Читать книгу The Anatomy of the Human Peritoneum and Abdominal Cavity - George S. Huntington - Страница 11

Table of Contents

Continuing our consideration of the development of the alimentary canal we find that changes from the simple primitive straight tube below the stomach depend upon two factors:

1. The increase in the length of the intestinal tube, which exceeds relatively the increase in the length of the body cavity in which it is contained.

2. The differentiation into small and large intestine, the development of the cæcum and ileo-cæcal junction, and the development of the accessory digestive glands, liver and pancreas, by budding from the proximal portion of the primitive entodermal intestinal tube.

Fig. 97.—Alimentary canal of human embryo of 5 mm. × 15. (Reconstruction after His.)

Fig. 98.—Schema of human embryonic intestinal canal, with intestinal umbilical loop, but before differentiation of the large and small intestine.

Fig. 99.—Viscera of Necturus maculatus, mud-puppy, in situ. (Columbia University Museum, No. 1175.)

1. In embryos up to 5 mm. cervico-coccygeal measure (Fig. 97) the intestinal tube follows the body curve without deviation. Subsequently the elongation of the intestine causes a small bend, with the convexity directed ventrad, to appear in the umbilical region. This bend gradually increases until the gut forms a single long loop, beginning a short distance below the pylorus and directed ventro-caudad. The apex of the loop, to which the vitello-intestinal duct is attached (Fig. 98) (cf. p. 34) projects beyond the abdominal cavity into the hollow of the umbilical cord, constituting the so-called “umbilical or embryonal intestinal hernia.” This entrance of the apex of the intestinal umbilical loop into the umbilical cord begins in embryos of about 10 mm. During the succeeding weeks—up to the tenth—the segment of the intestine thus lodged within the hollow of the umbilical cord increases. After this period the intestinal coils are gradually withdrawn within the abdomen. The explanation of this temporary extrusion of the intestine into the umbilical cord is probably to be found in the strain produced by the yolk-sac which is attached by the vitello-intestinal duct to the apex of the umbilical loop. As we have seen (p. 35) the site of the original apex of the loop may still be indicated in the adult by the persistence of a portion of the vitello-intestinal duct as a “Meckel’s diverticulum.”

In its simplest primitive condition the loop presents a proximal, descending or efferent limb, an apex, and an ascending, returning or afferent limb (Fig. 98). In the human embryo these segments of the loop furnish the jejuno-ileum and portions of the large intestine, in a manner to be subsequently detailed.

This stage in the development of the higher vertebrate intestine is well illustrated by the alimentary tract of the mud-puppy, Necturus maculatus, shown in Fig. 99, which represents the entire situs viscerum of an adult female animal.

The stomach is tubular, not distinctly differentiated from the œsophagus, placed vertically in the long axis of the body. The pyloric end is marked by a constriction separating stomach from midgut and immediately beyond this point the pancreas is applied to the intestine. The rest of the intestinal canal forms a simple loop, the descending limb presenting one or two primitive convolutions. There is no marked differentiation between large and small intestine, the canal possessing a nearly uniform caliber from pylorus to cloaca.

2. The differentiation of the small from the large intestine, marked by the appearance of the cæcal bud or protrusion (Fig. 100), takes place in the ascending segment of the umbilical loop a short distance from the apex. In the human embryo the cæcal bud appears in the 6th week as a plainly marked protuberance, which grows very slowly in length and circumference. It shows very early an unequal rate of development; the terminal piece, not keeping pace in growth with the proximal portion, is converted into the vermiform appendix, while the proximal segment develops into the cæcum proper. The increase in the length of the loop, which begins to be marked in the 7th week, is not uniform. The apex is the first portion to present the evidences of this growth. Subsequently the descending limb grows in length very rapidly and is early thrown into numerous coils of the future mobile portion of the small intestine (jejuno-ileum). Even before the withdrawal of the apex of the loop within the abdominal cavity a prominent coil of these convolutions is found protruding in the umbilical region (Fig. 544). The ascending limb of the loop from which a portion of the large intestine is developed, grows comparatively slowly at this time.

Fig. 100.—Schema of human embryonic intestinal canal after differentiation of the large and small intestine.

Fig. 101.—Human embryo of 2.15 mm., twelve days old. Seessel’s sac is the cephalic blind termination of the embryonic foregut before the communication with the ectodermal invagination of the stomadæum has been formed. (Reconstruction after His.)

Fig. 102.—Representation of alimentary canal and appendages of human embryo of 4.1 mm.; isolated. × 15. (Kollmann, after His.)

