Chapter X: Part II (2)

The same process of fixation, which resulted in the anchoring of duodenum and head of pancreas, extends to the body of the gland and the investing omentum. The peritoneum lining the original left, now the dorsal surface of the gland, fuses with the primitive parietal peritoneum covering the diaphragm and the left kidney. The main body of the pancreas in the adult appears prismatic, giving a triangular sagittal section. The dorsal surface is adherent to the ventral surface of the left kidney; the ventral surface is covered by the secondary parietal peritoneum (original right layer of mesogastrium) which lines the dorsal wall of the retrogastric space and omental bursa (lesser peritoneal sac). The great omentum now appears to take its dorsal point of departure along the sharp margin which separates this ventral surface of the pancreas from a third narrower surface directed caudad. This surface, under the conditions which we are at present examining, would be lined by the peritoneum continued onto it from the dorsal layer of the great omentum. This peritoneum merges along the dorsal margin of this caudal surface of the pancreas with the general parietal peritoneum covering the left lumbar region and the caudal part of ventral surface of the left kidney. We have, therefore, along this line a secondary transition from visceral to parietal peritoneum, obtained by the obliteration of the original visceral peritoneum investing the dorsal surface of the pancreas before adhesion to the parietal peritoneum.

The pancreas assumes, therefore, in the adult a secondary retro-peritoneal position, covered on its ventral surface by peritoneum of the "lesser sac," while the caudal surface is lined by part of the general peritoneal membrane of the "greater sac." The dorsal surface, denuded of serous covering by obliteration, is adherent to the crura of the diaphragm, the aorta and the ventral surface of the left kidney.

It is now proper to compare the conclusions just derived from the study of the development of the human dorsal mesogastrium and connected structures (spleen and pancreas) with the conditions presented by the corresponding parts in one of the lower mammalia, which illustrate some of the human embryonal stages. Here again the abdominal cavity of the cat forms an instructive object of study.

The purpose of the following comparison should be twofold:

I. The mesogastrium, spleen and pancreas in the cat will clearly illustrate the process of human development above outlined.

II. The abdominal viscera of the cat, if properly arranged, will enable us to complete the consideration of this region by including the very important relations which the transverse colon and third portion of the duodenum bear in man to the great omentum and pancreas.

I. SPLEEN, PANCREAS AND GREAT OMENTUM OF CAT.

After opening the abdominal cavity it will be seen that the great omentum can be lifted up, exposing the subjacent coils of the small and large intestine, to which it adheres at no point. In other words the entire dorsal surface of that part of the original mesogastrium which forms the great omentum is free. It will be remembered that this is not the case in the adult human subject, because here the dorsal surface of the great omentum adheres to the transverse colon. Consequently in man only that portion of the dorsal surface of the omentum can be seen which extends between the transverse colon and the caudal free edge of the membrane.

It will be noted that on the left side the spleen is connected by its mesal surface to the omentum and through it with the stomach (gastro-splenic omentum). In other words the cat illustrates the human embryonal stage in which the spleen has appeared between the layers of the dorsal mesogastrium at the extreme left or blind end of the retrogastric pouch formed by the rotation of the stomach and elongation of the mesogastric membrane, but _before_ the adhesion has taken place between the original _left_ (now _dorsal_) layer of the vertebro-splenic segment of the mesogastrium and the primitive parietal peritoneum apposed to it (Fig. 219). Consequently the dorsal wall of the "lesser" sac in the cat is still composed of the two layers of the free vertebro-splenic segment of the mesogastrium, the primitive right (now ventral) layer not having been converted, as is the case in man, into secondary parietal peritoneum by adhesion of the original left (now dorsal) layer to the primitive prerenal parietal peritoneum.

If we now examine the relation of the pancreas to the peritoneum we can establish the following facts:

1. The portion of the gland adjacent to the duodenum, corresponding to the "head" of the human organ, is included between the two layers of the mesoduodenum. This membrane is free, so that the dorsal surface of this portion of the pancreas is seen to be invested by the dorsal layer of the mesoduodenum (Fig. 223). The duodenum and the mesoduodenum, the latter containing the head of the pancreas between its layers, can be turned toward the median line, so as to expose the entire ventral surface of the post-cava and right kidney. To illustrate the arrangement which is found in the adult human subject the descending duodenum and pancreas should be allowed to fall over to the right so as to cover the vena cava and the mesal part of the ventral surface of right kidney. The adult human condition will now be produced if we assume that the structures are fixed in this position by the obliteration of the apposed serous surfaces, viz., the parietal peritoneum over kidney and vena cava on the one hand and the right layer of the mesoduodenum and the dorsal visceral peritoneum of the duodenum on the other.

2. In following out the pancreas of the cat in its entire extent, proceeding to the left of the pylorus, it will be seen that the body of the gland has extended between the two dorsal layers of the great omentum (primitive dorsal mesogastrium) over to the spleen (Fig. 223). Consequently the arrangement in the cat corresponds to the stage in the human development shown in Fig. 219 and Fig. 221 in which adhesion of the dorsal surface of the pancreas to the parietal peritoneum has not yet taken place.

It will be quite easy to reconstruct from the facts as demonstrated by the arrangement of the parts in the cat, the stage in the development of the lesser peritoneal sac in which the dorsal wall of the space is still formed by the proximal portion of the free dorsal mesogastrium (great omentum) and the structures included between its two layers.

It must then become apparent that the entire serous surface which in the adult human subject we regard as "parietal peritoneum of the lesser sac" lining the dorsal wall of the retrogastric space is derived from what originally was the right layer of the primitive sagittal dorsal mesogastrium.

