280 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



lymphoid structure is present generally in mammals. The ap- 

 pendix is a lymphoid diverticulum of the caecal apex (E. J. 

 Berry). It must be regarded as a lymphoid structure, and 

 although it can be dispensed with, is not therefore to be regarded 

 as vestigial in nature any more than is the tonsil. The 

 caecum is largest, as is also the bowel, in herbivorous animals. 



Ileo-caecal Fossae. — When the caecal diverticulum grows out 

 from the hinder limb of the U-shaped loop it carries with it 

 three folds (see Fig. 224): 



1. The ileo-colic fold, a process from the right side of the 

 mesentery containing the anterior caecal artery ; 



2. The bloodless fold, a process from the coat of the ileum ; 



3. The mesentery of the appendix, a process from the left side 

 of the mesentery, containing the artery to the appendix (Fig. 

 225 A). 



These three folds give rise to three fossae (Fig. 225 5): 



1. The ileo-colic, between the termination of ileum and ileo- 

 colic fold ; 



2. The ileo-caecal, between the bloodless fold and mesentery of 

 the appendix ; 



3. The retro-caecal, between the mesentery of the appendix and 

 commencement of the ascending meso-colon. 



The Duodenum. — The part of the duodenum above the 

 entrance of the common bile-duct is formed from fore-gut, the 

 part behind from the mid-gut. The liver and pancreatic buds 

 arise from its ventral border at the junction of these two parts. 

 At first it is entirely covered by peritoneum and suspended in the 

 mesentery, in which it forms a minor loop (Figs. 219, 222). In 

 its concavity rests the head of the pancreas. 



In the *7th week the U-shaped loop of intestine rotates so as to 

 bring the ascending colon, with its mesentery, against the duodenal 

 loop, which, with the head of the pancreas, is thus pressed 

 against the right kidney and inferior vena cava (Fig. 226). The 

 peritoneum covering the right aspect of the head of the pancreas 

 and duodenal loop adheres to the parietal peritoneum covering 

 the kidney, and disappears. The transverse meso-colon gains an 

 attachment to the front of the duodenum. 



The Duodeno-jejunal Fossa is formed as the U-shaped loop of 

 bowel rotates, dragging the transverse meso-colon after it (Fig. 



