Chapter VII: Part I (2)

1. Vessels pass from the proximal part of the abdominal aorta to the stomach and pyloric portion of the duodenum. This set of vessels forms the rudiment of the future coeliac axis. With the development of the liver and pancreas by budding from the duodenum, and with the appearance of the spleen in the mesoderm of the dorsal mesentery, branches corresponding to these organs (hepatic and splenic arteries) are added to the gastric and duodenal vessels and the adult arrangement of the coeliac axis is thus obtained (Figs. 130, 131, 132 and 133).

These vessels have an important bearing on the formation of the adult peritoneal cavity in the retro-gastric space, and will be considered in detail below with that portion of the subject.

2. The next vessel in order derived from the aorta and supplying the duodenum, pancreas, the small and a part of the large intestine is the above-mentioned superior mesenteric artery, which arises from the aorta a short distance caudad of the coeliac axis (Figs. 130, 131, 132 and 133).

At the time when the intestine still presents the primitive arrangement of the umbilical loop (Figs. 104 and 130) this vessel passes between the layers of the dorsal mesentery through the narrow duodeno-colic neck to reach the two limbs and the apex of the intestinal loop. In its course it gives off successively branches to the gut from each side. Those from the right side of the main vessel pass to the duodenum, pancreas, jejunum and ileum. Those from the left side of the main vessel accede in succession to the colic angle of the isthmus, the proximal portion of the colon, the caecum and the ileo-colic junction. The terminal portion of the superior mesenteric artery supplies the ileum near the ileo-colic entrance. After rotation it will be found that the turn has occurred at the point _X_ (Fig. 130), _i. e._, in that part of the vessel which occupies the duodeno-colic isthmus. Hence it will be found that the first branches derived from the right side of the primitive superior mesenteric artery, supplying the duodenum and pancreas (Art. pancreatico-duodenalis inferior) still arise after rotation from the right side. They are succeeded, beyond the point _X_, by the original highest _left_ branches passing to colon, caecum and ileo-colic junction, while all the original right-sided vessels, except the inferior pancreatico-duodenal, appear now as branches from the left side of the main artery, supplying the coils of the jejuno-ileum. Hence in the adult (Fig. 133) the succession of branches derived from the right or concave side of the superior mesenteric artery is as follows:

1. Arteria pancreatico-duodenalis inferior.
2. Arteria colica media.
3. Arteria colica dextra.
4. Arteria ileo-colica.

On the other hand, the first branches from what has now become the left or convex side of the vessel are the original lower right-hand vessels to the small intestine developed from the descending limb of the loop. Hence in the adult the left side of the superior mesenteric vessel gives rise to the vasa intestini tenuis.

3. The caudal intestinal arterial branch derived from the aorta is the inferior mesenteric artery supplying parts of the transverse colon, the descending colon, sigmoid flexure and rectum (Figs. 130, 131, 132, and 133).

On the other hand in the cases of non-rotation of the intestine as above described in Figs. 118-122, the embryonic type of the intestinal arterial supply persists, as indicated schematically in Fig. 134. Not only the pancreatico-duodenalis inferior, but all the remaining branches to the small intestine are derived from the right side of the superior mesenteric artery. The terminal branches of the main artery supply the ileo-colic junction, while the arterial supply of the large intestine, A. colica dextra and media, are given off from the left side of the parent vessel.

II. =Demonstration of Intestinal Rotation in the Cat.=--The changes in the relative position of the different intestinal segments and the final disposition of the mesenteries and blood vessels can best be understood by the direct examination of the abdominal contents in an animal whose permanent adult arrangement corresponds to one of the early embryonal human stages, and in which the necessary manipulations can readily be carried out and their results noted.

It is doubtful if the above detailed developmental stages in man can ever be clearly comprehended unless the student will for himself examine the conditions and perform the manipulations in one of the lower mammals.

The necessity of keeping the three dimensions of space in mind and the fact that certain structures during and after rotation cover and obscure each other, make diagrams and drawings unsatisfactory unless the actual examination of the object itself is combined with their study. Fortunately, among the common domestic animals of convenient size easily obtained the cat answers every purpose of this study admirably. The student is earnestly urged to pursue his study of the development and adult arrangement of the human abdominal viscera and peritoneum in the light which the anatomy of this animal can shed on the complicated and obscure conditions encountered in the human subject. The plan of having the opened abdominal cavity of the cat directly side by side with the human subject, while the arrangement of the abdominal viscera and peritoneum is considered, cannot be recommended too highly.

=Directions.=--After killing the animal with chloroform the abdominal cavity is to be freely opened by a cruciform incision and the skin flaps turned well back and secured in this position. It is well to select a male animal or an unimpregnated female, as the size of the pregnant uterus in the later stages renders the examination of the abdominal viscera and peritoneum more difficult.

For purposes of careful study and comparison of the vascular relations of the abdomen, it is highly desirable to inject the animal with differently colored gelatine, starch or plaster of Paris mass. The arterial injection can be made through the carotid artery, the systemic venous injection through the femoral vein, and the portal circulation can be filled after opening the abdomen, by injection through the superior mesenteric or splenic veins. Animals prepared in this manner are especially useful for the study of the upper portion of the abdominal cavity and of the peritoneal relations of liver, stomach, spleen, pancreas and duodenum. They may be kept for permanent reference in a 5 per cent. solution of formaline or 50 per cent. alcohol.

