Chapter XII: Part II (4)

To the left of these structures the omental tuberosity of the pancreas projects above the level of the lesser curvature under cover of the secondary parietal peritoneum forming the dorsal wall of the lesser sac, while the lower edge of the Spigelian lobe appears in the upper angle of the incision.

If we remember that the liver is itself welded to the diaphragm between the layers of the coronary ligament (Fig. 280), it will become apparent that the serous surface of the Spigelian lobe forms part of the ventral wall of a peritoneal recess situated behind the lesser omentum, between this membrane and the diaphragm. Access to this recess, without the division of peritoneal layers, can only be obtained by passing from right to left, along the caudate lobe, between the vena cava behind, covered by parietal peritoneum, and the free right edge of the lesser omentum in front. (In the reverse direction of the arrow shown in Fig. 284.) This hepatic or Spigelian recess of the lesser peritoneal cavity has categorically the following boundaries (Figs. 282 and 283):

Dorsal: Parietal peritoneum, reflected along the line CD, from the caudal layer of the coronary ligament to the diaphragm.

Ventral: Visceral peritoneum investing the Spigelian lobe and the gastro-hepatic omentum.

Right: Reflection of peritoneum along the line DE (caval fissure) to become the parietal peritoneum covering the diaphragm.

Left: Right layer of lesser omentum, reflected along the fissure for the ductus venosus (CB) to the cardiac portion of the lesser curvature, continuous with the dorsal layer of the lesser omentum reflected from the transverse fissure to the pyloric segment of the lesser curvature (AB).

We will presently see that certain relations of the vessels connected with the liver at the transverse fissure and of the duodenum prevent the finger, when passed from right to left behind the free right edge of the lesser omentum and along the caudate lobe of the liver, from proceeding downward at this point. A narrow channel of communication is thus formed between the Spigelian recess and rest of the lesser sac on the one hand, and the general greater peritoneal cavity on the other. This channel is the so-called foramen of Winslow.

Having once passed this narrow space the finger will be in the Spigelian recess and can palpate its boundaries. Further progress cephalad and to the right is barred by the diaphragmatic adhesions of the liver just detailed. But in the direction downward behind the lesser omentum and along the dorsal surface of the stomach, as well as to the left toward the spleen the excursion is limited only by the length of the examining finger.

After opening the abdominal cavity of the human adult, elevating the liver and depressing the stomach, the hepatic attachment of the lesser omentum can be traced as already described. It will then be observed that the gastric attachment of the membrane lies in one plane following the lesser curvature while the hepatic attachment forms a broken line, with the angle situated at the left extremity of the transverse fissure. The vertical segment of the hepatic attachment, occupying the fissure for the ductus venosus, turns at this angle into the transverse segment which follows the transverse fissure to its right extremity where the two layers pass into each other around the right free omental margin (hepato-duodenal ligament). Consequently we overlook, in an abdominal cavity thus exposed, the entire caudal surface of the liver, including the caudal surfaces of right, left, and quadrate lobes. The junction of right and caudate lobes can be seen between vena cava and right edge of the omentum, or rather, it can be felt at this point. But the Spigelian lobe, turning its surface dorsad against the parietal peritoneum covering the diaphragm, forms part of the "posterior" liver surface and is not visible, although--as just stated, it can be palpated by passing the finger through the foramen of Winslow. The Spigelian lobe cannot be overlooked in its entire extent until the liver is removed from the body and regarded from behind. The caudal edge (continuation of its right angle into the caudate lobe and papillary tubercle) can be seen by tearing through the layers of the lesser omentum and lifting the liver up forcibly (Fig. 286).

=Caudal Boundary of Foramen of Winslow.=--We have above referred to the fact that the finger introduced through the foramen of Winslow meets in this canal with resistance if an attempt is made to pass downwards. After passing this constricting point the free excursion into the Spigelian recess and behind the omentum and stomach and toward the spleen can be performed.