Fig. 103.—Alimentary canal and appendages of human embryo of 12.5 mm. × 12. (Kollmann, after His.)

Fig. 104.—A. Schematic representation of alimentary canal, with umbilical loop and mesenteric attachments in human embryo of about six weeks. B and C, stages in the intestinal rotation.

The future portions of the human adult alimentary tract below the stomach may be referred, in reference to their derivation, to this primitive condition of the tube as follows:

1. The segment of small intestine situated between the pylorus and the beginning or point of departure of the proximal or descending limb of the umbilical loop, develops into the duodenum. This portion of the small intestine is indicated early in embryos of 2.15 mm. (Fig. 101), by the origin of the hepatic duct from the intestinal tube. Somewhat later, in embryos of 4.10-5 mm. length, (Fig. 102) it becomes additionally marked by the origin of the pancreatic diverticulum. The duodenum, at first straight, now begins to curve, forming a short duodenal loop or bend. In embryos of 6 weeks the duodenum forms a simple loop placed transversely below the pyloric extremity of the stomach (Figs. 103 and 104).

2. The descending limb, the apex and a small part of the ascending limb of the umbilical loop form the jejuno-ileum.

3. The remainder of the ascending limb forms the cæcum and appendix, the ascending and transverse colon.

4. The distal straight portion of the primitive tube forms the terminal portion of the transverse colon (the splenic flexure), the descending colon, sigmoid flexure and rectum.

The primitive condition of the embryonal mammalian alimentary tract, after differentiation of the large intestine is well illustrated by some of the lower vertebrates in which development never proceeds beyond this stage. Fig. 112 shows the entire alimentary canal of a teleost fish, the conger eel (Echelus conger) isolated.

The preparation forms a good illustration of the embryonal stage of the higher vertebrates in which development has not proceeded beyond the formation of the simple umbilical loop, about corresponding to the schematic Fig. 98. The stomach is differentiated both by its caliber and by the formation of a pyloric ring valve.

The midgut forms a simple loop with a descending and ascending limb closely bound together by mesenteric attachment. Different from the course of development followed in the human embryo is the situation of the ileo-colic junction. The same appears in the terminal straight segment of the canal—corresponding to the human descending colon—while in the human embryo the differentiation of small and large intestine takes place in the course of the ascending limb of the loop. This condition depends upon the relatively much shorter extent of the teleost endgut compared with the human large intestine. Other examples are afforded by the alimentary tract of some of the Amphibia and Reptilia. Fig. 105 shows the alimentary canal of Rana catesbiana, the common bull frog. The stomach, fairly well differentiated, is succeeded by the small intestine of considerable length and uniform caliber. The proximal portion of the small intestine is characterized as duodenum by its connection with liver and pancreas. In the remaining portion of the intestinal canal it is not difficult to recognize the elements of the umbilical loop of the higher mammalian embryo. The larger mass of the jejuno-ileal coils is developed from the descending limb of the loop; a smaller number of convolutions belong to the returning or ascending limb, which also includes the ileo-colic junction. The very short large intestine of the frog passes straight down to enter the cloaca. Another example, in which the early embryonal stages of the higher mammalia are illustrated by the permanent structure of one of the lower vertebrates, is given in Fig. 106, which shows the alimentary tract of a chelonian, Pseudemys elegans, the pond turtle. The bilobed liver fits over the well-differentiated stomach in the manner of a saddle. The stomach itself, as in chelonians generally, has a markedly transverse position and passes under cover of the right lobe of the liver into the duodenum. The coils of small intestine form a prominent mass, which, however, when unravelled as shown in the figure, permits us to recognize its identity with the mammalian embryonic umbilical loop. The well-marked ileo-colic junction is situated at the termination of the returning limb of the loop, close to the beginning of the descending limb. This close approximation of the duodenum and colon (duodeno-colic isthmus) forms one of the most important factors in the further development of the mammalian intestinal canal and will again be referred to below.

Fig. 105.—Rana catesbiana, bull-frog. Alimentary canal and appendages. (Columbia University Museum, No. 1454.)

Fig. 106.—Pseudemys elegans, pond turtle. Alimentary canal. (Columbia University Museum, No. 1437.)

Fig. 107.—Abdominal viscera of Tamandua bivittata, the little ant-eater, seen from the left, with the intestines turned to the right. (From a fresh dissection.)

Fig. 108.—The same view, from another specimen. Figures 107 and 108 should be studied and compared together, as each supplements the other.