II. RELATION OF GREAT OMENTUM TO TRANSVERSE COLON, TRANSVERSE MESOCOLON AND THIRD PART OF DUODENUM.

The second purpose to be accomplished by the study of the cat's abdominal cavity at this stage is the correct appreciation of the adult human conditions which are produced by areas of adhesion between the transverse colon, transverse mesocolon and third part of the duodenum on the one hand, and the dorsal mesogastrium, as great omentum, with the structures contained between its layers, on the other.

Perform the manipulations of the large and small intestine in the cat (see p. 67) which are required in order that the tract may be arranged so that it will correspond in general to the topographical conditions presented by the adult human subject. Locate the transverse colon and mesocolon and the third portion of the duodenum produced by these manipulations in imitation of the corresponding human structures. Then proceed to plot the different parts out successively as they would appear in a sagittal section (Fig. 224).

The following facts are to be noted and indicated on the plan of the section:

1. The great omentum is free, hanging down from the greater curvature of the stomach over the coils of intestine. Turning the omentum up it will be observed that the body of the pancreas is included between the two dorsal layers of the membrane.

2. The omentum, containing the pancreas, can be lifted up, exposing the next succeeding structure, viz., the transverse colon and mesocolon. In the cat the large intestine has been brought over, by the manipulations above indicated, into a transverse position so as to represent the human transverse colon and its mesocolon. It is therefore necessary to remember that in this mammal the fixation of the transverse mesocolon in the position indicated, by adhesion of ascending and descending mesocola to the parietal peritoneum of the abdominal background, has not yet occurred. Consequently the membrane must be held in the transverse position in order to represent the human arrangement.

It will of course be observed that both surfaces of the transverse mesocolon established in this way are free, not adherent to either omentum or pancreas on the one hand, nor to the transverse duodenum on the other.

3. The third or transverse portion of the duodenum is seen to be attached by the distal part of the mesoduodenum, both of the serous surfaces of the membrane being free. The duodenum having been brought from right to left transversely across vertebral column and aorta, underneath the superior mesenteric artery, the mesoduodenum, in the segment corresponding to the transverse duodenum, exchanges its original sagittal position for one in a horizontal plane, with cephalic (primitive left) and caudal (primitive right) surfaces.

Now compare the above arrangement of the intestines and peritoneum in the cat at once with the conditions presented in the adult human subject, reserving certain intermediate stages, as exhibited by some of the lower monkeys, for subsequent study.

1. Area of adhesion between opposed surfaces of great omentum and transverse mesocolon and colon.

2. Area of adhesion between parietal peritoneum, duodenum, and caudal layer of transverse mesocolon.

3. Adhesion of opposed walls of omental bursa leading to obliteration of distal portion of pouch and producing "gastro-colic" ligament of adult human subject.]

The examination of a similar sagittal section representing schematically the adult human arrangement of the parts (Fig. 225) will reveal the following points of difference as compared with the cat:

1. The peritoneum covering the dorsal surface of the pancreas, derived from the primitive dorsal mesogastrium, has become adherent to the parietal peritoneum, as previously described.

2. The cephalic surfaces of the transverse colon and mesocolon fuse with the corresponding area of the dorsal (4th) layer of the great omentum (dorsal mesogastrium).

In the human foetus in the 4th month the connection is still so slight that the omentum can readily be separated from the transverse colon and mesocolon.

Further dorsad the cephalic layer of the transverse mesocolon adheres to the serous investment of the caudal surface of the pancreas, derived, as we have seen, from the same dorsal layer of the great omentum.

3. The duodenum and mesoduodenum are fixed by adhesion on the one hand to the parietal peritoneum, on the other to the caudal layer of the transverse mesocolon near the root of that membrane.

4. The cavity of the omental bursa is usually obliterated in the adult caudad of the level of the transverse colon, by adhesion of the apposed surfaces of the two intermediate omental layers.

We have therefore three general areas of secondary peritoneal adhesion to deal with (Fig. 225), viz.:

1. Dorsal layer of primitive } { Parietal peritoneum, cephalic
mesogastrium (great } { layer of transverse
omentum) including the } { mesocolon and cephalic surface
serous investment of the } to { of transverse colon.
dorsal and caudal surfaces } {
of the pancreas (Fig. 225, } {
1). } {

2. Transverse duodenum } { Parietal peritoneum and
and mesoduodenum (Fig. } to { caudal layer of transverse
225, 2). } { mesocolon.

3. Between the apposed serous surfaces of the intermediate omental
layers (Fig. 225, 3).

.

These areas of adhesion result naturally in the production of secondary lines of peritoneal transition as follows:

1. Figs. 225, 1; 226, 1, from the omentum, dorsal layer, to the caudal surface of transverse colon, caudal layer of transverse mesocolon and caudal surface of the pancreas.

2. Figs. 225, 2; 226, 2, from the caudal layer of the transverse mesocolon across the transverse portion of the duodenum to the parietal peritoneum and mesentery of the jejuno-ileum.

3. Figs. 225, 3; 226, 3, between the intermediate omental layers, forming the secondary caudal limit of the lesser sac.

These changes consequently result in the rearrangement of the adult human peritoneum in accordance with the following schema (Fig. 226):

We trace the peritoneum as the ventral or superficial layer of the great omentum from the greater curvature of the stomach caudad around the distal free edge of the omentum and cephalad, as the dorsal layer, to the ventral border of the transverse colon. Here apparently this layer is continued across the caudal surface of the large intestine and beyond as the caudal layer of the transverse mesocolon. While this condition obtains practically in the adult it is to be remembered that the adhesion (at 1 in Fig. 225) prevents us from lifting the omentum away from the colon, and that consequently the apparent continuity of the dorsal layer of the great omentum with the caudal layer of the transverse mesocolon is the result of this peritoneal fusion.

