DEVELOPMENT OF THE ALIMENTARY CANAL. 673 



foetus is continuous with the arnnion. The intestine makes at first a 

 single angular turn forward, and at its most prominent portion gives 

 off the stem of the umbilical vesicle (g). A short distance below this 

 point it subsequently enlarges in calibre, and the situation of this 

 enlargement marks the commencement of the colon. The two por- 

 tions of the intestine, after this period, become widely different from 

 each other. The upper portion, which is the small intestine, grows 

 most rapidly in the direction of its length, becoming- a long, narrow, 

 convoluted tube ; while the lower portion, which is the large intestine, 

 increases mainly in diameter. The lowermost part of the large intes- 

 tine, which alters least in form and position, becomes the rectum. It 

 elongates comparatively little, retains its position for the most part on 

 the median line, and, as its name indicates, continues nearly straight ; 

 presenting only a moderate antero-posterior curvature corresponding 

 with the hollow of the sacrum, and a slight lateral obliquity. It at 

 first forms the blind extremity of the large intestine, but subsequently 

 communicates with the exterior by the perforation of the anus. In 

 the embryo chick, according to Burdach,* the perforation of the anus 

 takes place on the fifth day of incubation ; in the human embryo it 

 appears during the seventh week. In certain instances, the opening 

 fails to take place, and the rectum is still closed at. birth; presenting 

 the malformation known as imperf orate anus. If the rectum be other- 

 wise fully developed, it may sometimes be felt, distended with meco- 

 nium, immediately under the integument ; and an artificial opening 

 may be successfully made by incision at the anal region. In other 

 cases the rectum is also more or less deficient, the large intestine ter- 

 minating in the upper part of the pelvic cavity. 



Just beyond the point of junction between the small and the large 

 intestine, the colon presents a rounded diverticulum, which increases 

 in length until the ileum, instead of forming a continuous tube with 

 the colon, seems to join it by an oblique lateral insertion. The diver- 

 ticulum of the colon is at this tiine conical in shape ; but afterward 

 that portion which forms its free extremity becomes elongated into the 

 appendix vermiformis; while the remaining portion is enlarged into 

 the caput coli. 



The caput coli and appendix vermiformis are at first situated near 

 the umbilicus ; but between the fourth and fifth months (Cruveilhier) 

 their position is altered, and they become fixed in the right iliac 

 region. During the first six months the internal surface of the small 

 intestine is smooth. At the seventh month the valvulae conniventes 

 begin to appear, after which they increase in size, though still com- 

 paratively undeveloped at birth. The colon is at first smooth and 

 cylindrical in form, like the small intestine ; its division into sacculi 

 by longitudinal and transverse bands takes place during the latter half 

 of foBtal life. 



* Traite de Physiologic. Paris, 1838, tome iii., pp. 274, 468. 

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