Chapter IX: Part II (1)

ANATOMY OF THE PERITONEUM IN THE SUPRA-COLIC COMPARTMENT OF THE ABDOMEN.

We have already seen that the transverse colon and mesocolon effect a general division of the adult human abdominal cavity into a cephalic supra-colic compartment, situated between the diaphragm and the level of the transverse colon and mesocolon, comprising in general the hypochondriac and epigastric regions, and a larger caudal infra-colic space which includes the entire rest of the abdominal cavity and is continued caudad into the pelvic cavity. The arrangement of the peritoneum and viscera in this latter space has just been considered. The fact will be recalled that the second or descending portion of the duodenum, passing dorsad of the hepatic colic flexure, forms so to speak the visceral connection between the portions of the alimentary tube situated in the supra-colic compartment and those situated in the infra-colic space. The fixation of this segment of the duodenum and its consequent secondary retroperitoneal position in the adult human subject masks this continuity of the alimentary canal to a certain extent so that it requires more than a superficial examination in order to trace correctly the course of the duodenum from the pylorus to the duodeno-jejunal angle, dorsad of the colon, root of transverse mesocolon and mesentery, and under cover of the secondary parietal peritoneum.

We have now to turn our attention to the viscera contained in the cephalic or supra-colic compartment of the abdomen and to consider the disposition of the serous membrane investing them and connecting them with each other and with the abdominal parietes.

The visceral contents of the supra-colic compartment comprise the liver, pancreas, spleen, stomach and the proximal portion of the duodenum, including the hepatic angle and the supra-colic part of the descending duodenum. Less directly the cephalic portions of the right and left kidney and the corresponding suprarenal capsules belong to this visceral group.

In this region of the abdomen we meet with the most extensive modifications of the primitive dorsal peritoneal membrane, producing conditions which, considered without reference to development and comparative anatomy, are complex and difficult of comprehension. These changes lead to the formation of the so-called "lesser sac," a term which in some respects is unfortunate as it implies a more complete degree of separation from the general peritoneal cavity or "greater sac" than actually exists.

In order to clearly understand the adult arrangement of the peritoneum in this region it is advisable to consider the subject in two distinct subdivisions, dealing successively with the two cardinal facts which contribute to effect the change from the simple primitive to the complicated adult condition.

These two main elements are:

1. Developmental changes in the position of the stomach, alterations in the disposition of the proximal part of the primitive dorsal mesentery attached to the stomach, and the development of pancreas and spleen in connection with this membrane.

2. The development of the liver and the successive stages in the production of the final adult vascular and serous relations of this organ.

=1. Stomach and Dorsal Mesogastrium.=--We have already considered the early stages in the differentiation of the stomach from the primitive intestinal tube of uniform caliber (p. 40). It will be recalled that the stomach at a certain period, while it already presents the main structural features familiar in the adult organ, occupies a vertical position in the abdominal cavity, turning its concave margin (lesser curvature) ventrad, while the convex dorsal border (greater curvature) is directed toward the vertebral column, being attached to the same by the layers of the proximal part of the primitive dorsal mesentery. At this time the stomach presents right and left surfaces, and the oesophageal entrance is at the highest or cephalic point of the organ, while the pyloric transition to the small intestine occupies the distal caudal extremity.

The primitive dorsal mesentery, as already stated, passes as a thin double-layered membrane between the ventral surface of the vertebral column and the dorsal border of the stomach, which, as we will presently see, becomes during the later stages of development the caudal (lower) margin or greater curvature.

It will be seen that the embryonic differentiation of the intestinal tract into successive segments justifies the application of a terminology based on this differentiation to the corresponding portions of the primitive common dorsal mesentery.

Thus the proximal portion extending between the vertebral column and the dorsal border or greater curvature of the stomach becomes the _mesogastrium_; we differentiate this portion still further as the "_dorsal mesogastrium_" to distinguish it from a "_ventral mesogastrium_" which we will presently encounter in considering the development of the liver and the connected peritoneum.

In the same way the section of the primitive common dorsal mesentery attached to the duodenal loop becomes the _mesoduodenum_, that connected with the mobile part of the small intestine (jejuno-ileum) the _mesentery_ proper, while the portion passing to the colon forms the _mesocolon_, to be subsequently still further subdivided, after the different segments of the large intestine have become mapped out, as the _ascending_, _transverse_ and _descending mesocolon_, the _mesosigmoidea_ and the _mesorectum_.

In tracing the development of the adult human peritoneum it is well to consider certain stages, which we will find illustrated by the permanent conditions presented by some of the lower vertebrates:

These stages comprise:

(_a_) Changes in the position of the stomach.

(_b_) Changes in the direction and extent of the dorsal mesogastrium.

(_c_) Development of the pancreas and spleen in connection with the mesogastrium.

A. Changes in the Position of the Stomach.

The primitive position of the organ above outlined (p. 41) is changed during the course of further development by a twofold rotation.

1. The primitive vertical position, in which the oesophageal entrance occupies the highest cephalic extremity, while the pyloric opening is at the opposite caudal end, is exchanged for one directed more transversely, approximating the two gastric orifices to the same horizontal level. In human embryos of 13.9 mm. the fundus has already descended, the pylorus moving cephalad and to the right, while the cardia becomes shifted more to the left. At the same time the greater growth and prominence of the convex border or greater curvature becomes marked in comparison with the relatively short extent of the opposite margin or lesser curvature.

2. Coincident with this change in position is a rotation around the vertical axis, by means of which the original left side of the stomach is turned ventrad, becoming the ventral or "anterior" surface, while the original right surface of the organ now looks dorsad toward the vertebral column, becoming the dorsal or "posterior" surface of human anatomy. The oesophageal or cephalic end is placed to the left of the median line, while the caudal or pyloric end is situated on the right side (Figs. 169 and 170).

