DEVELOPMENT OF THE ALIMENTARY SYSTEM. 407 



caecum, or caput coli, as we have seen, is at the umbilicus, and 

 the large intestine extends in a straight line toward the anus ; 

 at the third month, it is situated at about the middle of the 

 abdomen ; and it gradually descends, until it reaches the right 

 iliac region at about the seventh month. Thus, at the second 

 month, there is only a descending colon ; the transverse colon 

 is formed at the third month ; and the ascending colon, at the 

 fifth month. The ileo-csecal valve appears at the third month ; 

 the rectum, at the fourth month ; and the sigmoid flexure of 

 the colon, at the fifth month. 1 During this time, the large 

 intestine increases more rapidly in diameter than the small 

 intestine, while the latter develops more rapidly in its length. 



In the early stages of development, the surface -of the in- 

 testines is smooth ; but villi appear on its mucous membrane 

 during the latter half of intra-uterine existence. These are 

 found at first both in the large and the small intestine. At 

 the fourth month, they become shorter and less numerous in 

 the large intestine, and are lost at about the eighth month, 

 when the projections which bound the sacculi of this portion 

 of the intestinal canal make their appearance. The valvulse 

 conniventes appear, in the form of slightly-elevated, trans- 

 verse folds, in the upper portion of the small intestine. 2 The 

 villi of the small intestine are permanent. 



The mesentery is first formed of two perpendicular folds, 

 attached to the sides of the spinal column. As the intestine 

 undergoes development, a portion of the peritoneal mem- 

 brane extends in a quadruple fold from the stomach to the 

 colon, to form the great omentum, which covers the small in- 

 testine in front. 



As the head undergoes development, a large cavity ap- 

 pears, which is eventually bounded by the arches that are 

 destined to form the different parts of the face. This is the 

 pharynx. It is entirely independent, in its formation, of the 

 intestinal canal, the latter terminating in a blind extremity at 



1 BURDACH, TraitS de physiologic, Paris, 1838, tome iii., pp. 476, 477. 



2 Ibid, p. 470. 