After opening the abdominal cavity turn the great omentum up over the ventral surface of the thorax and secure it in this position, thus exposing the underlying intestines completely (Fig. 135). Trace in the first place the entire course of the intestinal tube from the pyloric extremity of the stomach down. It will be noticed that the first portion of the small intestine (duodenum) is freely movable, completely invested by peritoneum and attached to the dorsal midline by a mesoduodenum between the layers of which a portion of the pancreas is seen.

Following the duodenum caudad it will be observed that the gut can be traced directly continuous with the remaining coils of the small intestine. The ileo-colic junction and the beginning of the large intestine are marked by a short pointed caecum. The large intestine is short, as it is in all carnivore mammals, and passes from the caecum almost directly down into the pelvis.

Take the caecum and the first portion of the large intestine and turn them caudad and over to the left side as far as the peritoneal connections will permit.

Spread out the coils of the small intestine in the opposite direction, _i. e._, over to the right side.

The arrangement of the intestinal tract after these manipulations should appear as shown in Figs. 136 and 137.

It will be seen that all the essential features described for the corresponding stage in the human embryo (Fig. 104, _A_) exist here. The proximal portion of the small intestine (duodenum) retains its freedom and mobility, being attached to the ventral surface of the vertebral column by the portion of the primitive mesentery which now constitutes the mesoduodenum. The gut itself forms a bend with the convexity turned to the right.

Observe in the next place that the point (Fig. 136, _X_), where small intestine and colon approach each other closely, marks the situation of the foetal duodeno-colic isthmus. The small intestine at this point corresponds to the future duodeno-jejunal angle as will be seen after rotation has been accomplished.

Recalling the development of the jejuno-ileum it will not be difficult to recognize in the numerous coils of small intestine which succeed to the duodeno-colic isthmus the results of the increase in length of the descending or efferent limb of the human embryonal umbilical loop. Tracing these coils it will be found that the terminal portions of the ileum correspond to the apex and to the proximal part of the ascending or recurrent limb of the primitive loop, while the remainder of this limb furnishes the caecum and the next succeeding segment of the large intestine. Following the tube up to this point the colic boundary of the duodeno-colic isthmus will be reached; from here the short large intestine of the carnivore descends straight into the pelvis, attached to the ventral surface of the vertebral column by a mesocolon which corresponds to the distal part of the original primitive dorsal mesentery.

Now with the parts still in this position examine carefully the arrangement of the mesentery and of the intestinal blood vessels. Starting with the duodenum it will be seen that the primitive sagittal mesentery of this portion of the intestine has followed the gut in its turn to the right, so that the original right layer of the sagittal membrane is now directed dorsad and lies in contact with the parietal peritoneum which invests the background of the abdominal cavity in the right lumbar region below the liver and covers the ventral surface of the right kidney. Beneath this parietal peritoneum the inferior vena cava is seen, receiving the right renal vein and ascending to enter the dorso-caudal aspect of the right lobe of the liver. If now we assume that in the cat the opposed serous surfaces of the original right leaf of the mesoduodenum, now directed dorsad, and of the parietal peritoneum adhere to each other, and that the visceral peritoneum covering the dorsal surface of the descending duodenum likewise becomes obliterated by adhesion to the subjacent parietal peritoneum, we will obtain the arrangement found in the adult human subject, in which the descending duodenum is fixed by adhesion below the right lobe of the liver and ventrad of the medial portion of right kidney, right renal vein and inferior vena cava. During this process of anchoring the head of the pancreas, which is found between the two layers of the free mesoduodenum of the cat, would also become fixed to the abdominal background by adhesion of the original right leaf of the mesoduodenum, investing what has now become the dorsal surface of the pancreas, to the parietal peritoneum. The original left layer of the primitive mesoduodenum would then appear as _secondary_ parietal peritoneum covering what has now become the ventral surface of the transversely disposed head of the gland. The stages may be represented schematically in Figs. 138-140.

Figs. 138 and 139 shows the arrangement in the cat where a free duodenum and mesoduodenum exists, with the pancreas included between its layers.[2]

[2] The student should not be confused by the fact that a considerable portion of the pancreatic gland in the cat will be found included between the layers of the great omentum, extending over to the left side of the abdomen. This circumstance will be found of importance in studying the development of the dorsal mesogastrium and of the structures connected with it. For the present attention should only be given to the right extremity or head of the pancreas, situated close to the duodenum and included between the layers of the mesoduodenum.

It will be noticed that the duodenum in the cat can be carried over to the median line (Fig. 138) exposing the entire ventral aspect of the right kidney and the inferior vena cava beneath the primary lumbar parietal peritoneum. This manipulation will also expose the dorsal surface of the head of the pancreas, covered by what originally was the right leaf of the mesoduodenum.

Fig. 140 indicates the results of adhesion of the duodenum, pancreas and mesoduodenum to the parietal peritoneum as it normally occurs in the human subject. It will be seen that the primary parietal peritoneum can be traced mesad over the ventral surface of the right kidney as far as the point _X_, and that from here on to the median line the peritoneum is _secondary_ parietal peritoneum, consisting of the visceral peritoneal investment of the ventral surface of the duodenum and of the original left leaf of the mesoduodenum, beneath which the ventral surface of the pancreas is seen. Pancreas and duodenum occupy in the adult secondarily a "retro-peritoneal" position, _i. e._, the peritoneum now covering the ventral surface of these viscera appears as a continuation of the parietal peritoneum, the transition between primary and secondary parietal peritoneum occurring along the line marked _X_ in Fig. 140. The opposed peritoneal surfaces indicated by the dotted lines have become adherent and converted into loose connective tissue in which the pancreas and duodenum lie imbedded. In the human embryo this process of adhesion begins in the eighth week, starting at the duodeno-jejunal flexure and ascending gradually toward the pylorus. At the end of the fourth month the union is complete.

