior surface vertically downwards. The pyloric orifice typically lies 1-2 cm to the right of the midline in the transpyloric plane with the body supine and the stomach empty. The pyloric sphincter is a muscular ring formed by a marked thickening of the circular gastric muscle interlaced with some longitudinal fibres.

GASTRIC FORM AND INTERNAL APPEARANCES

It is clear from contrast radiographic studies that the form and position of the stomach are extremely variable depending on posture, the volume of its contents, and the surrounding viscera. They are also influenced by the tone of the abdominal wall and gastric musculature and by the build of the individual. The empty stomach is most commonly J-shaped and, in the erect posture, the pylorus descends to the level of the second or the third lumbar vertebra. The lowest part of the antrum often lies below the level of the umbilicus. The fundus usually contains gas. The overall axis of the organ is, therefore, slightly inclined from the vertical (Figs 5, 6). In short, obese individuals the axis of the stomach lies more towards the horizontal as a 'steer-horn' shape.

Variation caused by the contents of the stomach mainly affects the body because the pyloric part usually remains contracted during digestion. As the stomach fills, it expands forwards and downwards but, when the colon or small bowel is distended, the fundus enlarges towards the liver and diaphragm. As stomach capacity increases, the pylorus is displaced to the right and the axis of the whole organ lies in a more oblique direction (Figs 5, 7). In this position the anterior and posterior surfaces tend to face forwards and backwards and the lowest part is the pyloric antrum, which extends below the umbilicus. When intestinal distension interferes with downward expansion of the body, the stomach retains a horizontal position.

Figure 5 Axes of the empty and full stomach. As the stomach distends, the greater curvature 'rolls' downwards and the anterosuperior surface comes to lie almost completely vertical as the anterior surface.

During endoscopic examination (Fig. 6), the stomach is typically at least partially distended by air. The cardiac orifice and the lowest portion of the abdominal oesophagus viewed from above are typically closed at rest by tonic contraction of the lower oesophageal musculature. The gastric mucosa lining the orifice is puckered into ridges. It is present for a short but variable distance into the abdominal oesophagus and the transition between columnar and squamous epithelium is usually clearly visible. The presence of abnormal columnar epithelium within the anatomical oesophagus is referred to as Barrett's oesophagus but the precise definition of this condition is difficult. From within the distended stomach, the cardiac orifice appears in the medial wall of the fundus and is asymmetrical. The medial edge of the cardiac orifice is continuous with the medial wall of the body of the stomach. The mucosa is slightly thickened at this point with a raised profile, forming part of the 'mucosal rosette' that lines the orifice. The 'rosette' aids closure of the cardiac orifice and helps prevent reflux of stomach contents into the oesophagus. The medial edge of the orifice is more clearly visible than the lateral edge as it forms a more acute angle with the mucosal lining of the abdominal oesophagus.

Figure 6 Endoscopic appearance of the stomach: A, cardiac orifice from below; B, body greater curvature; C, body lesser curvature; D, pylorus.

In the partly distended stomach, the mucosa of the fundus is thrown into gentle folds with no particular pattern. As the stomach fills towards capacity, however, these folds rapidly become less pronounced, and the wall is nearly smooth when the stomach is over-inflated. The body of the stomach has the most pronounced mucosal folds. Even in moderate distension, they appear as long, broad mucosal ridges running in sinuous strips from fundus to pyloric antrum (Fig. 7). They are seen on all mucosal surfaces of the body but are most obvious on the anterolateral, lateral and posterolateral parts (which correspond to the inner surface of the anterior and posterior external surfaces and to the greater curvature). Here they are occasionally called the magenstrasse, a reference to their possible role in directing liquid entering the stomach immediately down into the pyloric antrum. These folds are least prominent on the medial surface (corresponding to the inner surface of the lesser curvature), which is much smoother, particularly when the stomach distends.

The areae gastricae within the antrum are small nodular elevations of the mucosal surface that are readily seen on double contrast barium meal (Fig. 7). The few folds present in the antrum when the stomach is relaxed disappear with distension. The antrum adjacent to the pyloric canal, the prepyloric antrum, has a smooth mucosal surface culminating in a slight puckering of the mucosa at the pyloric orifice caused by the contraction of the pyloric sphincter.

Figure 7 Double contrast barium meal in the erect position the stomach has a more 'J'-shaped configuration

GASTROSTOMY

Since the lower body and antrum of the stomach is related to the posterior aspect of the left anterior abdominal wall, it may usefully be accessed to form a gastrostomy. Its mobility enables the anterior surface of the stomach to be readily approximated to the parietal peritoneum on the posterior surface of the abdominal wall and a communication to be established between the lumen of the stomach and the surface of the skin. Although this may be performed as a direct open surgical procedure under general anaesthetic it is much more commonly performed using a percutaneous puncture guided by either endoscopic visualization of the stomach or radiological imaging. The procedure is made easier by the fact that the anterior surface of the stomach lies most nearly in the vertical plane when the stomach is distended. One of the main hazards of the procedure results from the occasional interposition of the transverse colon between the stomach and anterior abdominal wall. This may lead to inadvertent transfixion of the colon by the needle puncture system. The variable length of the transverse colonic mesentery means that it may sometimes lie adjacent to the anterior gastric surface when a subject is recumbent. These risks may be reduced by radiological guidance.