Near the dorsal attachment or "root" of the transverse mesocolon the caudal layer of the membrane becomes continuous with the parietal peritoneum investing the transverse portion of the duodenum on its ventral aspect, which peritoneum in turn passes into the free mesentery of the jejuno-ileum (Fig. 225, 2). Comparison with the previous figures will show that we are dealing here with another area of secondary peritoneal fusion.

If we now open the "lesser peritoneal cavity" by dividing the two layers of the omentum attached to the greater curvature of the stomach (Figs. 225 and 226 in direction of arrow) we will apparently reach the upper or cephalic surface of the transverse mesocolon. This layer can be followed dorsad to the sharp border which separates the ventral and caudal surfaces of the pancreatic body and the membrane can be traced thence over the ventral surface of the gland to the diaphragm. (The connections with the liver and stomach shown schematically in the diagram (Fig. 225) are to be considered in detail subsequently.)

In the adult the peritoneal surface just described appears as the cephalic layer of the transverse mesocolon and its continuation dorsad. From the facts previously considered it will be at once apparent that we are really dealing here with a part of the third layer of the primitive omentum. We do not see the original cephalic layer of the transverse mesocolon. This membrane has become fused with the fourth omental layer, and its free serous surface obliterated in the stretch between the vertebral column and the transverse colon. Hence the human adult transverse mesocolon is apparently composed of _two_ layers; the cephalic of these layers appears as peritoneum of the "lesser sac," in conformity with its derivation from the original third omental layer lining the interior of the omental bursa. The caudal layer, on the other hand, is a part of the general or "greater" peritoneal membrane. The entire adult transverse mesocolon, hence, comprises _four_ peritoneal layers, of which only two remain as permanently free serous surfaces. These differ in their derivation, the cephalic layer being a part of the primitive dorsal mesogastrium (third omental layer), while the caudal layer is part of the primitive mesocolon. Between these two layers of the adult transverse mesocolon are included the two obliterated embryonic membranes, _viz._, the fourth omental layer and the original dorsal layer of the transverse mesocolon.

Caudad the two layers of the adult transverse mesocolon surround the transverse colon and are continuous along the ventral margin of the intestine with the layers of the great omentum. Toward the vertebral column these layers again diverge. The cephalic layer, lining the "lesser peritoneal cavity" invests the ventral surface of the pancreas. The caudal layer continues over the caudal surface of the body of the gland and transverse portion of the duodenum into the parietal peritoneum and the free mesentery of the jejuno-ileum. Consequently the returning layers of the great omentum are said to surround the transverse colon and unite along the dorsal border of the intestine to form the transverse mesocolon, which membrane is continued dorsad toward the vertebral column as two layers. At the "root" of the transverse mesocolon these layers are then described as diverging, the cephalic passing up to line the ventral surface of the pancreas, while the caudal continues over the caudal surface of the pancreas and third portion of the duodenum into the parietal peritoneum and mesentery.

Wherever in this discussion of the transverse mesocolon the transition between the caudal layer of the membrane and the "parietal" peritoneum is referred to it is necessary to remember that this "parietal" peritoneum is the _secondary_ investment of the abdominal background, formed by the surface of the ascending and descending mesocolon which remains free after the opposite surface and the vertical segments of the large intestine have been anchored by adhesion to the _primary_ parietal peritoneum (cf. p. 81, Fig. 158).

A summary at this point of the course of the dorsal mesogastrium, in forming the great omentum and its subsequent connections, would show us that the membrane first enlarges and descends towards the transverse colon (Fig. 177). The omental bag is formed by the descending or superficial segment (starting from the greater curvature of the stomach), turned toward the observer in the figure, and by the ascending or deep layer which is attached above to the dorsal abdominal wall, in front of the vertebral column and aorta along the original line of origin of the dorsal mesogastrium. Gradually growing and descending further, the deep segment becomes attached to the transverse colon. It also becomes connected, especially on the left side, with the diaphragmatic peritoneum (phrenicocolic lig.), so that its original starting point is no longer distinct. Finally the development of the spleen and pancreas between the layers of the dorsal segment and their subsequent connections obscure the original conditions.

Fig. 297 shows the primitive condition at a time when the connection with the transverse colon and mesocolon has not yet taken place.

The omental bag or bursa epiploica develops in the region of the dorsal mesogastrium and the viscera included between its layers, by changes in the position and extent of the membrane which finally result in placing a part of the right half of the primitive coelom cavity behind the stomach. Up to the sixth week the line of origin of the dorsal mesogastrium is from the mid-dorsal line of the abdomen. It deviates from this origin to the left because the great curvature of the stomach to which it is attached turns in this direction. On this account, and because of the rapid growth of this portion of the mesogastrium, a bag or space is formed behind the stomach. The entrance into this space is situated to the right of the lesser curvature, behind the peritoneal layers connecting the same with the liver (lesser or gastro-hepatic omentum and hepato-duodenal ligament). The ventral wall of this space is formed by the dorsal surface of the stomach itself, the dorsal wall by the mesogastrium, turning to the left and presenting its original right surface, now directed ventrad. The caudal limit of the retro-gastric space is given by the turn of the mesogastrium to reach its attachment along the greater curvature of the stomach (rudiment of great omentum).

The stomach, in contributing to produce these changes, passes from the vertical to the oblique and finally into the transverse position. The pylorus, formerly directed caudad, passes up and to the right. The fundus develops and the original left side of the stomach becomes the ventral, the right side the dorsal. The original dorsal border, now the greater curvature, moving caudad, carries the attached dorsal mesogastrium with it into its new position. The mesogastrium now pouches to form the great omentum and rapidly enlarges. At first hardly projecting beyond the greater curvature, it increases in length until it forms a four-layered apron which hangs down as a loose sac over the transverse colon and the coils of the small intestine (Fig. 177). In the foetus of six months the cavity of the omental bag extends caudad as far as the lower edge of the omentum. Later adhesions between the peritoneal surfaces lining the interior of the bursa limit this extension.