Fig. 109.—Abdominal viscera of Tamandua bivittata, the little ant-eater, seen from the right, with the intestines turned to the left. (From a fresh dissection.)

Fig. 110.—The same view, from another specimen.

Fig. 111.—Schematic representation of the development of the mesentery of the umbilical loop.

Fig. 112.—Alimentary canal, isolated and in section, of Echelus conger, the conger eel. (Columbia University Museum, No. 1812.)

Fig. 113.—Chelydra serpentina, snapping turtle; intestinal canal, pancreas, and spleen, isolated. (Columbia University Museum, No. 1369)

From the ileo-colic junction the large intestine of the turtle continues caudad to the cloaca in a nearly straight line. The same primitive condition of the intestinal canal may be observed in some members of man’s own class, the mammalia—as in certain edentates. Figs. 107 and 108 show the entire abdominal portion of the alimentary tract in Tamandua bivittata, the little ant-eater of Brazil. The stomach is turned cephalad and the great omentum elevated. The intestines are turned over to the right side.

It will be observed that in spite of the numerous coils of the small intestine the general arrangement of the alimentary canal corresponds to the primitive scheme shown in Fig. 98. The entire intestinal canal is attached by a continuous vertical mesentery to the dorsal median line of the abdominal cavity ventrad of the vertebral column and aorta. The growth in length of the small intestine has necessitated a corresponding lengthening of the attached border of the mesentery—consequently the membrane presents a pleated or crenated appearance. The cæcum is well developed, the ileo-cæcal junction being situated within the returning limb of the loop, a little distance from the apex.

In Figs. 109 and 110, taken from the same specimens, the entire mass of the small intestines has been turned to the left so as to exhibit the right leaf of the common dorsal mesentery and the mesoduodenum, the latter containing the head of the pancreas. It will be noted that the mesentery, expanding beyond the duodeno-colic isthmus, is common to the small and to the proximal portion of the large intestine, i. e., to those segments of the alimentary canal which are developed from the two limbs of the umbilical loop. Figs. 107-110 should be studied and compared together, as each supplements the others.

It will be observed, in reference to the change from the primitive loop to the subsequent increase in the length of the tube and the resulting arrangement of the mesentery, that three successive stages are to be considered, represented schematically in Fig. 111. In the earliest stage (Fig. 111, I.) the two segments of the loop are of equal length, parallel to one another, the distance between the beginning and termination of the loop (1-2) being maintained throughout its extent. Hence the mesentery is of equal width in all its parts within the loop, only drawn out, i. e., away from the vertebral column, in accordance with the length of the loop. In the next stage (Fig. 111, II.) the increase in the length of the intestine is accompanied by a corresponding widening of the mesentery. The points 1 and 2 are still approximately the same distance apart as in the earlier stage, but the increase in the length of the tube between these points forces the two limbs of the loop to abandon their early parallel course, and to form curved lines with the concavity turned toward the mesenteric attachment. In this condition the mesentery consequently forms a widely expanded membrane framed by the intestine and narrowing between the points 1 and 2 to a neck or isthmus which effects the transition between the expanded segment surrounded by the intestine and the rest of the dorsal primitive mesentery. Finally in the stage represented in Fig. 111, III., the increase in the length of the small intestine has reached a point where a single curve is no longer sufficient for the accommodation of the growth. Consequently the tube now appears coiled and convoluted, and the mesentery, as it is attached to the gut, of necessity follows all the twists and appears fluted or pleated in its distal attached portion.

If we now carefully examine the conditions presented by the intestine and mesentery in a form like Tamandua (Figs. 107 and 108) we will find that they correspond to the developmental facts thus far considered. The termination of the duodenum (1) and the bend in the colon (2) mark the two points at which in the primitive schema (Fig. 111, I.) the umbilical loop begins and terminates. The proximal of these two points (1) corresponds to the termination of the duodenum, which segment extends from here cephalad to the pyloric extremity of the stomach. The distal point (2) is placed on the colon where the returning limb of the loop resumes the original median vertical course of the large intestine. These two points mark the neck of the loop, which we can describe as the duodeno-colic neck or isthmus.

The same condition is well shown in the intestinal canal of the snapping turtle (Fig. 113). The duodenum and colon approach each other very closely at the isthmus and between these points the convolutions of the intestine extend in a wide circle. We will find this approximation of duodenum and colon a feature which persists throughout all the later developmental stages of the higher vertebrates and has an important bearing on the final arrangement of the intestinal canal in the human adult.