The original ventral border, now the "lesser curvature" or "upper border," looks cephalad and to the right, toward the caudal surface of the liver, while the original dorsal border, as the "greater curvature" or "lower border" is directed in the main caudad and to the left.

The prominence of this border is still further increased by the greater development of the stomach to the left of the oesophageal entrance resulting in the formation of the "fundus" or "great cul-de-sac."

This rotation of the stomach explains the asymmetrical position of the vagus nerve in the adult, the left side of the embryonic stomach, innervated by the left vagus, becoming the "anterior" surface of adult descriptive anatomy and _vice versa_.

It will be readily appreciated that a comparatively flat organ like the stomach, will, as long as it occupies a sagittal position, with right and left surfaces, help to divide the upper part of the abdominal cavity to a certain extent into a right and left half, even if the peritoneal connections of the organ are left out of consideration. As soon, however, as the above-described changes in position take place and the surfaces of the stomach are directed ventrad and dorsad, the relative arrangement and extent of this right and left abdominal space becomes altered by the different disposition of the septum, _i. e._, the stomach. The original right side of the organ is now directed dorsad, and the rotation of the organ has created a space between this dorsal or "posterior" surface of the stomach and the background of the abdominal cavity, which is the inception of the "lesser peritoneal cavity" or retrogastric space. We will find that this space becomes well defined and circumscribed by the peritoneal connections of the stomach, but we will realize, even at this stage, that the _dorsal_ surface of the stomach will form a part of the general _ventral_ wall of the lesser peritoneal space.

On the other hand, the partial division of the abdomen into a right and left half, effected by the stomach in its primitive sagittal position, disappears after rotation of the organ. We now pass uninterruptedly from left to right across the ventral (original _left_) surface of the stomach.

B. Changes in the Direction and Extent of the Dorsal Mesogastrium.

The effects of the altered position of the stomach on the disposition of the abdominal space have just been considered in relation to the organ itself, without reference to its natural connections with the parietes and with adjacent viscera. Their true significance and their influence on the adult anatomical arrangement of the abdomen is, however, only appreciated when the changes in the arrangement of the peritoneal membrane which they involve, are taken into account.

The dorsal mesogastrium changes more than any other portion of the peritoneum in the course of development. It not only becomes displaced and altered in direction by the rotation of the stomach, but in addition it grows so extensively that it finally hangs down like an apron over the entire mass of small intestines, forming the great omentum.

If we begin with the primitive disposition of the sagittal stomach and dorsal mesogastrium shown in Fig. 171 it will be observed that both structures together actually divide the dorsal portion of the abdominal cavity into symmetrical right and left halves (Fig. 172).

After rotation of the stomach (Fig. 173) the mesogastrium loses its original sagittal direction. It follows the altered position of the original dorsal border of the stomach, which has now become the caudal margin or "greater curvature," by turning caudad and to the left, being at the same time considerably elongated. This occurs during the second month. Hence the dorsal mesogastrium, after leaving the vertebral column, turns ventrad and to the left to reach its gastric attachment along the greater curvature. This is the first indication of the formation of the great omental or epiploic bursa.

The stomach is here considered as developing in situ and as influencing by its growth and change of position the arrangement and direction of the peritoneal layers with which it is connected. As a matter of fact it is well to note that the stomach at first lies above the primitive diaphragm or septum transversum, migrating, however, at an early period into the subhepatic abdominal position. This migration produces a corresponding increase in the length of the oesophagus (Fig. 34) and the stomach, in consequence of this change in position, acquires its ventral and dorsal mesogastrium. For the purpose of explaining the adult peritoneal relations of the organ it is, however, more convenient to regard the stomach as an abdominal organ from the beginning and to deal with the subsequent changes in position from this standpoint. The inaccuracy is slight and renders the comprehension of the succeeding stages easier.

It will be noticed (Fig. 173) that the rudimentary retro-gastric space or "lesser peritoneal sac" is bounded ventrally by the dorsal (the primitive _right_) surface of the stomach, while its dorsal boundary is furnished by the ventral (originally _right_) layer of the dorsal mesogastrium.

In the primitive condition, therefore, dorsal mesogastrium and stomach form together a straight line sagittal in direction and placed in the median plane of the body. As the result of the developmental changes above outlined this straight line becomes bent at the point where the mesogastrium reaches the stomach (Fig. 173, x). The two component elements of the line (stomach and mesogastrium) hinge on each other here, and the angle which they form opens to the right.

The changes which are to be observed in the later stages depend principally upon a peculiar feature characteristic of the development of the dorsal mesogastrium. This feature consists in the extreme redundancy of the membrane which grows out of proportion to the requirements of its visceral connections, and to a certain extent becomes independent of the direct mechanical purpose of carrying blood vessels to the viscera. Hence in a transverse section at this period (Figs. 174 and 175) the mesogastrium no longer passes in a direct line between its points of attachment, viz. the greater curvature of the stomach and the vertebral column, but extends beyond the stomach to the left. We will appreciate the significance of this extensive growth of the mesogastrium especially in considering the development of the spleen and pancreas. For the present it will suffice to note (Figs. 174 and 175) that the growth has carried the mesogastrium well to the left of the stomach, consequently the retrogastric space is now bounded toward the left by the bend which the original right leaf of the primitive sagittal mesogastrium takes in order to reach its gastric attachment. The retrogastric space therefore terminates toward the left in a blind pocket formed by this reduplication of the mesogastrium.