Proceeding caudad it will next be observed that the peritoneum of the mesentery occupies the narrow neck of the duodeno-colic isthmus, and that large vessels (the superior mesenteric) pass between its two layers at this point to supply the segments of the intestine forming the loop. In conformity with the greatly increased length of the intestine it will be found that the mesentery expands from the narrow pedicle at the neck in a fan-shaped manner in order to develop a sufficiently long margin for attachment to the intestine. The following points should be carefully borne in mind in studying the mesentery with the intestines in this position:

1. The mesentery presents two free surfaces, right and left. With the coils of the small intestine turned over to the right, the left leaf of the mesentery is turned toward the observer.

2. Inasmuch as the descending limb of the embryonic loop has developed the greater part of the small intestine, while a portion of the large intestine (caecum and colon up to the isthmus) is the result of differentiation within the ascending or returning limb of the loop, it will be at once apparent that the double peritoneal layer which extends between the duodeno-colic isthmus and the attached border of the gut is partly mesentery of the small intestine, partly mesocolon passing to the large intestine (caecum and proximal colon). This condition may be indicated schematically in Fig. 141.

The curved line _A_ may be taken as an arbitrary division between the portion of the membrane which on the right of the figure passes to the small intestine, and the portion which proceeds to the left to be attached to the large intestine. In other words the line will schematically separate the true mesenteric from the mesocolic segment of the primitive membrane.

With the parts in their present position this line might be assumed to indicate a strip along which the opposed serous surfaces of the parietal peritoneum and the right leaf of the primitive mesentery became adherent. In that case an actual division into a mesenteric and mesocolic segment would have been effected.

Ventrad and to the right of this line of adhesion we would trace that portion of the primitive membrane which now passes to the coils of the small intestine as the true mesentery, having an apparent origin in the background of the abdomen to the dotted line of adhesion. In the same manner the peritoneal layers passing to the left to reach the caecum and beginning of the colon would appear as a free mesocolon with the same line of apparent origin from the background of the abdomen. (cf. p. 80.)

These considerations should be followed out in the dissection of the cat in order to become familiar with the principle of _secondary lines of origin_ for peritoneal layers. As we will see later this factor is of importance in correctly estimating the value of the human adult conditions.

3. A brief consideration of the mechanical conditions and comparison with the earlier stages will show why the peritoneal layers which occupy the bight of the fully developed umbilical loop are especially prone to develop secondary lines and areas of adhesion to other serous surfaces. If we compare the dorsal mesentery in its primitive condition, before the straight intestinal tube has become differentiated into the subsequent segments, and before the umbilical loop has been formed (Fig. 142), with the later stages represented by the intestines of the cat as now arranged (Figs. 143 and 144), it will be seen that the vertical line of attachment to the ventral surface of the vertebral column, between the points _a_ and _b_ corresponds in the advanced stages to the interval _ab_ separating the two points of the duodeno-colic isthmus; also that the entire mesenteric peritoneal surface beyond the isthmus is the result of drawing out and lengthening the intestinal tract. Consequently folding or overlapping of this extensive membrane affords opportunities for adhesions between its own serous surfaces or between it and the remaining visceral and parietal peritoneum of the abdomen.

Moreover, it will be appreciated that the entire extensive coil of intestines extending between the two boundaries of the duodeno-colic isthmus (_a_, _b_) is suspended from the back part of the abdomen by a narrow pedicle and that consequently rotation will readily occur around the axis drawn through the neck of the isthmus.

Now proceed to illustrate on the cat the result of the rotation as it occurs normally during the development of the primate intestinal tract. Take the caecum and commencement of the colon and draw the same over to the right across the duodeno-colic isthmus and the duodenum. Twist or rotate the entire mass of small intestines around the isthmic pedicle, so that the original left leaf of the mesentery will look to the right and _vice versa_ (Fig. 145). The conditions thus established will be found to correspond to the schemata shown in Figs. 114 and 115. The main features of the intestinal tract in the rearranged position will be as follows:

1. The two points, _a_ and _b_, of the duodeno-colic isthmus (Fig. 145) are still close together, but reversed in position, _b_ is in front and to the right, _a_ behind and to the left, whereas before the rotation _b_ was situated below and to the left, _a_ above and to the right (Fig. 135).

2. The direction of the ileo-colic entrance is reversed, the ileum now entering the large intestine from below and the left upwards and to the right, instead of from right to left.

3. The descending duodenum is now situated dorsad to the colon.

4. The original left leaf of the mesentery has become the right, and _vice versa_.

5. The superior mesenteric artery crosses over the transverse portion of the duodenum, and with the exception of the inferior pancreatico-duodenal artery the original right-sided branches now arise from the left side of the vessel and _vice versa_.

It is now time to compare the conditions established in the cat by the manipulations just detailed with the arrangement of the adult human intestinal tract and peritoneum below the level of the transverse colon and mesocolon.