In considering the elements which produce this narrowing of the communication between the two peritoneal sacs at the foramen of Winslow we have to deal with two factors, one primary and constant, the other secondary and inconstant.

1. The first of these is afforded by the arrangement of the arterial vessel supplying the liver. The hepatic artery is a branch of the coeliac axis, furnishing arterial blood to the liver tissues and supplying, in addition, branches to the stomach, duodenum and pancreas.

This vessel is, of course, placed primarily, like all other arterial branches supplying the alimentary tract, between the layers of the primitive dorsal mesentery. Originally the vessel supplies the distal (pyloric) portion of the stomach along its dorsal attached border (subsequently the greater curvature) corresponding to the adult gastro-epiploica dextra of the hepatic (gastro-duodenalis).

It likewise gives branches to the adjacent pyloric portion of the duodenum and the pancreas, as that gland develops from the intestine, corresponding to the adult superior pancreatico-duodenal branch, and to the ventral border (lesser curvature) of stomach, corresponding to the adult pyloric branch of the hepatic.

With the development of the liver from the duodenum arterial branches derived from this primitive gastro-duodenal vessel pass to the sprouting hepatic cylinders by continuing around the duodenum, beneath its serous investment, to reach the interval between the two layers of the ventral mesogastrium, in which the liver develops, near the free margin of this membrane.

After the rotation, which turns the right side of the stomach, duodenum and mesoduodenum dorsad, the branch which passes over the dorsal surface of the duodenum to reach the liver becomes more favorably situated and develops into the main hepatic artery which reaches the liver at the transverse fissure between the folds of the lesser omentum. The original right side of the duodenum, now turned dorsad, adheres to the parietal peritoneum. The hepatic artery which reached the liver by passing over this surface of the duodenum, beneath its visceral serous covering, becomes imbedded in connective tissue by the adhesion of the visceral duodenal and the primitive parietal peritoneum. Hence in the adult the hepatic artery courses imbedded in the connective tissue which binds the duodenum to the abdominal background to reach the interval between the two omental layers which carry it to the transverse fissure.

The hepatic artery, therefore, derived from one of the primitive intestinal branches (gastro-duodenal) is, notwithstanding its hidden position in the adult, originally situated between the layers of the free primitive dorsal mesogastrium.

It now becomes necessary to regard the development of the great omentum from the primitive dorsal mesogastrium in relation to this course of the hepatic artery. We have seen that the great omentum and the cavity of the omental bursa is produced by the extension of the dorsal mesogastrium to the left and caudad, subsequent to the rotation of the stomach. The splenic artery and the left gastro-epiploic branch pass from the coeliac axis to the left between the layers of the mesogastrium, as previously seen (Figs. 291 and 292).

The hepatic artery, however, is so to speak placed on the border line between the portion of the primitive mesentery which, as dorsal mesogastrium, is to turn to the left and caudad to form the great omentum, and the portion which, as mesoduodenum, turns to the right and passes to the duodenal loop (Fig. 287).

In the further course of development the dorsal mesogastrium grows more and more, forming the omental bag, while the mesoduodenum on the other hand becomes anchored early and obliterated as a free membrane by adhesion of its original right layer to the primitive parietal peritoneum. The hepatic artery runs on the line dividing these two different mesenteric segments. We can imagine, so to speak, that the redundant growth of the omentum to the left and caudad, takes place over the hepatic artery as a resistant support (Figs. 288 and 289). Cephalad of the hepatic artery is the developing omentum, caudad of the vessel the mesoduodenum. The artery follows the cephalic limit of the mesoduodenum and becomes, as stated, adherent to the abdominal background in the segment between its origin from the coeliac axis and the point where, after having crossed the dorsal surface of the duodenum, it enters the right edge of the lesser omentum on its way to the liver.

