(ESOPHAGUS, STOMACH, AND INTESTINES. 



107 



The large intestine is not at first marked off from the small by any difference in 

 calibre. Its commencement is distinguishable about the sixth week in the human 

 embryo by the appearance of the caecum, which gradually grows out (tigs. 125, D, and 

 127), forming at first a lateral protrusion of uniform calibre, but subsequently re- 

 maining narrow at its blind extremity to form the vermiform appendix, while the 

 remainder of the caecum and the colon increase in size. This protrusion occurs on 

 the U" sna P e( i 1P above described, and a little beyond the attachment of the 

 vitclline duct. 



With the increasing length of the gut it becomes thrown into coils, and the 

 earliest and most important of these is that by which the limb of the U~ sna P e( i 1P 



diapt^ 



snail intestine. 



Fig. 129. DIAGRAMS ILLUSTRATING THE DEVELOPMENT OK THE GREAT OMENTUM. (0. Hertwig.) 



A, earlier stage. 



B, later stage. 



st, stomach ; s.o, small omentum ; s'.o', oinental sac ; o', mcsogastriuin, springing from the 

 posterior wall of the abdomen, near which in A it encloses the pancreas ; o 2 , attachment of mesogastrium 

 to greater curvature of stomach ; o 3 , fold of mesogastrium or great omentum growing over coils 

 of small intestine; me, mesentery; in. c, transverse mesocolon: o 4 (in B), dotted line showing the 

 situation of that lamella of the mesogastrium which at first assisted in enclosing the pancreas but 

 which has now disappeared. The next part of this lamella has coalesced with the adjacent lamella of 

 the transverse mesocolon, and has also disappeared. The coalescence is indicated by the black line. 



with which the large intestine is continuous turns over on to the right side of the 

 peritoneal cavity, and thus throws the colon in an archlike disposition across the 

 commencement of the small intestine, and parallel with the longitudinal axis of the 

 stomach (commencement shown in fig. 126). Within this arch of the large intestine 

 the coils of the jejunum and ileum become disposed as the intestine lengthens. 

 Their mesentery spreads out at its intestinal attachment so as to adapt itself to the 

 increasing length of the gut, while its vertebral attachment, relatively much shorter, 

 loses to a great extent its primitive disposition, and acquires oblique and transverse 

 lines of attachment ; this is notably the case with the transverse mesocolou. 



Although the mesentery in most parts increases in length and expansion with the further 

 growth of the intestine, the contrary is the case with the mesentery of the duodenum, and of 

 the ascending and descending 1 colon. All these parts possess at first a complete mesentery like 

 the rest of the intestine, but that of the duodenum disappears entirely, so that this part of the 

 intestine becomes fixed to the posterior wall of the abdomen, and the same process takes place 

 to a lesser and variable extent with the ascending and descending mesocolon. Since the trans- 

 verse colon lies across the abdomen immediately below the stomach, it and its mesentery, trans- 

 versely disposed, also lie immediately below and behind the mesogastrium (now folded into the 

 great omentum). The two membranes come in fact into close contact, and eventually com- 