I. The shortness of the large intestine in the cat will require careful manipulation in order to produce a disposition in conformity with the arrangement of this portion of the human intestinal tract. By stretching the gut somewhat and pulling it well out of the pelvis sufficient length will be obtained to establish an ascending, transverse and descending colon. Move the caecum from the subhepatic position which it occupies immediately after rotation (Fig. 145) down to the lower and right-hand corner of the abdomen. Pull the distal portion of the large intestine well out of the pelvis and obtain thus sufficient length to establish an ascending, transverse and descending division each provided with a free mesocolon (Fig. 146). In the formation of the three definite main segments of the human large intestine, ascending, transverse and descending colon, the following stages may be recognized:

1. Immediately after rotation the large intestine lies transversely along the greater curvature of the stomach, with the caecum on the right side in front of the duodenum and closely applied to the caudal surface of the right lobe of the liver (Fig. 147).

PERSISTENCE OF SUBHEPATIC POSITION OF CAECUM IN ADULT.--The period at which the caecum descends into the iliac fossa is liable to a considerable range of variation.

Treves found in two foetus, measuring respectively 41/2" and 51/2", the caecum on a level with the caudal end of the right kidney, while in several individuals at full term the caput coli was still placed immediately below the liver, with no large intestine in the place of the ascending colon. This condition is well illustrated in the foetus shown in Fig. 124.

The caecum may remain undescended throughout life. Treves, in an examination of 100 bodies, found this condition in two subjects, both females, one 41, the other 74 years of age. Both cases presented an identical disposition. There was no large intestine in the place of the ascending colon. The caecum was placed on the right side, immediately underneath the liver, just to the right of the gall-bladder; it was quite horizontal in position, continuing the long axis of the transverse colon and included between the layers of the transverse mesocolon. From the extremity of the caecum a horizontal fold was continued to the abdominal parietes and upon it the edge of the liver rested. In one of these instances the colon from the tip of the caecum to the splenic flexure measured 38". The great omentum was attached only to the left half of this portion. The descending colon was very long, measuring 15".

In the other case the distance from the tip of the caecum to the splenic flexure was 27", the great omentum commencing 5" from the former point. The descending colon was of normal length.

In both bodies the remaining viscera were normal.

2. The caecum next descends ventrad of right kidney to the iliac fossa. The future ascending colon is at this time placed very obliquely on account of the large size of the foetal liver, and passes without a marked angle into the transverse segment. Thus in Fig. 148, from a foetus 5" in length, the descending colon is vertical and the splenic flexure well marked, forming the highest point of the colic arch. There is no hepatic flexure, and no ascending and transverse colon, but instead of these an oblique segment passing upwards and to the left between caecum and splenic flexure.

This disposition, due to the large size of the liver, is still marked at times in the foetus at term, and occasionally even in children up to 2 or 3 years of age.

3. The ascending colon is subsequently differentiated from the transverse segment and the hepatic flexure formed consequent upon the diminution of the relative size of the liver, which permits the foetal oblique segment of the colon extending in the earlier stages between the right iliac fossa and the spleen to become divided by a right-angled (hepatic) bend or flexure into an ascending and a transverse segment (Fig. 149).

4. The splenic flexure develops early and is well marked. It indicates the point of transition of the original ascending limb of the umbilical loop into the remaining vertical median segment of the large intestine, from which the descending colon is formed.

In the adult the ascending and descending portions of the colon are vertical. The transverse colon is not quite horizontal since the splenic flexure is higher and placed more dorsally than the hepatic flexure. In the embryo the rapidly-growing coils of the small intestine push the descending colon to the left and dorsad into close contact with the dorsal abdominal wall.

A small bend which appears about the middle of the third month in the left iliac fossa indicates the rudiment of the future sigmoid flexure or omega loop.

The rest of the endgut follows the body wall in a well-marked curve, whose termination lies within the concavity of the caudal portion of the embryo (Fig. 150). From this terminal part the rectum develops after the division of the cloaca and the union of the proctodaeum with the entodermal intestinal pouch has taken place as detailed above.

The early position of the colon produced by the large size of the foetal liver, and before the descent of the caecum has occurred, is shown in Fig. 124. In Fig. 123, where the liver has regained its normal proportions with reference to the abdominal cavity and viscera, and the caecum has descended into the right iliac fossa, the hepatic flexure is well marked and the first segment of the colon has acquired the vertical position on the right side, the single obliquely transverse segment of Fig. 124, having become divided into an ascending and a transverse colon.

[Fig. 124. Early stage. Liver relatively large. Proximal portion of the colon extends obliquely between the right lumbar region and the spleen. The caecum has not yet descended.

Fig. 123. Later stage. The caecum occupies the right iliac fossa. Relative reduction in the size of the liver allows the colic segment to be divided by the hepatic flexure into an ascending colon and a transverse colon.]

At times the transverse colon, whose normal average length in the adult is 20", greatly exceeds this measurement and forms an arch which hangs down or makes a well-marked V-shaped bend with the apex directed toward the pubes. This is the normal arrangement of this portion of the large intestine in many of the lower primates. Fig. 151 shows the abdominal viscera of _Macacus rhesus_, hardened _in situ_, seen from the front and the right side, with the omentum turned up over the stomach. The transverse colon forms an extensive V-shaped bend, whose apex reaches to the pubes, from which point the large intestine turns again cephalad and dorsad to form the splenic flexure and then descends to the pelvis.

The average length of the ascending colon in the adult, measured from the tip of the caecum to the hepatic flexure, was found by Treves in his series of 100 bodies to be 8", while the descending colon, from the splenic flexure to the beginning of the sigmoid loop, measured 81/2".

The descending colon may at times be much longer, up to 15", and become convoluted.

