DIGESTIVE SYSTEM 727 



main digestive tract almost entirely but are still connected with it by 

 small ducts. 



The glands push their way through both submucosa and musculosa 

 but, as they push against the serosa this seems to stretch out ahead of 

 all these outpushings, forming a covering for the outgrowths. This is 

 why not only the liver and pancreas, but every organ in the abdominal 

 cavity, is completely covered by this serous layer, which when thought 

 of in its entirety is called the peritoneum. 



The kidneys form a single exception to the statement that all organs 

 in the abdominal cavity are completely covered with peritoneum. These 

 do not spring from the digestive tract, however, and will be discussed 

 later with the uro-genital system. 



The entire digestive canal is covered with this serous layer. Figure 

 427 shows just how this develops and why it is that, while there is a 

 single layer of serosa over the ventral side of the intestinal tract there 

 are two layers running dorsalward which are attached close to the ven- 

 tral portion of the spinal region forming the sustaining ligaments. 



Probably this will be made clear if one place an ordinary sheet of 

 paper on the desk before him and lay a pencil at right angles to the long 

 axis of the sheet. By picking up the two ends of the paper so that the 

 pencil is held within the fold it will be seen that under the pencil there 

 is only one layer of paper but above it there are two. The various out- 

 pushings of the intestinal tract push the serosa before them just as the 

 pencil does the paper in this case. 



The two layers running dorsalward from the organ and forming the 

 sustaining ligament are called the mesentery, and it is between these two 

 sheets of mesentery that the blood supply of the organ is carried. 



If it be remembered that the digestive tract begins as a single tube, 

 approximately the same length as that of the body in which it grows, 

 and if the various elongations, outpushings, and inpushings are then fol- 

 lowed through the embryonic period, considerable light will be thrown 

 upon our understanding of the adult structure (Fig. 428). 



One must, however, be wary in comparing different type-forms of 

 animals, as well as animals of the same species at different stages of their 

 development, or there will be little validity in the comparisons. 



The first portions of the digestive tract to differentiate are the 



long bag g which is the greater omentum ; the mesogaster and mesocolon are fusing 

 at i. C, completion of the fusion of mesogaster and mesocolon at i. a, liver ; b, 

 serosa of the liver ; c, lesser omentum or gastro-hepato-duodenal ligament ; d, 

 stomach ; e, lesser peritoneal sac or cavity of the greater omentum ; /, mesocolon ; 

 g, portion of the mesogaster which forms the greater omentum ; h, intestine ; 

 i, fusion of the mesogaster and mesocolon. ( From Hyman after Hertwig ) . 



II. Scheme of digestive canal and mesenteries in human embryos, 30 and 50 

 mm. long, ac, ascending colon ; c, caecum ; co, colon ; d, duodenum ; dc, descending 

 colon ; k, kidney ; r, rectum ; rd, recto-duodenal ligament ; rl, recto-lienal ligament ; 

 rrd, recto-duodenal recess; s, stomach; sp, spleen; tc, transverse colon. (From 

 Kingsley after Klaatsh). 



III. Transverse section of a salamander embryo in the region of the liver. 

 (Redrawn from Maurer). 



IV. Schematic arrangement to show the development of the omental bursa. 

 (After Corning). P, Pancreas; Ao, Aorta; L.H.G., Hepatogastric ligament. 