Further Changes in the Development of the Human Alimentary Canal. Rotation of the Intestine. Formation of the Segments of the Colon. Final Permanent Relations of the Segments of the Intestinal Tube.—The next important stage leading up to the final adult disposition of the intestine in man and the higher mammals is the rotation of the portions developed from the two limbs of the primitive loop around an oblique axis drawn from the duodeno-colic isthmus to the apex of the loop. The portion of the large intestine, developed from the ascending limb of the loop, moves in the third month to the middle line, coming into contact with the ventral abdominal wall. From here the large intestine passes, ventrad of the jejuno-ileal coils, toward the cephalic end of the abdominal cavity and lies transversely along the greater curvature of the stomach. The growing coils of the small intestine crowd the colon more and more cephalad. In the fourth month the cæcum turns to the right, coming into contact with the caudal surface of the liver, ventrad of the duodenum, and subsequently reaches the ventral surface of the right kidney. As the result of this rotation the ileo-colic junction, cæcum and succeeding portion of the colon are carried from the original position in the distal and left part of the abdomen cephalad and to the right across the proximal (duodenal) portion of the small intestine, while the coils of the jejuno-ileum, developed from the descending limb and apex of the loop, are turned in the opposite direction, caudad and to the left underneath the preceding (Figs. 114 and 115). This change in the relative position of the parts of the intestinal tract and the resulting altered bearing of the colon to the duodenum will be best appreciated by considering in the first place the effect of the change on the arrangement of the primitive mesentery and the intestinal vessels, and secondly by repeating actually the rotation in the intestinal tract of a mammal (cat) in which the adult arrangement of the intestine and peritoneum permits us to perform the manipulations and note the result.

Fig. 114A.—Intestinal canal in stage of umbilical loop—before rotation.	Fig. 114B.—First stage in rotation, colon crossing duodenum.
Fig. 115A.—Second stage in rotation—rotation of small intestine.	Fig. 115B.—Schema of intestinal canal after complete rotation and descent of cæcum.

I. Effect of Rotation on the Disposition of the Primitive Mesentery and on the Relative Position of Duodenum and Colon, and Consequent Arrangement of the Intestinal Blood Vessels.—It will be appreciated that in Fig. 111, representing a profile view of the original arrangement, or in Figs. 107 and 108, showing the intestinal canal of Tamandua, the left layer of the primitive mesentery is turned toward the observer. The membrane is seen to pass from the ventral aspect of the vertebral column and aorta, through the narrow neck of the duodeno-colic isthmus, to expand in the manner already indicated toward its intestinal attachment. In the rotation of the intestine the twist takes place at the duodeno-colic neck, carrying, as already stated, the large intestine cephalad and to the right, while the jejuno-ileum is turned in the opposite direction caudad and to the left. During this rotation the duodeno-jejunal angle (Figs. 114, B and 115, A) passes to the left underneath the proximal segment of the colon, which now lies ventrad and to the right of the duodenal portion of the small intestine. The mesenteric peritoneum, occupying the bight of the umbilical loop, will, after the rotation, in the left profile view shown in Fig. 104, A and B, turn its original right leaf toward the beholder, i. e., toward the left, while the original left leaf is turned toward the right.

Observation of the difference in the position of the ileo-colic junction will still further accentuate the change in the relative position of the parts which has been effected by the rotation. In the primitive condition shown in Fig. 104, A, the ileum enters the large intestine from right to left, and the concavity of the cæcal bud turns its crescentic margin ventrad and to the right.

After rotation is accomplished (Fig. 104, B and C, and Fig. 115) the ileo-colic entrance takes place in the opposite direction, from left to right and the cæcum turns its concave margin caudad and to the left.

Fig. 116.—Abdominal viscera of Tamandua bivittata, with the intestine rotated to correspond to the development in the human subject. (From a fresh dissection.)

Fig. 117.—The same view as Fig. 116, from another specimen.

Figs. 116 and 117 show the intestinal tract of Tamandua bivittata arranged so as to correspond to the human embryonic condition after rotation. The cæcum has been brought up and to the right across the proximal duodenal portion of the small intestine, while the jejuno-ileal coils have been turned down and to the left. The rotation has been accomplished by a twist at the duodeno-colic isthmus, and the original right leaf of the mesentery has become the left and vice versa. Comparison with Figs. 107 and 108, representing the condition before rotation in the same animal, will indicate the changes which have been accomplished by imitating the course of development followed in the higher mammals.

Failure of rotation and arrest of development at the primitive stage, with consequent persistent embryonic condition of the mesentery, occurs occasionally in man. Such cases have been reported by W. J. Walsham, in St. Barthol. Hosp. Rep., London, Vol. 16. The following four instances of this condition, taken from the Columbia University museum, will illustrate the disposition of the abdominal contents.