One more factor is to be taken into consideration, namely the tendency, already noted, of peritoneal surfaces to become adherent to each other. Such adhesion involves the apposed surfaces of the mesogastrium and of the primitive parietal peritoneum to the left of the vertebral column. The dorsal (original _left_) layer of the mesogastrium adheres to the parietal peritoneum covering the left side of the abdominal background and the cephalic portion of the ventral surface of the left kidney up to the end of the blind pouch which forms the extreme left limit of the retrogastric space. Hence, after this process of adhesion is completed, the dorsal wall of the retrogastric space is lined by secondary parietal peritoneum covering the left kidney (original right leaf of primitive mesogastrium) (Fig. 175). We obtain (Fig. 175 at x) an apparent continuity of the parietal peritoneum with that portion of the mesogastrium which, derived from the original left layer of the membrane, appears now to extend, as the ventral one of two layers, between the stomach and the abdominal parietes near the lateral border of the left kidney. (Primitive gastro-splenic omentum.)

It should be remembered that the disposition of the peritoneum just indicated is modified by the development of the pancreas and spleen, both of which organs are intimately associated with the mesogastrium. The foregoing statements and diagrams are therefore merely given for the purpose of affording a general view of the extent, growth and changes of the dorsal mesogastrium before proceeding to consider the development of the pancreas and spleen in and from the membrane itself.

In the view directly from in front the redundancy of the peritoneum forming the mesogastrium is shown in Figs. 176 and 177. Just as the membrane extends further to the left than required by its visceral connection with the stomach, so the downward growth exceeds the demand made by the rotation of the attached border (greater curvature) caudad and to the left. The mesogastrium, forming, as it now does, the great omentum, enlarges in descending toward the transverse colon (Fig. 177). The bag thus formed can be distended with air in a foetus of from 8 to 9 cm. vertex-coccygeal measure, as shown in the figure. Consequently in sagittal section the membrane is seen to extend caudad beyond the level of the greater curvature, and must turn on itself and pass again cephalad in order to reach the stomach (Fig. 178). By reason of this excessive growth the limits of the primitive retrogastric space are enlarged, not only toward the left, but more especially in the caudal direction. The bend made by the mesogastrium in returning to the stomach forms the blind extremity of a pouch which continues the retrogastric space caudad beyond the stomach, and whose dorsal and ventral walls are formed by the reduplicated mesogastrium. This pocket or pouch constitutes the _omental_ or _epiploic bursa_ of the lesser peritoneal cavity, for the great omentum is the direct product of this redundant growth of the mesogastrium caudad. It will be observed that the great omentum is made up of four peritoneal layers, the folding of the double-layered mesogastrium naturally producing this result. The first or ventral and the fourth or dorsal layer are derived from the original left layer of the primitive sagittal mesogastrium; the intermediate second and third layers, separated from each other at this stage by the cavity of the omental bursa, are products of the primitive right leaf of the mesogastrium. Since the entire retrogastric space with its extensions becomes the "lesser cavity" of the human adult peritoneum, it will be seen that its serous membrane is derived from the original right leaf of the mesogastrium (second and third omental layers). After the above-described adhesion of the mesogastrium to the parietal peritoneum overlying the ventral surface of the left kidney, the membrane would be traced in sagittal section (Fig. 179) from the dorsal surface of the stomach caudad, lining the interior of the omental bursa (second layer) to the turn or blind end of the pouch; thence cephalad as the third omental layer, forming the dorsal wall of the epiploic bursa, to invest, as secondary parietal peritoneum, the cephalic segment of the ventral surface of the left kidney.

C. Development of Spleen and Pancreas in the Dorsal Mesogastrium and Changes in the Disposition of the Great Omentum.

In order to obtain a correct conception of the adult human conditions it is finally necessary to consider the development of the spleen and pancreas in their connection with the dorsal mesogastrium and to note the changes which are produced by adhesion of portions of the great omentum to adjacent serous surfaces. It will be advisable to discuss these subjects at first separately, and to subsequently combine all the facts in an attempt to gain a correct impression of their share in determining the disposition of the adult human peritoneum.

=1. Development of Spleen.=--The spleen develops from the mesoderm between the layers of the dorsal mesogastrium, near its point of accession to the greater curvature, in the region of the subsequent fundus. It has therefore, like the stomach, originally free peritoneal surfaces. After rotation of the stomach the organ lies between the two layers of the membrane at the extreme left end of the retrogastric space (Fig. 180).

=Vascular Connections.=--The splenic artery accedes to the mesal surface of the spleen from the vessel which originally passed directly to the dorsal border (subsequent greater curvature) of the stomach, between the layers of the mesogastrium.

With the further growth of the spleen the segment of this vessel situated between its origin from the coeliac axis and the hilum of the spleen becomes relatively larger, forming the adult splenic artery, while the continuation of the original vessel to the greater curvature of the stomach appears now as a branch of the splenic artery, viz., the arteria gastro-epiploica sinistra.

Through the development of the spleen the dorsal mesogastrium has been subdivided into a proximal longer vertebro-splenic, and a distal shorter gastro-splenic segment. The former, as we have seen, loses its identity as a free membrane in the human adult, by fusing with the parietal peritoneum investing the ventral surface of the left kidney. Hence, after this adhesion has taken place, the splenic artery courses from the coeliac axis to the spleen behind peritoneum which functions as part of the general parietal membrane, but which is derived from the original right leaf of the proximal vertebro-splenic segment of the primitive mesogastrium (Fig. 181). On the other hand the distal segment of this membrane, beyond the spleen, remains free, carrying, as the gastro-splenic omentum, the left gastro-epiploic artery between its layers from the splenic artery to the greater curvature of the stomach.