II. In the next place, in order to understand the arrangement of the peritoneum in this lower larger compartment of the abdomen, disregard for the present the peritoneal connections of the stomach, liver, pancreas and spleen, and the folds of the great omentum entirely. This latter membrane is adherent in the adult human subject by its dorsal surface to the upper margin of the transverse colon, so that in turning the omentum up over the ventral chest wall the transverse colon will be carried with the omentum and the lower layer of the transverse mesocolon will be put upon the stretch. This membrane forms in adult man by its transverse attachment to the abdominal background the cephalic limit of the larger lower compartment of the abdomen, which is framed laterally by ascending and descending colon, continuous below with the pelvic cavity and occupied chiefly by the freely movable coils of the jejuno-ileum.

Remember that the duodenum starting from the pyloric extremity of the stomach first turns cephalad and dorsad in contact with the caudal surface of the right lobe of the liver, forming the first portion or hepatic angle of the duodenum; that in the next place the second or descending portion of the duodenum passes down in front of the medial part of the ventral surface of the right kidney and the inferior vena cava, but _behind_ the right extremity (hepatic flexure) of what after rotation and formation of the ascending colon appears as the transverse colon; that consequently the descending duodenum is divided by its intersection with the transverse colon into a cephalic supra-colic and a caudal infra-colic segment.

Also remember that the second angle of the duodenum (transition between the descending and transverse portions) is consequently situated to the right of the vertebral column below the level of the transverse colon and the secondary attachment presently to be considered of the transverse mesocolon to the background of the abdominal cavity.

The third portion of the duodenum extends from this point more or less transversely--depending upon the type--to the left, across the vertebral column and aorta. This transverse portion, after the rotation of the primitive loop at the duodeno-colic angle, is crossed in the direction caudad and ventrad by the superior mesenteric vessels, which hence divide this portion of the intestine into a right and left segment.

The latter turns cephalad and ventrad on the left side of the vertebral column (4th or ascending portion) to become continuous at the duodeno-jejunal angle with the free or floating small intestine (jejunum).

If we imagine in the cat the duodenum anchored or fixed by adhesion of the dorsal (originally right) leaf of the mesoduodenum and of its own dorsal visceral peritoneum to the abdominal parietal peritoneum in the manner above indicated (p. 70) as far as the duodeno-jejunal angle we will have conditions established which correspond to those found in the human adult abdominal cavity.

III. It is next necessary to study carefully the disposition of the primitive dorsal mesentery connected after rotation with the different segments of the intestinal tube, ascending, transverse and descending colon and free small intestine.

In order to obtain in the cat a cephalic limit to the region now under consideration which will correspond to the arrangement of the adult human peritoneum, we will begin with the peritoneal membrane attached to the portion of the colon which in the rearranged intestinal tract represents the human transverse colon. This transverse segment of the large intestine is now made to extend directly across the abdomen from the liver to the spleen. The two layers composing the transverse mesocolon are an upper or cephalic and a lower or caudal layer.

Now it will be seen in the cat that the upper or cephalic layer of the transverse mesocolon thus established is continuous on each side with the dorsal (originally right) leaf of the ascending and with the dorsal (originally left) leaf of the descending mesocolon, which peritoneal layers are in direct opposition to the parietal lumbar and prerenal peritoneum. On the other hand, the inferior or ventral layer of the transverse mesocolon is continuous on each side of the median line with the ventral (originally respectively left and right) leaves of the same mesocola, while at the site of the duodeno-colic isthmus the two layers of the transverse mesocolon are continuous as originally with the two layers of the mesentery of the jejuno-ileum (Fig. 146).

Now fix the transverse mesocolon firmly against the background of the abdomen and place the ascending and descending colon as far as possible over to the right and left side respectively. We will assume a line of secondary adhesion between the transverse mesocolon and the parietal peritoneum investing the dorsal abdominal wall. Along this line the upper or cephalic surface of the transverse mesocolon would become continuous with the dorsal parietal peritoneum, while the lower or caudal layer would still be continuous with the left leaf of the ascending and the right leaf of the descending mesocolon. We have already seen that the duodenum and mesoduodenum become anchored in the subhepatic region and that the visceral ventral peritoneum of the gut and the original left leaf of the mesoduodenum appear then as secondary parietal peritoneum. Hence a sagittal section through the right lumbar region, right kidney and descending duodenum would, immediately after rotation and establishment of the transverse mesocolon, show the peritoneal arrangement indicated in Fig. 153. After adhesion of the transverse mesocolon continuity would be established between its upper or cephalic layer and the secondary parietal peritoneum investing the supra-colic portion of the descending duodenum (Fig. 154) while its caudal layer becomes continuous with the secondary parietal peritoneum covering the infra-colic segment of the duodenum and the lower portion of the ventral surface of the right kidney.

Reference to the schematic Figs. 152, 153 and 154, will show that the adult duodenum becomes fixed to the posterior parietes of the abdomen by adhesion of its visceral serous covering and of the dorsal layer of the mesoduodenum to the primitive parietal peritoneum. The supra-colic segment of the adult descending duodenum lies under cover of a single peritoneal layer, derived from its own visceral investment and appearing as secondary parietal peritoneum by continuity laterad along the line of adhesion with the primitive parietal peritoneum covering the upper part of ventral surface of right kidney, while mesad, the layer covering this segment of the duodenum, is continued into the secondary parietal peritoneum derived from the left or ventral leaf of the mesoduodenum and covering the ventral surface of the pancreas (cf. Figs. 138-140).