The omental bursa is therefore formed by a ventral lamella, consisting of two peritoneal layers, which hangs down from the greater curvature of the stomach and passes around the caudal free edge of the omentum into the double-layered dorsal lamella, which ascends, over the transverse colon, to the original starting point of the dorsal mesogastrium along the front of the vertebral column and aorta. Hence the "great omentum" is originally composed of four layers of peritoneum.

The dorsal double lamella becomes adherent over a considerable area to the parietal peritoneum of the dorsal abdominal wall. In this way the organs developed between the two layers of the lamella obtain their final fixed position. The pancreas becomes anchored and appears in the adult as a "retro-peritoneal" structure, while the spleen is attached by the "phrenico-lienal ligament" to the diaphragm.

In addition the dorsal omental lamella adheres in the fourth month to the cephalic layer of the transverse mesocolon and to the transverse colon.

Important illustrations of some of the intermediate stages in the human development of this portion of the peritoneal tract are afforded by the permanent adult conditions found in the abdominal cavity of some of the lower primates, notably certain of the cynomorphous monkeys.

Fig. 227 shows the abdominal cavity and disposition of the peritoneum in a macaque monkey (_Macacus rhesus_, male) in the ventral view, with the coils of small intestines removed and the omentum lifted up and reflected upon the ventral body wall. The following important points of difference from the arrangement in the _cat_ on the one hand, and in _man_ on the other, are to be noted:

1. The large intestine presents the typical primate course, with an ascending, transverse and descending colon. The ileo-caecal junction is situated in the right iliac fossa.

2. The ascending and descending mesocola are still _free_, not having become adherent to the parietal peritoneum along the dorsal abdominal wall. Hence the caudal portions of the ventral surfaces of the two kidneys are still covered by the _primitive parietal peritoneum_.

3. The great omentum is not yet adherent to the transverse colon and mesocolon except for a short distance on the extreme right. At this point the dorsal layer of the omentum has begun to contract adhesions to the hepatic flexure of the colon and ascending colon, but the rest of the transverse colon is free. Differing from the human arrangement is a line of adhesion, uniformly present in these monkeys, between the dorsal surface of the omentum along its right edge and the ventral surface and right border of the _caecum_ and _ascending colon_, parts which normally are not adherent to the omentum in man.

4. Hence in tracing the omentum to the left of the limited adhesion to the hepatic flexure and ascending colon, _i. e._, nearly throughout the entire extent of the transverse colon, we find the membrane passing freely without adhesion over the cephalic surface of the transverse mesocolon, which preserves its original free condition, independent of the omentum. This arrangement is shown in the schematic sagittal section in Fig. 230.

5. Tracing the omentum dorsad beyond the transverse colon and mesocolon the pancreas is reached. Here we encounter the first extensive area of omental or mesogastric adhesion. The omental peritoneum continues over the ventral and caudal surfaces of the gland, investing the same, but the dorsal surface has lost its serous covering and is anchored to the ventral surface of the left kidney. Hence a sagittal section would show the arrangement of the monkey's omentum as indicated in the schematic Figs. 229 and 230. Making now a general comparison of the peritoneal membrane of this animal with that of man, and of both with the preceding common embryonal condition, we can draw the following conclusions, indicated schematically in the five figures 228-232.

1. The dorsal layer of the monkey's omentum in its proximal segment behaves in the same way as in man, _i. e._, it becomes adherent to the primitive parietal peritoneum down as far as the caudal margin of the dorsal surface of the pancreas included between the primitive mesogastric layers forming by their further growth the omental apron.

Therefore we find, as in the human subject,

(_a_) The pancreas adherent to the ventral surface of the left kidney.

(_b_) A portion of the ventral surface of the kidney, cephalad of the pancreas, and the dorsal wall of the retrogastric (lesser peritoneal) space lined by secondary parietal peritoneum derived from the third layer of the omentum (original right layer of dorsal mesogastrium).

2. The monkey differs from adult man in the behavior of the dorsal omental layer in relation to the cephalic surface of the transverse mesocolon. The adhesion, which in the human subject fuses this layer with the transverse colon and mesocolon, does not occur in the monkey.

Hence we have in this animal the following conditions:

(_a_) The omentum is non-adherent to the transverse colon and transverse mesocolon.

(_b_) The caudal surface of the pancreas is lined by its original mesogastric peritoneum.

(_c_) The transverse mesocolon is formed by the original two layers of the primitive dorsal mesentery; hence its cephalic layer is not "peritoneum of the lesser sac" as is the case in man.

(_d_) The caudal part of the ventral surface of the left kidney below the pancreas, is covered by the original parietal peritoneum.

(_e_) Only one point or line of _secondary peritoneal transition_ exists, where the dorsal layer of the omentum in the adult becomes continuous with the parietal peritoneum covering the caudal surface of the pancreas and the ventral surface of the left kidney.

_Note_: In the schematic sections shown in Figs. 228 to 232 the transverse colon is represented as far removed from the ventral surface of the left kidney, in order to make the peritoneal lines of the mesocolon more clear. Actually a sagittal section which would divide the kidney would cut the transverse colon at its extreme left end, where it turns close to the ventral surface of the left kidney and then follows its lateral border to form the splenic flexure (Fig. 235). The caudal part of the ventral surface of the left kidney in the adult human subject is covered by the peritoneum which, as secondary parietal peritoneum, is derived from the upper part of the right leaf (later ventral leaf) of the descending mesocolon. Hence it should be remembered that these diagrams present _combinations_ of sections. A section which will show the full development of the transverse mesocolon is mesad of the kidney; while a section through the kidney would be too far laterad to show the transverse mesocolon.