To be Continued... (The Vascular, Lymphatic and Innervation)

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Anatomy of the Neonates

The Anatomy of Full Term Neonate
Gray's Anatomy 39th edition. Elsevier. 2008

Figure 1 Timetable of development of the body systems. The development of individual systems can be seen progressing from left to right. Embryonic stages, weeks of development and embryo length are shown. Embryonic stages are associated with external and internal morphological features rather than embryonic length. To identify the systems and organs at risk at any time of development, follow a vertical progression from top to bottom. (Click Image to Enlarge)Figure 2 The two timescales used to depict human development. Embryonic development, in the upper scale, is counted from fertilization (or from ovulation, i.e. in postovulatory days; see O'Rahilly & Muller 1987). Throughout this book, times given for development are based on this scale. The clinical estimation of pregnancy is counted from the last menstrual period and is shown on the lower scale; throughout this book, fetal ages relating to neonatal anatomy and growth will have been derived from the lower scale. Note that there is a 2 week discrepancy between these scales. The perinatal period is very long, because it includes all preterm deliveries.

Immediately after parturition the fetus, once it has been exposed to the environment external to the maternal uterus, becomes a neonate. In Western societies, technological advances have enabled successful management of preterm infants, many at ages that were considered non-viable a decade or two previously. Now, the study of neonatology very much overlaps the later stages of fetal development. Preterm infants, although obviously past organogenetic processes, are still engaged in maturational processes with local interactions and pattern formation driving development at local and body-system levels. The sudden release of such fetuses into a gaseous environment, of variable temperature, with full gravity and a range of microorganisms promotes the rapid maturation of some systems and the compensatory growth, in terms of effect of gravity or enteral feeding or exposure to microorganisms, of others. To understand this multitude of mechanisms operating within a newly delivered fetus, as much information as possible concerning normal embryological and fetal development is required.

Details of the relative positions of the viscera and the skeleton in a full term neonate are shown in Figs 3A, B, C; 4. The newborn infant is not a miniature adult, and extremely preterm infants are not the same as full-term infants. Thus, just as there are immense differences in the relations of some structures between the full-term neonate, child and adult, so there are also major differences between the 20 week gestation fetus and the 40 week fetus, just before birth. The study of fetal anatomy at 20, 25, 30 and 35 weeks is vital for the investigative and life-saving procedures carried out on preterm infants today.

Figure 11.4 Topographical representation of the anatomy of a full-term neonate. The surface markings of all organs are shown, with some coloured and others only in outline. The female genital tract is shown on the right of the body in C, with the male tract on the left.Figure 4. The extent of the ossified skeleton in the full term neonate. Note the derivation of the parts of the skeleton: the skull is derived from paraxial mesenchyme and neural crest mesenchyme; the axial skeleton, vertebrae and ribs are derived from paraxial mesenchyme; the skeletal elements in the limbs are derived from the somatopleuric mesenchyme, which forms the limb buds.

Details of the relative positions of the viscera and the skeleton in a full term neonate are shown in Figs 3 and 4. The newborn infant is not a miniature adult, and extremely preterm infants are not the same as full-term infants. Thus, just as there are immense differences in the relations of some structures between the full-term neonate, child and adult, so there are also major differences between the 20 week gestation fetus and the 40 week fetus, just before birth. The study of fetal anatomy at 20, 25, 30 and 35 weeks is vital for the investigative and life-saving procedures carried out on preterm infants today.