=Pancreatico-gastric Folds.=--If we open the lesser peritoneal cavity by dividing the gastro-hepatic omentum and look into the background of the retro-omental space, we will see a fold of the secondary lining parietal peritoneum (derived from the mesogastrium), which can be traced from the cephalic border of the pancreas to the pyloric extremity of the stomach. This fold carries the hepatic artery to the lesser omentum behind the first portion of the duodenum, and is called the right or main pancreatico-gastric fold. A similar fold, further to the left, carries in a like manner the coronary artery of the stomach to the cardiac end of the lesser curvature. This fold forms the left or secondary pancreatico-gastric fold. Between the two folds the caudal margin of the Spigelian lobe projects into the lesser cavity.

The appearance of the two pancreatico-gastric folds in the adult human subject is well seen in Fig. 284.

Fig. 290 shows the abdominal cavity of _Nasua rufa_, with great omentum divided to bring into view the vessels passing from coeliac axis to liver and stomach and elevating the retrogastric parietal peritoneum to produce the pancreatico-gastric folds.

(The course of the hepatic artery from coeliac axis to liver in the dorsal view in the cat is seen in Fig. 223.)

Figs. 291 and 292 represent schematically cross-sections directly through the foramen of Winslow, showing the method by means of which the hepatic artery reaches the upper border of the duodenum and the effect of the adhesion of duodenum and mesoduodenum upon the disposition of the vessel.

The coronary artery, like the splenic, is at first situated between the layers of the dorsal mesogastrium (vertebro-splenic segment). Like the splenic the coronary artery becomes anchored to the abdominal background and placed secondarily behind the parietal peritoneum of the lesser sac by the adhesion of this mesogastric segment to the primitive parietal peritoneum. To reach the lesser curvature at the cardia and to run thence from left to right along the lesser curvature between the layers of the gastro-hepatic omentum, the vessel raises the investing parietal peritoneum (originally the right leaf of the dorsal mesogastrium) into a crescentic fold, extending between its origin from the coeliac axis at cephalic margin of pancreas and the beginning of the lesser curvature of the stomach. Hence this fold is called the left pancreatico-gastric fold. (Seen well in Fig. 284.)

In the next place it must be borne in mind that the relation of the primitive hepatic artery to the vascular supply of the stomach, pancreas and duodenum produces a permanent shortening of the primitive mesentery at this point. This result is indicated in the schematic figures 287, 288 and 289.

In the original condition the dorsal mesentery, passing to a practically straight intestinal tube, is of uniform sagittal measure (Fig. 287).

As development proceeds, and as the liver grows from the duodenum, the hepatic artery develops from the primitive pyloric vessel as above indicated. This vessel, assuming greater importance with the rapid growth of the liver, is not lengthened out as happens with the remaining purely intestinal branches which follow the increase in the length of the intestinal canal. The hepatic artery, therefore, will mark the point where the original short sagittal extent of the primitive mesentery will tend to be preserved. Cephalad of this point the dorsal mesogastrium grows out into the great omentum (Figs. 288 and 289); caudad of the same point the membrane, in following the development of the intestine, becomes drawn out into the permanent mesentery and mesocolon.

The hepatic artery, in addition, marks the cephalic limit of the adhesion which anchors the duodenum and mesoduodenum to the parietal peritoneum. Consequently in the adult the vessel courses in as direct a manner as possible, taking the shortest course from the coeliac axis to the liver, passing dorsad of the duodenum and giving what now appear as secondary branches to supply the intestine, the stomach and pancreas (pyloric and gastro-duodenal arteries (pancreatico-duod. superior and gastro-epiploica dextra)).

Even if no fixation of the duodenum and mesoduodenum takes place this course of the hepatic artery will produce a constricted passage between the liver (caudate lobe) cephalad, abdominal parietes and aorta dorsad, lesser omentum and pyloric duodenum ventrad, and hepatic artery caudad. This passage leading from the general peritoneal cavity into the retrogastric space is the _primitive foramen of Winslow_. This condition is well represented in the abdominal cavity of some of the lower mammalia, in which duodenum and mesoduodenum remain permanently free.

Fig. 293 shows a view of the abdominal cavity from the right side in a specimen of the ant-eater, _Tamandua bivittata_.