Figs. 233, 234 and 235 show sagittal sections through the left kidney with the adult arrangement of the peritoneum and colon and the embryonic and adhesion stages leading to the same.

It will be observed that in all the schematic sections of the early embryonic stages the two layers of the transverse mesocolon are shown without dorsal attachment, as turning with the formation of a fold (Fig. 228 at x) into two layers descending ventrad of the parietal peritoneum. This is because the dorsal attachment of the mesocolon is at this stage still in the median line and would hence not be encountered by a sagittal section through the kidney, and because the two layers of the transverse mesocolon, immediately after rotation of the large intestine, are still directly continuous with the two layers of the descending mesocolon. That is to say, the cephalic layer of the transverse mesocolon is continuous with the dorsal (originally the left) layer of the descending mesocolon, and the caudal layer of the transverse mesocolon with the ventral (originally the right) layer of the descending mesocolon, which is, in the human subject, to assume subsequently the character of parietal peritoneum after the dorsal layer and the primitive parietal peritoneum have become obliterated by adhesion (Fig. 235).

Fig. 236 shows this continuity of the descending and transverse mesocolon as a permanent adult condition in the macaque. The fold of transition between the two is seen at x in Fig. 228. It will be noticed that the ventral surface of the left kidney, caudad of the adherent pancreas, is covered by the primitive parietal peritoneum, corresponding to section in Fig. 230.

RELATIONS OF SPLEEN AND OMENTUM IN _MACACUS RHESUS_.

The spleen in this animal has not contracted any extensive adhesions to the parietal peritoneum (the phrenico-lienal lig. of anthropotomy is not developed). It can be turned mesad so as to expose the lateral border and an adjacent segment of the ventral surface of the left kidney, as well as the dorsal surface of the tail of the pancreas at its tip, still covered by mesogastric peritoneum. Hence in the monkey the adhesion of the original vertebro-splenic segment of the mesogastrium, including the pancreas, to the primitive parietal peritoneum is less complete than in man.

MEDIAN ATTACHMENT OF DESCENDING MESOCOLON AND ITS RELATION TO THE MESOCOLON OF THE SIGMOID FLEXURE IN THE _MACAQUE_.

Fig. 236 shows the abdominal viscera, hardened in situ, of _Macacus cynomolgus_, the Kra monkey, in the ventral view and from the left side.

The great omentum is lifted up, the pancreas is adherent to the ventral surface of the left kidney, the caudal portion of which is covered by the primary parietal peritoneum, which can be exposed by turning the still free descending mesocolon mesad. The mesocolon retains its primitive attachment to the median line ventrad of the large prevertebral blood vessels. It is readily seen that adhesion between the left leaf of this free descending mesocolon and the parietal peritoneum down to the level of the iliac crest would produce the conditions found in the human adult, with an attached descending colon and a free sigmoid flexure; also that limited adhesion of the mesocolon of the sigmoid flexure to the parietal peritoneum would produce, as previously explained (cf. p. 97), the intersigmoid peritoneal fossa.

=2. Ventral Mesogastrium and Liver.=--The peritoneal reflections from the stomach to the liver, and the arrangement of the membrane in connection with the latter organ, remain for consideration.

Certain complicated adult conditions, encountered in this part of the abdominal cavity, make it desirable to arrange the subject for purposes of study under the following subdivisions:

I. The development of the liver and of its vascular system, and the significance of the adult circulation of the liver and of the foetal remnants connected with the organ.

II. The anatomy of the ventral mesogastrium and the changes produced in the arrangement of the membrane by the development of the liver.

=I. A. Development of the Liver.=--The liver, like the pancreas, is developed from the duodenum as an outgrowth from the hypoblast lining the enteric tube. As we have previously noted, the first outgrowth of the hepatic diverticulum is closely associated with the distal pancreatic outbud; in fact the latter arises as a derivative from the hepatic duct rather than as a distinct outbud from the intestinal tube. (This close association of the hepatic duct with the pancreas is well seen in the arrangement of the concealed pancreas of some teleosts (cf. p. 117, Fig. 196).)

In point of time the liver is the first accessory structure to develop by budding from the primitive alimentary canal, the pancreas and lung following.

In the primitive type of development, as seen in _Petromyzon_ and in the Amphibia, the liver appears very early, as a diverticulum of the embryonic intestinal tube, near its cephalic extremity, projecting on the ventral aspect down into the mass of yolk-cells (Fig. 237). The short stretch of the primitive alimentary canal cephalad of the hepatic diverticulum corresponds to the foregut. With the development of the heart the primitive foregut becomes divided into pharynx and post-pharyngeal segment (oesophagus and stomach). The hepatic diverticulum then lies immediately dorsad of the caudal or venous extremity of the heart. Hence it is probable that the liver is an older organ in the ancestral history of the vertebrates than the pharynx or even the heart. The liver diverticulum lies in close connection with the omphalo-mesenteric veins which return the blood from the yolk-sac to the heart. In the course of further development, as will be seen below, the liver comes into very intimate relations with the venous circulation.

In human embryos of 3.2 mm. the primitive hepatic duct appears as a wide hollow pouch composed of hypoblast cells, growing between the two layers of the ventral mesogastrium, which membrane, extending between the ventral border of the primitive stomach and the ventral abdominal wall, will be subsequently considered in detail. The liver, in developing between the layers of the ventral mesogastrium, approaches very early the _septum transversum_ or rudimentary diaphragm and becomes connected with the same. A mass of mesodermal cells, derived from the mesogastrium and from the primitive mesodermal intestinal wall surrounding the hypoblastic lining of the tube, covers the caecal termination of the primitive hepatic duct, forming the so-called embryonic _hepatic ridge_. This mesodermal tissue accompanies the duct in its further growth and branching, forming the connective tissue envelope, known in the adult as the capsule of Glison. The primitive hepatic duct is directed cephalad in the mesogastrium between the vitelline duct and the stomach (Fig. 101).