Neonatal measurements and period of time in utero
The 10th to 90th centile ranges for length of full-term neonates are c.48 cm to c.53 cm Length of the newborn is measured from crown to heel. In utero, length has been estimated either from crown-rump length, i.e. the greatest distance between the vertex of the skull and the ischial tuberosities, with the fetus in the natural curved position, or from the greatest length exclusive of the lower limbs. Greatest length is independent of fixed points and thus much simpler to measure. It is generally taken to be the sitting height in postnatal life. This measurement is recommended by O'Rahilly and Muller (2000) as the standard in ultrasound examination. The 10th to 90th centile ranges for weight of the full-term infant at parturition ranges are c.2700 g to c.3800 g , the average being 3400 g; 75-80% of this weight is body water and a further 15-28% is composed of adipose tissue. After birth, there is a general decrease in the total body water, but a relative increase in intracellular fluid. Normally, the newborn loses c.10% of the birth weight by 3-4 days postnatally, because of loss of excess extracellular fluid and meconium. By 1 year, total body water makes up 60% of the body weight. Two populations of neonates are at particular risk, namely those who are preterm, and those who are small-for-dates, some of whom have suffered 'intrauterine growth restriction'.
Low birth weight has been defined as less than 2500 g, very low birth weight as less than 1500 g, and extremely low birth weight as less than 1000 g. Infants may weigh less than 2500 g but not be premature by gestational age. Measurement of the range of weights fetuses may attain before birth has led to the production of weight charts, which allow babies to be described according to how appropriate their birth weight is for their gestational age, e.g. small for gestational age, appropriate for gestational age and large for gestational age. Small for gestational age infants, also termed 'small-for-dates', are often the outcome of intrauterine growth retardation. The causes of growth restriction are many and various and beyond the scope of this text.
For both premature and growth-retarded infants, an assessment of gestational age, which correlates closely with the stage of maturity, is desirable. Gestational age at birth is predicted by its proximity to the estimated date of delivery and the results of ultrasonographic examinations during pregnancy. It is currently assessed in the neonate by evaluation of a number of external physical and neuromuscular signs. Scoring of these signs results in a cumulative score of maturity that is usually within ± 2 weeks of the true age of the infant. The scoring scheme has been devised and improved over many years. For an account of methods of assessing gestational age in neonates, consult Gandy (1992).

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Liver Anatomy

THE LIVER ANATOMY

Pictures from Lippincott Williams & Wilkins Atlas of Anatomy, 1st Edition. 2008

Fig. 1. The Liver (Click image to enlarge)

The liver takes up the majority of the right upper abdominal cavity and extends from the right lateral aspect of the abdomen 15 to 20 cm transversely toward the xiphoid. The weight of the adult liver varies from 1200 to 1800 g, dependent on the overall body size, and constitutes about 1.8% to 3.1% of the total body weight; however, at birth, the liver is larger compared to adjacent thoracic and abdominal viscera and constitutes about 5% to 6% of the body weight.The liver anatomically has four lobes: right, left, caudate, and quadrate. The right lobe accounts for one half to two thirds the total liver volume; however, functionally, the right and left lobes are of about equal size and are divided by a line extending from the inferior vena cava superiorly to the middle of the gallbladder fossa inferiorly. A total of eight functional segments are present, each demarcated by the vascular and biliary drainage: the lateral (segments VI and VII) and medial (segments V and VIII) divisions of the right lobe, the medial (segment IV) and lateral (segments II and III) divisions of the left lobe, and the caudate lobe (segment I), the latter a “watershed” area of the right and left lobe vasculature.

The hepatoduodenal ligament connects the liver to the superior aspect of the duodenum and supports the hilar vessels and duct structures. The transverse fissure separates anteriorly the right lobe from the caudate lobe, whereas the umbilical fissure is located to the left of the quadrate lobe, which itself is bordered on the right by the gallbladder. The peritoneal layers forming the falciform ligament, which extends between the liver and the anterior abdominal wall, separate to form the superior layer of the coronary ligament and the left triangular ligament. The ligamentum teres is located along the lower edge of the falciform ligament and contains the obliterated umbilical vein remnant. The total surface area of the liver is structured by direct continuity with the surrounding abdominal organs, ligaments, and fascia.

Fig.2. Liver Lobes and Ligaments

The portal vein is the main route of vascular drainage of the gastrointestinal tract and is formed through the merger of the superior mesenteric and splenic veins. It also receives blood from the coronary and cystic veins. The portal vein is located along the hepatoduodenal ligament posterior to the hepatic artery and common bile duct and ends at the porta hepatis at the main lobar fissure, dividing into the right and left main branches. The right branch divides early into anterior and posterior segments, whereas the left branch divides into two segments: the pars transversus which extends to the left in the porta hepatis, and the pars umbilicus which descends into the umbilical fossa in line with the left segmental fissure. The caudate lobe veins arise from both the right and left main portal vein branches.

The hepatic vein is composed of three major tributaries (right, middle, left), each having intrahepatic branches. The middle and left hepatic eins often converge to form a single outflow vessel before draining into the inferior vena cava, whereas the right hepatic vein opens through a separate ostium. The caudate lobe drains directly into the inferior vena cava.

Fig.3. Internal Features of Liver (Vascular)

The hepatic artery is a branch of the celiac artery and ascends along the hepatoduodenal ligament and eventually divides into the right and left main branches. The right hepatic artery is usually seen behind the common hepatic duct after giving rise to the cystic artery, and it eventually divides into the anterior and posterior segmental branches. The left hepatic artery obliquely passes upward and to the left in the porta hepatis, eventually dividing into the medial and lateral segmental branches. The quadrate lobe is fed by a middle hepatic artery branch, whereas the caudate lobe is fed by both right and left hepatic artery branches.

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