On the other hand, the infra-colic segment of the descending duodenum, as well as the lower and mesal angle of the ventral surface of right kidney, between ascending and transverse colon, is covered by a layer of secondary parietal peritoneum derived from the ventral layer of the ascending mesocolon and continuous with the caudal layer of the transverse mesocolon. Beneath this secondary parietal peritoneum are two obliterated layers, on the one hand the dorsal layer of the mesocolon, on the other the visceral infra-colic duodenal serosa and the primitive prerenal parietal peritoneum.

In the further development of the adult human arrangement the changes below the level of the transverse colon and mesocolon result in the fixation of the ascending and descending colon to the background of the right and left lumbar regions. The opposed serous surfaces of the ascending and descending mesocola and of the dorsal parietal peritoneum adhere and the process also usually involves the dorsal visceral peritoneum of the ascending and descending colon, so that these portions of the gut obtain a fixed position.

Adhesion of the mesocolon to the dorsal body wall (parietal peritoneum) does not occur at all points at the same time. Usually adhesion proceeds from the midline laterad. The fixation of the ascending colon in the human embryo begins about the fourth month.

In the descending segment by the same time adhesion has usually proceeded nearly up to the descending colon, but the intestine itself is as yet free. In the fifth month the descending colon has usually become fixed between the splenic flexure and the beginning of the sigmoidea. In the latter region a free mesocolon usually persists throughout life.

Differences in the rate of growth between the length of the body wall and the length of the mesocolon may play an important part in the production of peritoneal _fossae_, small pouches which in some regions of the abdomen may assume considerable proportions. Such fossae are found around the duodeno-jejuneal angle, the caecum and appendix, and the sigmoid flexure. They will be considered more in detail with these respective regions, especially in reference to their relation to retro-peritoneal hernia.

In a certain proportion of cases adhesion between the parietal peritoneum and the ascending and descending mesocolon is incomplete or entirely wanting, resulting in the formation of a more or less completely free ascending and descending mesocolon. Treves, in an examination of 100 bodies, obtained the following figures:

In 52 subjects there was neither an ascending nor a descending mesocolon, the intestine being fixed in the manner which is regarded as normal.

In 22 there was a descending, but no trace of an ascending mesocolon.

In 14 a mesocolon was found in both the ascending and descending segments of the large intestine.

In 12 there was an ascending mesocolon, but no corresponding fold on the left side. Hence from this series a mesocolon may be expected on the left side in 36 per cent., on the right side in 26 per cent.

Both development and comparative anatomy would lead us to expect that the descending mesocolon would be found more frequently than the ascending.

In the lower animals the descending mesocolon is always an extensive and conspicuous membrane. It is well developed in all monkeys and the anthropoidea, as the remains of the primary vertical fold of the dorsal mesentery, while the ascending mesocolon is a secondary production, acquired during the development of the bowel by rotation.

In most of the lower monkeys the ascending mesocolon is also largely or entirely free. The descending mesocolon can always in these animals be reflected to the median line (cf. Fig. 155).

The line of attachment in man of the descending mesocolon is usually along the lateral border of the left kidney and vertical, while the line of attachment of the ascending mesocolon is usually less vertical, crossing the caudal end of the right kidney obliquely from right to left and with an upward direction (Fig. 156).

In like manner when both the ascending and descending mesocola are absent as free membranes the left or descending colon is adherent along the lateral border of the kidney to the abdominal parietes, while the ascending colon is fixed at the hepatic flexure a little obliquely across the ventral surface of the caudal end of the corresponding gland ascending toward the medial margin.

Treves found in the cases of persistent ascending mesocolon in the adult that the membrane varied in breadth from 1" to 2" while the persistent fold on the left side varied between 2" and 3" in breadth.

In the foetus, up to 5"-6" in length, the descending mesocolon is usually an extensive fold. Its attachment is vertical, but nearer to the median line than in the adult, usually along the medial border of the left kidney. It is at times found attached along this line in the adult.

An ascending mesocolon is rare even in the foetus. The caecum and beginning of the ascending colon are completely invested by peritoneum, but above the parts so invested the colon is usually adherent along an oblique line to the ventral and medial aspect of the right kidney.

In the foetus at full term, if the caecum is still undescended and in contact with the liver, it is not uncommon to find the cephalic portion of the descending colon provided with a mesocolon, while the caudal part of the descending colon is fixed by adhesion to the ventral surface and lateral border of the left kidney. This free membrane is then really a part of the transverse mesocolon. Where the caecum descends to the iliac fossa the portion of the foetal descending colon so invested is drawn over to the right and incorporated in the transverse colon.

Treves in two out of 100 bodies found the caecum in the right iliac region, but both it and the whole of the ascending colon were entirely free from any peritoneal connections with the dorsal parietes of the abdomen.

The gut from the tip of the caecum to the hepatic flexure was entirely invested by peritoneum continuous with the mesentery. The ascending colon was covered in the same manner and by the same fold as the small intestine. The segment of large intestine thus free measured 8" in both instances.

The mesentery lacked the usual attachment to the dorsal abdominal wall and its root was represented by the interval between the duodenum and the transverse colon. The membrane had no other than its original primary attachment, and small intestine and ascending colon formed together a loop that practically represented the condition of the great primary intestinal loop. (Compare p. 73.)

The arrangement presented in these two subjects corresponds to that met in many animals, such as the cat.