In embryos measuring 4.25 mm. the duct is 0.24 mm. long. Later (in embryos of 8 mm.) the primitive single duct divides into two secondary branches, indicating, even at an early stage, the adult arrangement of the duct, as formed by the union of the right and left hepatic ducts (Fig. 185).

The gall-bladder in embryos of this size (8 mm.) is a well-defined caecal diverticulum, branching caudad from the main hepatic duct.

The vesicular mucous surface is thus derived from the enteric hypoblast in the same way as the epithelial lining of the bile-ducts and capillaries. The external muscular and fibrous coats of the gall-bladder are developed from the mesoderm of the mesogastrium.

It is to be noted that at an early stage the gall-bladder is derived from the main duct close to the intestine, the latter duct being very short. Later on the common duct grows in length, making the liver more and more a gross anatomical organ distinct from the intestine. The cystic duct develops as the result of a similar increase in length of the cystic diverticulum. The two principal secondary branches of the hepatic duct give origin to sprouts or buds. These are derivatives of the hypoblastic cells of the larger ducts and may from the beginning be hollow, possessing a lumen continuous with that of the parent duct (Selachians, Amphibians). In warm-blooded animals these sprouts are at first solid, forming the s. c. _hepatic cylinders_, and only subsequently become hollowed out with the further development of the biliary duct system of the liver. The rapid growth of the organ leads to a great increase in the number of the hepatic cylinders. They spread out on all sides, finally coalescing with adjacent buds so as to form an interlacing network whose meshes are filled by blood vessels. After the hepatic cylinders have become canalized they preserve the same arrangement, hence the resulting biliary capillaries of the adult form an anastomosing network. Amphioxus and the amphibians have a single hepatic outgrowth (Fig. 49).

In the Selachians the liver arises as a ventral outgrowth at the hinder end of the foregut immediately in front of the vitelline duct, thus bringing the liver from the beginning into close proximity with the vitelline veins entering the heart. Almost as soon as formed the outgrowth develops two lateral diverticula, opening into a median canal. The two diverticula are the rudimentary lobes of the liver and the median canal uniting them is the rudiment of the common bile-duct and gall-bladder.

In the Teleosts the liver arises quite late (in the trout about the 25th day) as a solid outgrowth from the intestinal canal close to the heart. In the Amniota the liver arises in the same position as in the Anamnia, but, at least in birds and mammals, shows its bifurcation almost, if not quite, from the start. The two forks embrace between them the omphalo-mesenteric or vitelline veins just before they empty into the sinus venosus of the heart.

In the chick the liver appears between the 56th and 60th hour, the right fork being always of greater length but less diameter than the left. The hepatic outbud in the rabbit appears during the 10th day, and during the 11th day begins to send out branches.

In man, as above stated, the bud appears well marked in embryos of 3 mm.

[Certain adult variations make it appear possible that there are two human embryonic hepatic buds, a cranial and a caudal, as is the case in birds.]

=I. B. Comparative Anatomy of the Liver.=--The liver, phylogenetically a very old organ, occurs in all vertebrates, for the caecal diverticulum of the intestine of amphioxus (Fig. 49) has probably the significance of a hepatic outbud.

The primitive form of the liver is symmetrically bilobed, a type which is seen well in the chelonian organ (Fig. 238).

In size the liver is subject to great variations. It is usually larger in animals whose food contains much fat. Hence carnivora in general have a larger liver than herbivorous animals.

Its shape also varies considerably, depending on the form of the body cavity and on the amount and disposition of the available space. Hence in the snakes the organ appears long drawn out, flattened, almost ribbon-like (Fig. 239), while the relatively very large coronal diameter of the body cavity in the turtles permits the liver to expand transversely (Fig. 238).

In general, when the liver is large and the available space for its reception limited, it is usually split into several (two to seven) lobes, which permit, by mutual displacement, the accommodation of the organ to varying space-conditions of the body cavity (Fig. 240). Under the opposite circumstances, on the other hand, even the primitive bilobed character may disappear and the liver is then unlobed (Fig. 241).

The presence or absence of a gall-bladder depends apparently largely on the character of the food and on the habitual type of digestion. In many vertebrates digestion is carried on nearly continuously, without marked interruption, especially in many ungulates, ruminants and rodents. In such animals the gall-bladder is absent. It is also absent in several birds (most Parrots, Doves, Ostrich, Rhea americana, the Cuculidae, Rhamphastos, etc.). This variability emphasizes the morphological fact that the biliary bladder is only a modified portion of the hepatic duct system, as shown by the development above outlined.

A great variety is observed in the arrangement of the biliary ducts, through which, at the period of intestinal digestion, bile passes from the liver and gall-bladder into the intestine, while in the intervals of digestion the secretion is only carried from the liver to the bladder. The following main types of the biliary duct system may be recognized:

1. The hepatic duct joins the cystic to form the common bile-duct, entering the duodenum by passing obliquely through the intestinal wall (Fig. 242). This form is encountered in man and in most mammals. It is also found in some birds (_Buceros_), many amphibians, and in some fish (_Lophius_). Instead of one hepatic duct two may join the cystic duct separately to form the common bile duct (_Phoca litorea_), or the number of hepatic ducts may be further increased. The separate hepatic ducts then unite successively with the cystic duct. This occurs in many mammals (as _Tarsius_, _Galeopithecus_, monotremes) and in some fishes (_Xiphias_, _Trigla_, _Accipenser_) (Fig. 243).

