DEVELOPMENT OF THE DIGESTIVE APPARATUS. 823 



the cavity of the pedicle of the umbilical vesicle is obliterated, at about the 

 tenth week. 



At the upper part of the abdominal cavity the alimentary canal presents 

 two lateral projections, or pouches. The one on the left side, as it increases 

 in size, becomes the greater pouch of the stomach, and the one on the right 

 side, the lesser pouch. 



At a short distance below the attachment of the pedicle of the umbilical 

 vesicle to the intestine, there appears a rounded diverticulum, which is 

 eventually developed into the caecum. The caecum gradually recedes from 

 the neighborhood of the umbilicus, which is its original situation, and finally 

 becomes fixed, by a shortening of the mesentery, in the right iliac region. 

 As the caecum is developed it presents a conical appendage, which is at first 

 as large as the small intestine and is relatively longer than in the adult. 

 During the fourth week this appendage becomes relatively smaller and more 

 or less twisted, forming the appendix vermiformis. At the second month 

 the caecum 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-caecal valve appears 

 at the third month ; the rectum, at the fourth month ; and the sigmoid flex- 

 ure of the colon, at the fifth month. During this time the large intestine in- 

 creases more rapidly in diameter than the small intestine, while the latter 

 develops more rapidly in its length. 



In the early stages of development the internal surface of the intestines is 

 smooth ; but villi appear upon its mucous membrane during the latter half 

 of intrauterine existence. These are found at first both in the large and 

 the small intestine. At the fourth month they become shorter and less 

 abundant in the large intestine, and they are lost at about the eighth month, 

 when the projections which bound the sacculi of this portion of the intes- 

 tinal canal make their appearance. The valvulae conniventes appear, in the 

 form of slightly elevated, transverse folds, in the upper portion of the small 

 intestine. 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 por- 

 tion of the peritoneal membrane extends in a quadruple fold from the stom- 

 ach to the colon, to form the great omeiitum, which covers the small intes- 

 tine in front. 



As the head undergoes development a large cavity appears, 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 the stomach ; and between the pharynx and the stomach there is at first 

 no channel of communication. The anterior portion of the pharynx pre- 

 sents, during the sixth week, a large opening, which is afterward partially 