The lateral limit of the area of adhesion between mesogastrium and parietal peritoneum is situated along the lateral border of the left kidney. Hence, in the final condition of the parts, the main splenic vessels at the hilum are situated between two peritoneal layers of which the ventral (Fig. 181) appears as the parietal peritoneum forming the dorsal wall of the retro-gastric space, while the dorsal layer (Fig. 181) forms a reflection from the mesal surface of the spleen, along the dorsal margin of the hilum, to the adjacent lateral border of the left kidney (lieno-renal ligament) and to the diaphragm. At this point of adhesion subsequently firmer strands of connective tissue develop in the serous reduplication forming the _ligamentum phrenico-lienale_ of systematic anatomy. This process of adhesion takes place during the second half of intra-uterine life. A connection with the colon, produced by adhesion of the mesogastrium to the splenic flexure of the large intestine, forms the adult _lig. colico-lienale_, while a similar adhesion between great omentum, transverse mesocolon and phrenic parietal peritoneum just caudad of the spleen, gives rise to the _colico-phrenic_ or _costo-colic "supporting" ligament_ of the spleen.

On the other hand, the ventral one of the two layers constituting the gastro-splenic omentum and including between them the left gastro-epiploic artery, is formed by the distal part of the primitive left layer of the mesogastrium, while the dorsal layer of the same fold is the portion of the primitive right layer beyond the spleen, which has not been converted into secondary parietal peritoneum, but forms now part of the ventral wall of the lesser peritoneal sac between the spleen and the stomach (Fig. 181) (lig. gastro-lienale). Since, therefore, the gastro-splenic omentum is a specialized part of the fully-developed dorsal mesogastrium, and since we have seen that the great omentum is formed directly by the excessive growth of this membrane caudad, it is not difficult to understand why in the adult human subject the ventral layer of the gastro-splenic omentum is directly continuous with the ventral layer of the great omentum along the greater curvature of the stomach to which both are attached. The dorsal layer of the gastro-splenic omentum would, in the same way, be continuous with the second layer of the great omentum, lining the ventral wall of the omental bursa, if it were not for the fact that in the adult adhesions usually obliterate the cavity of the bursa.

Fig. 182 shows the stomach, left kidney, spleen and splenic flexure of the colon hardened in situ and removed from the body of a two-year-old child. The great omentum has been divided along the line of adherence to the transverse colon.

In Fig. 183 the spleen has been removed from the preparation by division of its peritoneal and vascular connections, and is shown in its mesal aspect (gastric and renal surfaces, intermediate margin and hilum). It will be seen that the peritoneal reflections are arranged in the form of two concentric elliptical lines. The two ventral lines form the gastro-splenic omentum and correspond to the reflection of the peritoneum from spleen to left end of stomach carrying the gastric branches derived from the splenic artery. The third line from before backwards results from the division of the secondary parietal peritoneum of the lesser sac, covering splenic artery, and ventral surface of pancreas and derived from the dorsal mesogastrium; while the most dorsal fourth line represents the divided reflection of the peritoneum from the renal surface of spleen to lateral border of left kidney and diaphragm (lig. lieno-renale).

Between the second and third lines of peritoneal reflection appears the portion of the mesal surface of the spleen in contact with and invested by the extreme left end of the lesser peritoneal sac.

Fig. 184, taken from an adult human subject with the viscera hardened in situ, shows the left or splenic extension of the lesser peritoneal cavity.

=2. Development of the Pancreas.=--The pancreatic gland is derived from the hypoblast of the enteric tube. The secreting epithelium and that lining the ducts of the adult gland is formed by budding and proliferation of the intestinal epithelium. The gland develops primarily from two outgrowths which are at first separate and distinct from each other.

1. The proximal and dorsal bud grows directly from the hypoblast lining the duodenum immediately beyond the pyloric junction.

In embryos of 8 mm. (four weeks) (Fig. 185) it appears as a small spherical outgrowth connected by a slightly narrower stalk with the epithelial intestinal tube.

2. The distal and ventral outgrowth is separated from the preceding and is from the beginning closely connected with the similar embryonic outgrowth from the enteric tube which is to form the liver. This portion of the pancreas is, strictly speaking, derived primarily from the epithelium of the primitive hepatic duct and not directly from the duodenum. This primary arrangement of the gland, being formed of two main collections of budding hypoblastic cells, corresponds to the adult system of the pancreatic excretory ducts. The proximal or dorsal outgrowth furnishes that portion of the head of the gland whose excretory system terminates in the _secondary pancreatic duct_ or _duct of Santorini_, while the distal (ventral) outgrowth includes within its area the termination of the principal pancreatic duct or _canal of Wirsung_, which is closely connected with the end of the common bile-duct at the intestinal opening common to both (Figs. 186-187). The method of union of the two pancreatic outgrowths and their respective share in building up the adult gland explains the usual adult arrangement of the excretory system and its variations.

In the embryo of five weeks (Fig. 186) the two portions have grown in length. The dorsal or proximal outgrowth, developing between the layers of the mesoduodenum, is at this time the larger of the two, composed of a number of glandular vesicles clustered around the stalk represented by the parent duct.

The distal or ventral pancreatic growth, connected with the liver duct, is as yet small and presents only a few vesicular appendages. The duct of this portion empties in common with the hepatic duct into the duodenum.

In embryos of the sixth to seventh week (Fig. 187), the two glandular outgrowths have become connected with each other at a point which corresponds exactly to the divergence of the duct of Santorini from the main pancreatic duct (canal of Wirsung) in the adult gland (Fig. 188).

The secondary pancreatic duct (of Santorini) of the adult corresponds to that section of the proximal or larger embryonic outgrowth situated between the intestine and the point where the two glandular diverticula fuse with each other. Hence the canal of Wirsung in the adult is a compound product. It includes the duct system developed, in connection with the bile duct, in the head of the gland, forming the intestinal termination of the main duct. Its distal body portion on the other hand is derived from the duct system of the originally larger proximal outgrowth, including the entire peripheral portion which has become secondarily added to the duct of the ventral outgrowth to form together with it the canal of Wirsung. On the other hand the proximal portion of the duct system of this originally larger part becomes secondarily differentiated as the duct of Santorini.