The right kidney is seen in the background, covered by the parietal peritoneum. The duodenum and mesoduodenum are free and can be turned toward the median line. The opening of the foramen of Winslow leading into the retrogastric space is seen between the liver cephalad, kidney and vena cava dorsad, lesser omentum and pyloric extremity of the stomach ventrad, and a fold of peritoneum carrying the hepatic artery caudad. Exactly similar conditions prevail in the cat and in many other mammals.

It will be seen in all these instances that neither portal vein nor bile-ducts limit the foramen caudad. These structures can be lifted up and turned toward the median line with the free duodenum and mesoduodenum. But the hepatic artery must pass to the liver from the _retroperitoneal coeliac axis_. In doing this the vessel traverses the cephalic border of the pancreas, and the pyloric extremity of the stomach and duodenum, to reach the lesser omentum which conveys it to the liver.

Consequently there must always be a narrow peritoneal neck between the liver cephalad, aorta dorsad, hepatic artery caudad, and pyloric extremity of stomach and duodenum together with the lesser omentum ventrad. It should be remembered that the vessel which extends after the development of the liver into the lesser omentum as the _hepatic_ artery, was originally destined for the supply of these latter structures. In the adult these primary embryonic terminal branches to the intestine appear as secondary branches derived from the hepatic as the main vessel. Their origin, however, serves to keep the beginning of the small intestine in comparatively close connection with the hepatic artery which courses over the dorsal surface of the duodenum to reach the liver. The narrow space thus left between aorta, hepatic artery, duodenum, lesser omentum and liver forms the framework of the foramen of Winslow and appears always as a confined and narrow channel. This relation is shown in the accompanying schematic Figs. 294 and 295 which represent a sagittal section through the foramen. This primitive foramen is thus bounded cephalad by the liver (caudate lobe, connecting Spigelian and right lobes), ventrad by the first portion of the duodenum and the lesser omentum, with hepatic artery behind the intestine and between the omental layers; dorsad by the abdominal background and large retroperitoneal vessels, and caudad by the coeliac axis and beginning of the hepatic artery.

2. In the forms which possess in the adult an adherent duodenum and mesoduodenum, as in man, the foramen of Winslow obtains a secondary caudal limit by the agglutination of the descending duodenum and the parietal prerenal peritoneum. This is the secondary and inconstant factor referred to above in the caudal boundary of the foramen. The result of this anchoring of duodenum and mesoduodenum is to bring the margin of the foramen further to the right and to bury the hepatic artery still further from view. Thus in the adult human subject the structures bounding the foramen at the margin of the entrance into the narrow channel would be above caudate lobe of liver, behind postcava, below duodenum adherent to ventral surface of right kidney, in front first portion of duodenum and lesser omentum. The hepatic artery will be felt on introducing the finger through the foramen in its original position, but it will be seen that the actual boundaries of the foramen have been moved so to speak a little further to the right by the duodenal adhesion.

Fig. 296 shows a complete dissection of the adult human viscera and vessels concerned in the formation of the foramen, hardened in situ.

The stomach is removed, dividing of course the coronary artery and vein and the left gastro-epiploic artery. The portal vein, hepatic artery and bile-duct are seen entering and leaving the liver at the transverse fissure. Behind them and to the right the vena cava enters the liver. The hepatic artery distributes its pancreatico-duodenal branches to the duodenum and pancreas. The left angle of the Spigelian lobe and the fissure for the ductus venosus appear to the left of the portal vein and hepatic artery. The right angle of the Spigelian lobe and its continuation into the right lobe by means of the caudate lobe is hidden by the structures occupying the transverse fissure. We would enter the beginning of the foramen of Winslow by passing between the vena cava behind, the structures in the transverse fissure (portal vein, hepatic artery and duct) in front, caudate lobe of liver above and duodenum below, the latter in the undisturbed condition of the parts adherent to the right kidney. Continuing to the left the finger would pass between aorta behind, coeliac axis and hepatic artery below and in front, and liver above. These structures bound the permanent and primary narrow channel of communication between the retrogastric or lesser peritoneal space and the general peritoneal cavity, which exists even if a free duodenum and mesoduodenum allow us to lift the intestine away from vena cava and right kidney.