2. Of two hepatic ducts only one helps to form with the cystic duct the common duct, while the other leads from the liver transversely into the bladder, especially into the neck, forming the hepatico-cystic duct (Fig. 244). This arrangement is found in several mammals (calf, sheep, dog).

3. No common bile-duct is formed. The hepatic and cystic ducts each empty separately into the intestine (hepato-enteric and cysto-enteric ducts), while a hepato-cystic duct carries the bile directly from the liver to the gall-bladder (Fig. 245).

_Lutra vulgaris_ among mammalia, the majority of the birds and several reptilia present this type.

When the gall-bladder is absent a single large hepato-enteric duct is found, or instead a number of smaller ducts which enter the intestine successively.

=I. C. Development of Vascular System of Liver.=--In order to comprehend the peritoneal relations of the adult liver it is absolutely necessary to have a clear understanding of the development of the vascular system in connection with the gland.

For our purpose, in the first place, a serial consideration of the successive stages, illustrated by schematic diagrams, will prove most practicable. These diagrams represent the structures in the dorsal view, _i. e._, in the position which they would occupy in the adult liver with the gland resting on its upper or convex surface and with the ventral sharp margin turned toward the beholder (see Fig. 259).

The development of the venous system, especially in connection with the liver, presents a somewhat complicated series of successive conditions. After having become familiar with the principal typical embryonal stages, as shown in the following diagrams, the student is strongly recommended to cement this knowledge by the comparative examination of the venous system. The permanent veins of the lower vertebrates, while in many cases not strictly homologous to those of the higher forms, yet are excellent objects for study, since they serve to illustrate temporary stages in the development of the mammalian venous system, and to that extent are of aid in comprehending one of the most difficult and important chapters in human anatomy. At the conclusion of the diagrammatic consideration of the mammalian development a number of comparative facts will be put together for this purpose.

=1. Early Stage.=--In the earlier developmental stages in mammalian embryos the primitive dorsal aorta extends caudad along the ventral aspect of the vertebral axis, giving off paired vitelline or omphalo-mesenteric arteries to the yolk-sac and allantoic arteries to the embryonic urinary bladder or allantois (Figs. 246 and 247).

The blood is returned from the vascular area of the yolk-sac by two vitelline or omphalo-mesenteric veins, which unite near the heart to form a common trunk, continued as the _sinus venosus_ into the caudal or auricular extremity (venous end) of the primitive tubular heart (Figs. 246, 247 and 248).

Arteries in black.

Veins in outline.]

=2. Development of Allantois. Stage of Placental Circulation.=--The placental circulation, replacing the temporary vitelline circulation of the earliest stages, is inaugurated by the appearance of two umbilical veins, which pass cephalad, imbedded in the tissue of the ventral mesogastrium, to empty into the sinus venosus near the vitelline veins (Fig. 249). The umbilical veins return the oxygenated blood from the placenta to the embryo. At first the right umbilical vein is the larger of the two.

The sinus venosus at this time also receives two large veins, transversely directed, called the ducts of Cuvier, which are formed near the heart by the union of the anterior cardinal (primitive jugular) and posterior cardinal veins, draining respectively the head end of the embryo, and the body walls and Wolffian bodies.

The vitelline veins are placed on each side of the primitive small intestine, and become connected with each other by a broad anastomotic branch (Fig. 249). When the hepatic outgrowth buds from the duodenum the vitelline veins send out branches which break up into a wide-meshed capillary network in the mesodermic tissue enveloping the hepatic cylinders. Hence at this period the circulation in the vitelline veins is made up of three districts:

(_a_) Distal segment of veins, coursing along duodenum, and joined by a transverse anastomosis, before reaching the liver bud (subintestinal veins).

(_b_) Middle segment, from which capillary vessels are derived, ramifying upon and between the developing hepatic cylinders.

(_c_) Proximal segment, formed by the continuation of the proximal part of the vitelline veins into the sinus venosus of the heart.

=3. Formation of Portal Circulation. A.=--With the further development of the liver the direct connection of the distal segment of the vitelline veins with the sinus venosus becomes lost, the intermediate segment being entirely broken up into an intrahepatic network (Fig. 250). Hence all the blood brought to the liver by the vitelline veins (venae hepaticae advehentes) passes through the hepatic capillary circulation, before it is carried by the proximal segment of the vitelline veins (venae hepaticae revehentes) into the sinus venosus. The amount of this blood increases with new connections which the vitelline veins make with the venous radicles developing in the intestinal tract and its appendages. In proportion as, with the development of the placenta and reduction of the yolk-sac, the original significance of the vitelline veins as nutritive and respiratory vessels disappears, this secondary connection of the vitelline veins with the veins of the alimentary tract becomes more and more important, until finally the original vitelline veins, now properly called omphalo-mesenteric veins, return the blood from the intestinal tube, pancreas and spleen to the liver.

The venae hepaticae advehentes, becoming connected in this way with the developing intestine, pancreas and spleen, form the rudiments of the future portal system, while the venae hepaticae revehentes are prototypes of the hepatic veins of the adult circulation.

=B. Development of the Portal Vein.=--The distal subintestinal segments of the vitelline veins are early united by a transverse anastomotic branch. The section of the veins above this anastomosis is seen already in Fig. 250 to have assumed an annular shape, while the veins below the primary anastomosis are approaching each other to form a second ring-like junction.