Fig. 188 shows the normal adult arrangement of the pancreatic and biliary ducts in a corrosion preparation of the canal.

The duct of Santorini in this case opened by a separate orifice into the duodenum above the common opening of the biliary and pancreatic ducts (cf. p. 113).

=Explanation of Adult Arrangement of Human Pancreatic Ducts and Their Variations Dependent Upon the Embryonic Development.=--The smaller distal embryonic outgrowth is, as we have seen, from its inception in close connection with the duodenal end of the common bile-duct (Fig. 185).

The proximal outgrowth, situated nearer to pylorus and derived directly from the duodenal epithelium, is the larger and forms the greater part of the bulk of the adult pancreas (Figs. 186, 187).

If, notwithstanding this primitive arrangement, the distal duct (canal of Wirsung) appears as the main pancreatic duct in the adult, while the proximal (duct of Santorini) is secondary, this depends upon a union of the products of the two outgrowths in such a manner that the greater part of the duct system of the proximal and larger portion is transferred to the distal duct to form the adult canal of Wirsung, while the smaller segment of the proximal duct, between its opening into the duodenum and the point of fusion of the two outgrowths, forms the adult secondary duct of Santorini. This duct opens usually into the duodenum upon a small papilla situated about 2.5 cm. above the common duodenal termination of the bile-duct and canal of Wirsung (papilla Vateri) (Fig. 193). The duct of Santorini usually tapers toward the duodenal opening from its point of departure from the main duct, its caliber gradually diminishing in the direction indicated, so that it is smaller at the duodenal opening than at the point of confluence with the main duct (Fig. 189). Hence the secretion from the proximal head portion of the pancreas, conveyed by this duct and its tributaries, passes usually into the main pancreatic duct and not directly into the intestine through the duodenal opening of the duct of Santorini. The latter is, however, thus enabled to vicariously take upon itself the conduct of the pancreatic secretion in cases of obstruction or obliteration of the main duct (calculi, ulcers, cicatrices, etc.). In these cases of obstruction of the main duct the duct of Santorini enlarges and performs its functions.

Occasionally, without obstruction of the main duct, the duodenal opening of the duct of Santorini is large, and the flow of secretion evidently the reverse of the usual, _i. e._, directly into the intestine.

In other cases, also without pathological conditions, the proximal duct is the larger of the two and serves as the principal channel of pancreatic secretion, the canal of Wirsung being small. This is evidently a persistence and further development of the early embryonic relative condition of the two outgrowths above described (Fig. 190). On the other hand the duct of Santorini may not open at all into the duodenum, terminating in small branches which drain the proximal part of the head of the gland (Fig. 191).

Schirmer has examined the arrangement of the pancreatic ducts in 105 specimens. In 56 of these the duct of Santorini passed from the main duct into the duodenum, opening upon a papilla situated 2.5 cm. above the common opening of the bile duct and canal of Wirsung.

In 19 the duct of Santorini was well developed but did not open into the duodenum.

In but 4 cases the duct of Santorini formed the only pancreatic duct, the lower opening being occupied by the bile duct alone (Fig. 192). We may assume in these cases failure of development of the distal outgrowth connected with the primitive hepatic bud, leaving only the proximal duodenal outgrowth to form the entire adult gland.

Figs. 188 and 189 show the normal arrangement of the duodenal openings of the biliary and pancreatic ducts.

Figs. 190 to 192 show schematically the variations in the relative development and the adult arrangement of the pancreatic ducts.

=Diverticulum and Papilla Vateri.=--From what has been said regarding the embryonic union of the distal pancreatic outgrowth with the hepatic bud it will be easy to recognize the corresponding features in the arrangement of the adult duodenal termination of the common bile-duct and canal of Wirsung. The dilated interior of the duodenal papilla (diverticulum Vateri) corresponds to the embryonic segment between the intestinal opening of the primitive liver duct and the point when this duct gives off the distal larger pancreatic outbud (Figs. 186, 187, 188, 193 and 194).

The union of the pancreatic and biliary ducts to form the recess of the diverticulum Vateri, which then opens by a single common orifice into the duodenum, is better marked in some of the lower vertebrates than in man.

Fig. 195 shows the proximal portion of the duodenum of the cassowary (_Casuarius casuarius_) with the biliary and pancreatic ducts and the diverticulum at their confluence in section.

The development of these two main digestive glands as diverticula from the intestinal canal also explains the direct continuity of the mucous membrane of their ducts with that lining the duodenum, a fact which is of considerable importance in the pathological extension of mucous inflammations from the intestine to the duct system of the glands.

=Development of the Pancreas in Lower Vertebrates.=--In the embryo of the _sheep_ two pancreatic buds are found, but the duct of the dorsal (proximal) outgrowth (duct of Santorini) subsequently fuses entirely with the main duct.

In the _cat_ there are likewise two pancreatic outgrowths.

In the _chick_ three pancreatic buds are visible about the fourth day.

_Amphibia_ likewise present three embryonic pancreas buds.

The ventral (distal) outgrowth is double, the two portions proceeding symmetrically from each side of the hepatic duct. The single dorsal outgrowth is derived directly from the duodenal epithelium. Later on all these outgrowths fuse to form the single adult gland.

_Fish_ also possess several (up to four) embryonic pancreatic outgrowths.