The main facts pertaining to the structure of the lesser peritoneal sac and its connection with the greater peritoneal cavity by means of the foramen of Winslow may be summed up as follows:

The mesogastrium as a whole, expanding originally in the sagittal plane in a fan-shaped manner between the vertebral column and the ventral abdominal wall, from the level of the umbilicus to the septum transversum (diaphragm), divides the cephalic part of the abdominal cavity into a symmetrical right and left half.

Figs. 172 and 273 represent the membrane as seen in a profile view from the left side. We distinguish the segment dorsad of the stomach as the dorsal mesogastrium, directly continuous with the remaining segments of the common primitive dorsal mesentery, while the portion ventrad of the stomach forms the ventral mesogastrium in which the liver develops. The segment of the ventral mesogastrium between liver and stomach becomes the lesser or gastro-hepatic omentum, while that between liver and ventral abdominal wall forms the falciform or suspensory ligament.

A transection, showing the dorsal and ventral mesogastrium at the level of the fundus of the stomach, is given in Fig. 298. The mesogastria are here seen to be short, while in the schematic Figs. 291 and 292 the membrane is, for the sake of distinctness, represented as being of considerable extent.

The ventral mesogastrium surrounding the liver and stomach extends caudad to include the first portion of the duodenum. Beyond this point it terminates in a thickened free edge which includes the umbilical vein. This vein extends from the umbilicus to the transverse fissure of the liver (Fig. 297), lying within the umbilical fissure on the caudal surface of the gland.

At the point where the vein enters the liver the thickened margin of the ventral mesogastrium is continued, as ligamentum hepato-duodenale, to the upper part of the duodenum and forms the ventral boundary of the foramen of Winslow. Between the layers of the mesogastrium which meet in this margin are situated the portal vein, biliary duct and hepatic artery, together with the nerves and lymphatics of the liver.

The mesogastrium originally divided the abdominal cavity between umbilicus and diaphragm into symmetrical right and left halves of equal size and extent. This early symmetrical arrangement becomes disturbed about the seventh week by the rotation of the stomach and the resulting altered course of the mesogastrium, which render the two original equal halves of the abdominal cavity unequal and asymmetrical. The original right half becomes placed behind the stomach and is converted into a blind sac with its opening directed to the right.

The communication of the general abdominal cavity with the retrogastric space by means of this channel is still wide in the embryo, but gradually becomes narrowed in the course of further development to form the foramen of Winslow. This opening is situated between the hepato-duodenal ligament and the parietal peritoneum covering the vena cava. It is constricted from below by the curve of the hepatic artery as this vessel passes from the coeliac axis to reach the liver at the transverse fissure between the layers of the lesser omentum.

The earlier developmental stages of the higher mammalian embryos are in general well illustrated by the permanent adult conditions found in some of the lower vertebrates, in which development does not proceed beyond the primitive condition.

In reptiles, birds and mammals the epiploic bursa is generally formed, while in amphibia the dorsal mesogastrium is very short and connects the stomach directly to the dorsal midline of the abdominal cavity without forming the sac-like extension of the great omentum.

The dorsal mesogastrium with the stomach, and the ventral mesogastrium including the liver between its layers, divides in these animals the cephalic part of the body cavity into two halves, corresponding to the earlier embryonic stages in man and in the higher mammalia.

The foramen of Winslow of the higher forms appears in the lower vertebrates as the wide-open space leading from below into the right half of the coelom cavity. The dorsal mesogastrium remains short, not forming the pouch-like extension of the great omentum. The stomach retains more or less its primitive vertical position without rotation or elevation of the pyloric extremity, and the intestinal canal is simple, short and comparatively straight.