In Fig. 251 the subintestinal segments of the two vitelline veins are seen to have communicated with each other by transverse anastomotic branches around the duodenum, two of these branches being situated ventrad and one dorsad of the intestinal tube. These branches, and the portions of the primitive vitelline veins between their points of derivation, form two vascular loops or rings, encircling the primitive duodenum (Fig. 251).

The distal portions of the vitelline veins, before reaching the caudal annular duodenal anastomosis, next fuse into a single longitudinal vessel which also receives the veins from the stomach, intestine, spleen, and pancreas, and forms the beginning of the portal vein.

By atrophy of the right half of the lower, and of the left half of the upper duodenal venous ring (Figs. 252 and 253), the proximal portion of the portal vein is formed as a single vessel, taking a spiral course around the duodenum (Fig. 256). Hence in the adult the portal vein and its principal branch (the superior mesenteric vein) crosses over the ventral surface of the duodenum (third portion), turns along the mesal side of the second portion, and then continues to the liver along the dorsal aspect of the first portion (Fig. 254). _Note_--In comparing Fig. 254 with the schematic figures it should be noted that the same presents the parts in the _ventral_ view, while the schemata offer the _dorsal_ aspect.

=4. Changes Leading to the Final Arrangement of the Umbilical Veins.=--A very important rearrangement of the umbilical veins takes place. These veins originally course in the lateral abdominal wall, close to the fold of the amnion (Fig. 255), and then turn cephalad of the developing liver along the septum transversum to empty into the sinus venosus at each end (Figs. 249 and 250). The right umbilical vein is at first the larger.

This symmetrical arrangement, and the direct connection of the umbilical veins with the sinus venosus, now becomes lost by the occurrence of the following changes:

1. At first (Fig. 249) all the blood carried to the liver by the omphalo-mesenteric veins passes through the hepatic capillary network before being conducted by the venae revehentes to the sinus venosus. Very early, however, a new intrahepatic channel develops, the ductus venosus (Figs. 250-253), which passes obliquely between the entrance of the left omphalo-mesenteric vein into the capillary system (l. v. advehens) and the termination of the right omphalo-mesenteric vein (r. vena revehens) in the sinus venosus.

In human embryos of 4 mm. the ductus venosus can already be distinguished, and in embryos of 5 mm. the vessel has assumed considerable proportions.

2. A communication is next established on both sides between the capillary hepatic network in the portion of the liver nearest to the abdominal wall and the umbilical veins as they ascend imbedded in the abdominal wall (Fig. 251).

This connection is usually from the start larger on the left side and connects with the left omphalo-mesenteric vein just at the point where the same is about to be continued into the ductus venosus. This connection becomes rapidly larger, so that the ductus venosus, which at first appeared merely as an anastomotic channel between the left omphalo-mesenteric vein and the terminal portion of the right omphalo-mesenteric vein, now forms the main continuation of the left umbilical vein. This vessel grows very rapidly up to its connection with the ductus venosus and soon exceeds the right umbilical vein in size (Fig. 252). Beyond the ductus venosus on the other hand the proximal segment of the left umbilical vein diminishes in size, and loses its independent character by incorporation in the hepatic circulation. Only its terminal portion, emptying into the sinus venosus, is preserved. This is surrounded by the growing masses of hepatic cylinders and is converted into a vena revehens.

The connection of the right umbilical vein with the liver vessels is at first symmetrical to that on the left side, but less strongly developed. The effect of this connection is to reduce in the same way the proximal segment of the right umbilical vein and to convert its termination into a vena revehens. With the great development of the left vein, however, the vein on the right side gradually diminishes and finally loses its connection with the intrahepatic circulation altogether. The right umbilical vein is now reduced to a vessel of the ventral abdominal wall, which carries blood in the reverse of the original direction, _i. e._, from the abdominal wall caudad _into_ the left umbilical vein (Figs. 253 and 255).

The connection thus established between the umbilical vein and the portal circulation results in the formation of a single large (the original left) umbilical vein which, throughout the remainder of foetal life, returns all of the placental blood (Fig. 253).

The newly developed hepatic portion of the left umbilical vein becomes, however, not only connected with the ductus venosus, but also with the right part of the upper venous ring, derived from the right omphalo-mesenteric vein (Fig. 253). This connection forms the left portal vein of the adult, and enlarges rapidly.

The terminations of the ductus venosus and of the venae hepaticae revehentes undergo a number of secondary changes in relative position. The left hepatic vein loses its direct connection with the sinus venosus, and now opens into the termination of the ductus venosus, into which the right hepatic vein also empties. This common vessel (v. hepatica communis) subsequently forms the proximal segment of the postcava when this vessel develops (Fig. 256).

The blood, therefore, returned to the liver by the left umbilical vein divides at the transverse fissure into three streams. Two of these pass through the connection with the portal vein and through branches developed from the hepatic part of the umbilical vein into the capillary system of the right and left lobe. The third continues through the ductus venosus to the common hepatic vein and sinus venosus (Fig. 256). The ductus venosus thus becomes the chief vessel returning arterialized placental blood to the heart. When the postcava develops fully the hepatic segment of this vessel also joins the terminal part of the ductus venosus (Fig. 256) and gradually replaces the same as the main returning venous channel, the proximal part of the ductus venosus being incorporated in the vena cava (Fig. 257). The postcava then receives the right hepatic veins separately, while the left hepatic veins and ductus venosus open together into the main vein. This condition obtains up to the time of birth and the consequent interruption of the placental circulation.

While at first the ductus venosus communicates throughout its entire length with the meshwork of the hepatic capillary system, a separation into two segments, _i. e._, ductus venosus proper and intrahepatic segment of umbilical vein, is established after the free communication with the left umbilical vein takes place. This condition is exhibited in Fig. 258, which represents the corroded venous system of the foetal liver, and in Fig. 259, showing an injected liver in the foetus at term.