Recently in human embryos of 4.9 mm. cervico-coccygeal measure three pancreatic outgrowths have been observed, all entirely distinct from each other, one dorsal, budding from the epithelium of the primitive duodenum and two ventral, proceeding from the grooved gutter which represents the primitive ductus choledochus at this period. In embryos of from 6 to 10 mm. the two ventral outgrowths have already fused, hence only two buds, a single ventral and a dorsal, are now encountered.[4]

[4] Iankelowitz, Arch. f. Mikr. Anat., Bd. 46, 1895.

These observations place the development of the human pancreas in line with the triple pancreatic outgrowths, two ventral and one dorsal characteristic of the majority of the lower vertebrates, which have been hitherto carefully examined. The ventral or distal bud is probably double in the majority of vertebrates. The two segments fuse, however, so early that the derivation of the pancreas from a double outgrowth, as described above for the human embryo, practically obtains. In forms in which the adult gland presents a number of separate openings into the duodenum (cf. p. 118), the development would probably show multiple embryonic outgrowths from the intestinal hypoblast.

In any case the dorsal pancreatic bud appears to have developed in the vertebrate series before the ventral outgrowth and to be hence phylogenetically the older structure.

COMPARATIVE ANATOMY OF THE PANCREAS.

With the exception of _Amphioxus_ and probably also of the _Cyclostomata_, the gland appears to be present in all vertebrates, varying, however, much in size, shape and relation to the intestinal tube. Usually it appears as an elongated, flattened, more or less distinctly lobulated organ, in close apposition to the duodenum between the layers of the mesoduodenum. In all forms in which the gland is found it is connected with the post-gastric intestine and marks the beginning of the midgut. In structure the gland is usually acinous, resembling the salivary glands. It is well developed in the selachians, forming a triangular body connected with the beginning of the midgut (Fig. 202). In some instances the gland elements do not extend beyond the intestine itself, but remain imbedded in the wall of the midgut, as in _Protopterus_. In certain adult teleosts the pancreas is surrounded by the liver (Fig. 196), in others it does not appear as a compact gland but is distributed in the form of finely scattered lobules throughout the mesentery between the two layers of this membrane. On account of this concealed position of the gland it was formerly believed that the adult teleosts did not possess a pancreas. The pyloric caeca (cf. p. 119) found in these forms were consequently considered to be homologous with the pancreas of the higher vertebrates.

In _Myxinoids_ a peculiar lobulated glandular organ is found imbedded in the peritoneal coat of the intestine near the entrance of the bile-duct, into which its lobules open separately. This organ possibly corresponds to the higher vertebrate pancreas.

An organ which may represent a dorsal pancreas is also developed in _Ammocoetes_ (larva of _Petromyzon_), but its exact homology is still doubtful. It is possible that a true pancreas has not yet developed in the cyclostomata. In _Amphioxus_ no trace of a pancreas is found. In all other vertebrates the gland is present. In certain amphibians, as the frog, the single pancreatic duct opens into the common bile duct (Fig. 197).

In lacertilians and in some chelonians a lateral offshoot of the pancreas is directed transversely and is adherent to the spleen. Fig. 113 shows the gland in _Chelydra serpentaria_. While the gland usually has a single duct, yet two ducts are found in a number of animals (many mammals, birds, chelonians and crocodiles). At times three ducts are encountered, as in the chicken and pigeon.

The arrangement of the pancreatic duct system among mammalia presents the following variations:

1. Mammals with _one_ pancreatic duct, either connected with the bile-duct or entering the intestine independently:

Monkeys, most rodents (except the beaver), marsupials, carnivora (except dog and hyena), many ungulates (pig, peccary, hyrax, etc.), most ruminating artiodactyla.

(_a_) The pancreatic duct joins the common bile-duct before entering the duodenum in the monkeys, marsupials, carnivora, in the sheep, goat and camel.

The point of entrance of the combined duct into the intestine varies. In some forms it is near the pylorus, in others at some distance from the same. The common opening is situated 11/2" to 2" beyond the pylorus in carnivora, and one foot behind the same point in the goat and sheep.

(_b_) The pancreatic duct does not join the bile-duct, but empties separately into the intestine, in most rodents and in the calf and pig.

In the calf the pancreatic duct opens into the duodenum 15' beyond the bile-duct and 3' beyond the pylorus.

In the pig the pancreatic opening is 5"-7" beyond that of the bile-duct and 6"-8" behind the pylorus.

2. Mammals with _two_ pancreatic ducts, of which one usually joins the bile-duct: perissodactyla (except the ass according to Meckel), elephant, beaver, several carnivora, dog, hyena, and according to Bernard the cat. In the perissodactyla the proximal of the two pancreatic ducts empties, either combined with the bile-duct, or separate from it, but very close to it, 3"-4" behind the pylorus. The second distal duct is smaller and opens several inches further down.

In most rodents the pancreatic entrance is placed at some distance from the pylorus. Fig. 199 shows the arrangement of the parts in the rabbit, in which animal the main distal pancreatic duct empties at a distance of 13"-14" from the pylorus into the end of the duodenum, which intestine forms a very long loop, while the biliary duct, receiving the smaller proximal pancreatic duct, opens near the pylorus.

In the _beaver_ the smaller proximal duct joins the bile-duct or even enters the duodenum anterior to the bile-duct, nearer the pylorus, while the distal larger pancreatic duct opens into the intestine 16"-18" behind the biliary duct. Of the two ducts found in the dog (Fig. 200) the smaller proximal either joins the bile-duct or opens into the intestine close to it, 1" to 11/2" beyond the pylorus. The larger distal duct opens into the duodenum 1" to 11/2" behind the biliary duct. Fig. 201 shows the dog's stomach and proximal portion of the duodenum in section. The proximal smaller pancreatic duct here joins the biliary duct, and opens with it by a single orifice into the duodenum. The distal larger pancreatic duct opens independently into the intestine further caudad.

The parts in _Hyaena_ present a similar arrangement.