A cross-section of the cat's abdomen arranged as above would show the following disposition of the peritoneum, corresponding to the stage in the human development preceding the fixation of the two vertical colic segments (Fig. 157). It will be seen that the right and left mesocola can be reflected to the median line where they become continuous ventrad of the vertebral column and aorta with the mesentery of the small intestine. The ventral surfaces of both kidneys are seen to be covered by the primitive parietal peritoneum of the abdominal cavity.

Fig. 158 shows the adult human arrangement of the same parts, after fixation of the vertical colic segments by adhesion of the opposed surfaces of their mesocola and the primitive parietal peritoneum. The background of the abdomen is now seen to be covered by a layer of secondary parietal peritoneum, _viz._, the original left leaf of the ascending and right leaf of the descending mesocolon, continuous above with the lower or caudal layer of the transverse mesocolon.

This adhesion is so complete that the original condition is disregarded in adult descriptive anatomy. The layer which has adhered to the parietal peritoneum can no longer be recognized and the other has assumed the role of parietal peritoneum.

The connection of the transverse mesocolon with the dorsal lamella of the great omentum will be considered below.

The course of the vessels in the ascending and descending mesocola is not altered by the secondary adhesions. These vessels are in the adult situated behind the secondary parietal peritoneum derived from the mesocola.

The origin of the transverse mesocolon obtains by the fixation of the hepatic and splenic flexures high up in the abdomen a transverse course, and the transverse growth of the abdomen holds the membrane in this position cephalad of the duodeno-jejunal flexure, so that on elevating the transverse colon the mesocolon appears as separating the upper from the lower abdominal compartment. This posterior line of attachment or so-called "root of the transverse mesocolon," is nothing more than the upper limit of the area of adhesion between the primitive parietal peritoneum and the opposed surfaces of the ascending and descending mesocola. Reference to the abdominal cavity of the cat after complete rotation (Fig. 146) will show the original continuity of the three mesocola very clearly. A secondary connection is established along the lateral border of ascending and descending colon (Fig. 158), between the primitive parietal peritoneum and the ventral visceral peritoneal investment of the large intestine. Both of the vertical segments of the colon now appear fixed. Their dorsal surfaces are uncovered by peritoneum and can be reached in the lumbar region, laterad of the kidney, without opening the peritoneal cavity (lumbar colotomy).

The caudal portions of both kidneys are covered, beneath the secondary parietal peritoneum, by a layer of loose connective tissue representing the result of obliteration by adhesion of the first and second of the original three layers of prerenal peritoneum, _viz._, the primitive parietal (1) and the two layers of the mesocola (2 and 3).

LINE OF ATTACHMENT OF THE MESENTERY OF THE JEJUNO-ILEUM.--Examination of the caudal surface of the transverse mesocolon in the cat, with the parts in the above outlined position, will show how and why in the adult human abdomen the duodeno-jejunal angle appears to dip out from beneath the transverse mesocolon, becoming gradually more and more free until complete transition to the mobile jejunum is obtained. From this point, situated to the left of the second lumbar vertebra, the dorsal attachment of the adult human mesentery of the jejuno-ileum extends somewhat obliquely caudad and to the right to terminate in the right iliac fossa at the ileo-colic junction.

Returning to the conditions presented by the cat's intestines to obtain an explanation of this line of fixation we must recall the fact that in the peritoneum included within the limits of the umbilical loop, after differentiation of small and large intestine, but before rotation, we have both the elements of the mesentery of the small intestines and of the ascending and transverse mesocolon combined (Fig. 141). For it will be seen that this membrane carries at this time vessels both to the jejuno-ileum and to the segments of the large intestine (caecum, ascending and transverse colon). This fact will be at once recognized if the cat's intestines are arranged to correspond to the primitive condition (Fig. 136) and the mesentery examined.

After rotation and differentiation of the colic segments and after the adhesion of the ascending and descending colon in man, the course of the main trunk of the superior mesenteric artery passes, after crossing the third portion of the duodenum, down and to the right to terminate near the ileo-colic junction by anastomosis with its ileo-colic branch. The adhesion of the right and left mesocola to the dorsal parietal peritoneum proceeds mesad as far as this line, leaving free the mesentery of the small intestines, which contains the vasa intestini tenuis derived from the left side of the main vessel. The secondary line of attachment of the mesentery to the abdominal background is therefore along this line. To obtain a clear idea of these processes of development in man assume that in the cat, after rotation and establishment of the three divisions of the colon, the two vertical (ascending and descending) mesocola become adherent to the dorsal parietal peritoneum, leaving the mesentery of the small intestine free.

Fig. 159 illustrates schematically the area of mesocolic adhesion in the human subject after complete rotation, and the line of the mesentery of jejuno-ileum.

Fixation of the ascending and descending cola and of their mesocola proceeds cephalad as far as the line _AB_, which thereby constitutes the root of the free transverse mesocolon.

The secondary parietal peritoneum derived from the ventral layer of the ascending mesocolon covers the lower and inner portion of the ventral surface of the right kidney, the infra-colic division of the descending and the dextro-mesenteric segment of the transverse duodenum, while along the root of the jejuno-ileal mesentery it becomes continuous with the right layer of that membrane. The secondary parietal peritoneum derived from the ventral layer of the descending colon covers the lower part of the ventral surface of the left kidney and the sinistro-mesenteric segment of the transverse duodenum and becomes continuous along the mesenteric radix with the left layer of the jejuno-ileal mesentery.