Fig. 118 shows the arrangement of the abdominal viscera in an adult female body. Beginning at the pyloric extremity of the stomach the entire course of the duodenum can be overlooked and its continuation into the jejuno-ileal division traced. The small intestines occupy the ventral and right part of the cavity. The ileo-colic junction is placed in the lower left-hand corner of the abdomen and the small intestine enters the large from right to left, the ascending colon is situated to the left of the median line and at its point of transition into the segment representing the transverse colon is connected by several adhesions with the ventral surface of the duodenum. The transverse colon, folded into several coils bound together by adhesion, occupies the upper left portion of the abdomen.

Fig. 119, taken from the same specimen, shows the entire mass of intestines lifted up and turned to the left, exposing the background of the abdominal cavity lined by parietal peritoneum. The duodenum is still entirely free and non-adherent to the parietal peritoneum. The continuity of the mesoduodenum with the jejuno-ileal mesentery is well shown. The primitive right leaf of the mesentery is turned to the observer. This layer after completed rotation would form the left layer of the adult mesentery of the jejuno-ileum.

Fig. 118.—Abdominal viscera of adult human female, in a case of arrested rotation of the intestines. (Columbia University Museum, Study Collection.)

Fig. 119.—The same preparation with the intestinal coils displaced upward and to the left.

Fig. 120 illustrates another instance of the same condition in the adult. In this case the duodenum was coiled twice upon itself and adherent to the prerenal parietal peritoneum.

Fig. 121, presenting the same adhesion of the duodenum, illustrates very perfectly the persistence of the narrow duodeno-colic isthmus in cases of non-rotation, as well as the development of the different segments of the adult tract from the limbs of the embryonal umbilical intestinal loop.

It will be observed that beyond the duodeno-colic isthmus the coils of the jejuno-ileum have resulted from the increase in length of the descending limb, the apex and the proximal part of the ascending or recurrent limb, carrying the ileo-colic junction and cæcum. The remainder of the ascending limb, terminating in the embryonic condition at the splenic flexure by passing into the descending colon, has in the course of further development in this individual produced a straight segment—the misplaced ascending colon—and a convoluted and bent representative of the normal transverse colon.

The same disposition of the large intestine may be noted in the other preparations.

Fig. 120.—Abdominal viscera of adult human male; non-rotation of intestine. (Columbia University Museum, Study Collection.)

Fig. 121.—Abdominal viscera of adult human male; non-rotation of intestine. (Columbia University Museum, Study Collection.)

Fig. 122.—Abdominal viscera of child, two years old; non-rotation of intestine. (Columbia University Museum, Study Collection.)

Fig. 123.—Human fœtus at term; abdominal viscera, hardened in situ; non-rotation of cæcum. (Columbia University Museum, No. 1813.)

Fig. 122 shows an instance of non-rotation observed in the human infant at two years of age.

Fig. 123, taken from a fœtus at term, shows the result of failure to completely rotate in the region of the cæcum and ileo-colic junction. The rest of the large intestine has rotated as usual and assumed the normal position. The terminal ileum, however, passes behind the cæcum and enters the large intestine on its right side; the cæcum is turned upwards and to the right and the appendix lies ventrad of the beginning of the ascending colon. In order to produce the normal arrangement, shown in Fig. 124, taken from another fœtus at term, it would be necessary to turn the cæcum and ileo-colic junction in Fig. 123 through half a circle. The cæcum would then turn upwards and to the left, the ileum entering the large intestine from left to right, and the appendix would be placed behind the cæcum and ileo-colic junction. Figs. 125 and 126 show the normal and abnormal arrangement presented by these two preparations diagrammatically. The instances in which in the adult the ileo-colic entrance is placed on the right side of the large intestine and in which the appendix is situated laterad of the ascending colon unquestionably find their explanation in the failure of the intestine to completely rotate at the ileo-colic junction.

Fig. 124.—Human fœtus at term; abdominal viscera, hardened in situ; normal position of completely rotated cæcum and appendix. (Columbia University Museum, No. 1814.)		Figs. 125, 126.—Schematic representation of final stages in rotation of cæcum and large intestine.
	Fig. 125.—Just before final rotation of cæcum and terminal ileum. Concavity of cæcum directed cephalad and to right. Terminal ileum enters colon from right to left.		Fig. 126.—Rotation completed. Concavity of cæcum turns caudad and to left. Terminal ileum enters colon from left to right.