Bernard always found _two_ pancreatic ducts in the _cat_, one large principal duct and a second smaller accessory duct. Of these, the one situated nearest to the pylorus always united with the bile-duct. The pancreatic duct thus joining the bile-duct was sometimes the main duct, sometimes the accessory smaller duct.

Since the main function of the pancreatic juice is the conversion of starch into sugar, the gland appears better developed in general in herbivora than in carnivora, without, however, disappearing in the latter. In fact it is of considerable size in the carnivora, because the secretion also acts on the albuminous food substances and, though to a lesser degree, on the fats.

PYLORIC CAECA OR APPENDICES.

In the _Cyclostomata_ and _Selachians_ the intestinal canal is in the main free from caecal appendages, while a large portion of the tube is provided with a special fold of the mucous membrane which projects into the lumen of the gut (spiral valve). Fig. 43 shows the straight intestinal tract with the spiral valve of the longer distal segment in a cyclostome, _Petromyzon marinus_ or lamprey. In Figs. 202 and 203 the selachian (shark) intestine is represented in two examples, while the similar spiral valve in a Dipnoean or lung fish, _Ceratodus_, is seen in Fig. 204.

On the other hand in the Ganoids and in many Teleosts longer or shorter finger-shaped diverticula of the midgut are found immediately beyond the pylorus in the region of the bile-duct.

These pouches or diverticula of the intestine form the so-called pyloric caeca or appendices of these fish. They vary very much in length, diameter and number in different forms.

Thus but a single diverticulum appears in _Polypterus_ and _Ammodytes_ (Fig. 205). _Rhombus maximus_ and _Echelus conger_ (Figs. 112 and 206) have two, and the same number appear in _Lophius piscatorius_ (Fig. 207). Perca has three and the _Pleuronectidae_ have three to five.

Fig. 208 shows the stomach and the beginning of the midgut with four pyloric caeca in _Pleuronectes maculatus_, and Fig. 209 the same parts of this animal in section.

Fig. 210 shows the stomach and midgut of _Paralichthys dentatus_, the summer flounder, with three well-developed conical pyloric caeca. On the other hand in some forms the number of pyloric appendices is enormously increased, while their caliber diminishes. Thus 191 caecal appendages are found surrounding the beginning of the midgut in _Scomber scomber_. A well-marked example of prolific development of the pyloric appendages is furnished by the common cod, _Gadus callarias_ (Fig. 211). The appendices are in the natural condition bound together by connective tissue and blood vessels, so as to form a compact organ, resembling a gland (Fig. 211, A), and a similar arrangement is found in _Thynnus vulgaris_ and _alalonga_, _Pelamys_ and _Accipenser_ (Fig. 212).

_B._ Dissected to show confluence of caeca to form a smaller number of terminal tubes of larger calibre entering the intestine.]

In the smaller upper figure on the left the stomach, mid-gut, and pyloric caeca are seen in section, showing the lumen of the latter and their openings into the mid-gut.

The lower left-hand figure shows the mid- and end-gut in section, the latter provided with a spiral mucous valve.]

In some Teleosts (Siluroidea, Labroidea, Cyprinodontia, Plectognathi and Leptobranchiates) the appendices are entirely wanting. If there are not more than 8-10 appendices they usually surround the gut and empty into the same in a circle. In other cases they are arranged in a single line, or in a double row, opposite to each other (Fig. 213). Each appendix may open into the intestine independently, this especially where the number is limited and the individual pouches large (cf. Figs. 206-210), or several may unite to form a common duct.

Fig. 211, _B_, shows the appendices in _Gadus callarias_, the cod, freed by dissection from the investing connective and vascular tissue. It will be noticed that a considerable number of the tubes unite to form ducts of larger caliber which open into the intestine, as seen in the section shown in Fig. 214.

The pyloric appendices apparently have the same _significance_ as the spiral intestinal fold of the Selachians, Cyclostomes and Dipnoeans, _i. e._, the production of an increase in the area of the digestive and absorbing surfaces of the intestinal mucous membrane. Hence, as stated, the appendices and the spiral fold are found to vary in inverse ratio to each other. Thus, for example, _Polypterus_ (Fig. 205) still has a fairly well developed spiral fold and only a single pyloric appendix, while _Lepidosteus_, with but slightly developed spiral fold, has numerous appendices. It was formerly held that the pyloric caeca and the pancreas were mutually incompatible structures, and that where one is found the other will be wanting.

Hence the appendices were regarded as homologous with the pancreas of the higher forms. Recent observations have shown that this view is not strictly and entirely correct, while at the same time it merits consideration in several respects.

It is true that the pancreas in certain teleosts is now known to be present although concealed from observation in the liver or scattered in the form of small lobules between the layers of the mesentery (cf. p. 117), and that in a number of fish, such as _Salmo salar_, _Clupea harengus_, _Accipenser sturio_, both the appendices and the pancreas are encountered. Consequently these structures are not identical or even completely homologous, since they occur side by side in the same form.

On the other hand Krukenberg has demonstrated that the appendices pyloricae may function physiologically as a pancreas by yielding a secretion which corresponds to the pancreatic juice in its digestive action. In the majority of forms, however, they apparently merely increase the intestinal absorbing surface, secreting only mucus.

These structures are nevertheless very interesting and instructive since they furnish a perfect gross morphological illustration of the embryonal stages just considered in connection with the development of the mammalian pancreas. In the adult ganoid or teleost these blind diverticula or pouches, varying greatly in shape, number and size, protrude from the intestine immediately beyond the pylorus, usually in close connection with the duodenal entrance of the bile-duct. Two or more of these pouches may unite to form a common duct or canal opening into the intestine.