Caudad the adhesion of the descending colon and mesocolon to the parietal peritoneum proceeds only to the point _C_, following the dotted line mesad and resulting in the formation of the free mesocolon of the sigmoid flexure.

=Resume of the Adult Arrangement of the Human Peritoneum in the Lower Compartment of the Abdomen, Below the Level of the Transverse Colon and Mesocolon.=--We should now consider the arrangement of the human peritoneum in the adult below the dorsal attachment of the transverse mesocolon in the light of the embryological and comparative anatomical facts just stated. In doing this it will be advisable to study both the actual conditions encountered and their significance in the sense of determining the derivation of the peritoneal layers from the primitive dorsal mesentery. Open the abdominal cavity in the usual manner by a cruciform incision.

Turn the great omentum up on the chest wall, exposing the underlying intestines. This manipulation, as already stated, will cause the omentum to carry the transverse colon with it, on account of the adhesion, in the adult, of the gut to the dorsal layer of the omentum. Hence the cephalic or upper layer of the transverse mesocolon will not be seen at this stage because the omental adhesion just referred to prevents us from passing between the greater curvature of the stomach and the transverse colon without tearing peritoneal layers. It will, however, be possible to trace on the right side the duodenum from the pylorus down ventrad of the right kidney until the descending portion disappears behind the hepatic flexure of the colon. With the omentum and transverse mesocolon turned up, as stated, and the transverse mesocolon put upon the stretch, it will be seen that the abdominal space now overlooked is bounded cephalad by the lower layer of the transverse mesocolon and its attachment to the dorsal abdominal wall. The lateral limits of the space are given by the ascending and descending colon respectively. The attachment of the mesentery of the small intestine to the oblique line extending from the left of the vertebral column at about the level of the second lumbar vertebra to the right iliac fossa subdivides the entire space into a secondary right and left compartment.

Begin by following the caudal layer of the transverse mesocolon dorsad on the right side. In the angle between ascending and transverse colon (hepatic flexure) pressure will locate the caudal portion of the ventral surface of the right kidney. Remember that the peritoneum touched in these procedures appears in the adult as parietal prerenal peritoneum, but that in reality it is the left leaf of the originally free ascending mesocolon, whereas the original right leaf of this membrane and the primitive parietal peritoneum have, by adhesion of their serous surfaces, been converted into the loose subserous connective tissue covering the ventral aspect of the kidney beneath what now appears as parietal peritoneum.

Mesad of the resistance offered to the finger by the right kidney the caudal (infra-colic) portion of the descending duodenum and the angle of transition between it and the third or transverse portion will be found, invested in the same way by secondary (mesocolic) parietal peritoneum. It will be seen, especially if the duodenum is injected or inflated, that the hepatic flexure of the colon lies ventrad of the vertical descending second portion of the duodenum, so that one part of this intestine is situated cephalad the other caudad of the colon. (Supra- and infra-colic segments of descending duodenum.)

Individual differences are observed in the area of colic attachment to the duodenum. Usually the two intestines are in contact with each other and adherent over a considerable surface. Exceptionally the transverse mesocolon extends across to the right so as to include the hepatic flexure. In this latter case the uncovered non-peritoneal surface of the descending duodenum is small, represented by the interval between the layers of the transverse mesocolon, and the hepatic flexure is then not directly adherent to the gut.

If we now trace the transverse duodenum from right to left we will encounter the right layer of the root of the jejuno-ileal mesentery. The caudal layer of the transverse mesocolon, the right leaf of the mesentery and the secondary parietal peritoneum investing the ventral surface of the transverse duodenum all meet at this point. Surround the mesentery of the free small intestine with the fingers of one hand so that the entire mass of intestinal coils can be swung alternately from side to side.

Turning them over to the left, as already stated, the proximal portion of the transverse duodenum can be traced from right to left as far as the root of the mesentery. Here the peritoneum investing the ventral surface of the duodenum becomes continuous with the right leaf of the mesentery. Now swing the whole mass of small intestines over to the right, exposing the parietal peritoneum in the space to the left of the vertebral column, between the attachment of the mesentery to the median side, the root of transverse mesocolon cephalad and the descending colon to the left. Remember that the same significance attaches to this secondary parietal peritoneum as on the right side. It appears in the adult as parietal peritoneum, but is in its derivation the original right leaf of the descending mesocolon. Close to the root of the mesentery the continuation from the right side of the transverse duodenum will be seen, crossing the median line from right to left ventrad of aorta and vertebral column and usually turning cephalad on the left side of the lumbar vertebrae, as the fourth or ascending duodenum, to reach the caudal surface of the transverse mesocolon near its attachment, where the gut turns ventrad to form the duodeno-jejunal angle and become continuous with the free small intestine.

From the fact that the transverse duodenum is thus seen on each side of the root of the mesentery it will be recalled that after rotation of the primitive intestine the superior mesenteric artery crosses the transverse portion of the duodenum to reach its distribution between the leaves of the mesentery. Hence this portion of the small intestine consists of a dextro- and sinistro-mesenteric segment. This intersection of mesentery and duodenum marks the site of the primitive duodeno-colic isthmus through which the superior mesenteric artery passed to reach its distribution to the gut composing the embryonic umbilical loop.

To the left of the ascending duodenum a portion of the caudal surface of the pancreas will be seen, covered by the continuation of the caudal leaf of the transverse mesocolon into the parietal peritoneum. The consideration of this relation of peritoneum and pancreas will profitably be deferred until we have studied the developmental changes in the region of the dorsal mesogastrium and great omentum.