These forms, therefore, offer direct and valuable morphological illustration of the manner in which the pancreas of the higher vertebrates develops, _i. e._, as a set of hollow outgrowths or diverticula from the hypoblast of the primitive enteric tube. We can establish a consecutive series, beginning with forms in which only one or two diverticula are found, and extending to types in which the number of the little cylindrical pouches reaches nearly two hundred and in which they are bound together by connective tissue and blood vessels so as to closely resemble the structure of a glandular pancreas. This is one of the most striking instances in which the minute embryological stages of the higher types are directly illustrated by the permanent adult conditions found in the lower vertebrates. [The same statement, as we will see, holds good in reference to the development of the _liver_.]

RELATION OF THE PANCREAS TO THE PERITONEUM.

The gland becomes very intimately connected with the serous layers of the primitive dorsal mesentery. In order to clearly comprehend the adult serous relations it is necessary to make a distinction between two divisions or portions of the gland, based upon the altered relations of the primitive dorsal mesentery which result from the differentiation of the primitive simple intestinal tube into stomach and duodenum.

1. The primary outgrowth of the pancreatic tubules from the duodenum, _i. e._, the part which is to form the "head" of the adult gland, is situated between the two layers of that division of the primitive dorsal mesentery which forms, after differentiation of stomach and small intestine, the _mesoduodenum_. Coincident with the rotation of the stomach, as we have seen, the duodenum and mesoduodenum exchange their original sagittal position in the median plane of the body for one to the right of the median line, balancing, so to speak, the extension of the stomach to the left (Fig. 218).

The original right layer of the mesoduodenum and the right surface of the duodenum now look dorsad and rest in contact with the parietal peritoneum investing the right abdominal background and the ventral surface of the right kidney and inferior vena cava. We have already seen that the descending portion of the duodenum in man becomes anchored in this position by adhesion of these apposed peritoneal surfaces. This fixation includes, of course, the structures situated between the layers of the mesoduodenum, _i. e._, the head of the pancreas. Consequently, after rotation and adhesion, this portion of the gland turns one surface ventrad, invested by secondary parietal peritoneum, originally the left leaf of the free mesoduodenum, while the original right surface of the gland has become the dorsal and has lost its mesoduodenal investment by adhesion to the primary parietal peritoneum.

2. In order to understand the way in which the body and tail of the pancreas obtain their final peritoneal relations it is necessary to consider the development of the dorsal mesogastrium to form the omental bag. If we regard the primitive dorsal mesentery in the profile view from the left side (Fig. 215) it will be seen that, as already stated, the mesoduodenum is the first part of the membrane to be invaded by the pancreatic outgrowth from the intestine. Cephalad of the mesoduodenum the primitive dorsal mesogastrium (Fig. 215) is seen to protrude to the left and caudad to form, as already explained, the cavity of the omental bursa of the retrogastric space ("lesser peritoneal sac"). The further growth of the pancreas carries the developing gland from the district of the mesoduodenum into that portion of the dorsal mesogastrium which now forms the dorsal wall of the omental bursa (Fig. 216).

This double relation of the pancreas to the mesoduodenum and to the mesogastrium forming the omental bursa is well seen in foetal pigs between two and three inches in length (Fig. 217).

The head portion of the pancreas is seen developing between the layers of the mesoduodenum, while the body and tail of the gland, extending to the left, grows between the two dorsal layers of the omentum bursa towards the spleen, which organ is found connected with the left and dorsal extremity of the omental sac derived from the dorsal mesogastrium.

Before the growth of the great omentum is pronounced the continuity of the mesoduodenum and dorsal mesogastrium can be readily appreciated (Fig. 218). But after the redundant growth of the membrane has carried the great omentum further caudad, the stomach and the two omental layers attached to the greater curvature lie in front of the structures included between the two dorsal layers and conceal them from view (Fig. 177).

In sagittal sections to the left of the median line (Figs. 221 and 222) the pancreas now appears included between the layers of the great omentum near their point of departure from the vertebral column. (This point is of course identical with the prevertebral attachment of the primitive dorsal mesogastrium from which the omentum is developed.)

The foregoing considerations will, therefore, lead to the conclusion that the pancreas presents, in regard to its peritoneal relations, two distinct segments:

1. The portion adjacent to duodenum (head and neck of the gland) is developed between the layers of the mesoduodenum.

2. The distal portion of the gland, comprising the body and tail, develops between the layers of the great omentum (dorsal segment), derived from the primitive dorsal mesogastrium.

The transections of the dorsal mesogastrium shown in Figs. 180 and 181 will now have to be amplified by the introduction of the body of the pancreas between the two layers of the vertebro-splenic segment, in addition to the splenic artery (Figs. 219 and 220).

Hence the following facts will be understood:

1. In the adult the splenic artery supplies a series of small branches to the pancreas as it courses along the cephalic border of the gland on its way to the spleen.

2. After the above-described adhesion of the original left leaf of the dorsal mesogastrium (vertebro-splenic segment) to the parietal peritoneum (Fig. 220), the dorsal surface of the body of the pancreas loses its peritoneal investment and becomes attached by connective tissue to the ventral surface of the left kidney.

3. The ventral surface of the body of the pancreas is in the adult lined by peritoneum of the "lesser sac"; in other words the organ has practically assumed a "retro-peritoneal" position, its ventral peritoneal covering appearing now as the dorsal parietal peritoneum of the retro-gastric space.

4. When completely developed the extreme end (tail) of the pancreas extends to the left, following the splenic artery, until it touches the mesal aspect of the spleen at the hilus.

5. If we, therefore, leave out of consideration for the moment the transverse colon and duodenum, which will be taken up presently, and confine ourselves to the arrangement of the stomach, pancreas and great omentum, a sagittal section to the left of the median line would result as shown in Fig. 222, after the adult condition of adhesion has been established.