776 DEVELOPMENT OF THE ALIMENTARY CANAL 



leaving a wide opening at a, 6, where the integument of the foetus is 

 continuous with the commencement of the amniotic membrane. The 

 intestine makes at first a single angular turn forward, and opposite the 

 most prominent portion of this angle is to be seen the stem of the um- 

 bilical vesicle (g). A short distance below this point the intestine sub- 

 sequently enlarges in calibre, and the situation of this enlargement 

 marks the commencement of the colon. The two portions of the intes- 

 tine, after this period, become widely different from each other. The 

 upper portion, which is the small intestine, grows most actively in 

 the direction of its length, and becomes a long, narrow, convoluted 

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

 rapidly in diameter, but elongates less than the former. The rectum is 

 the part of the large intestine which alters least its form and position. 

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

 upon the median line, and as its name indicates, continues to follow a 

 nearly straight course; presenting only a moderate antero-posterior 

 curvature corresponding with the hollow of the sacrum, and a slight 

 lateral obliquity, from its upper portion which is placed a little toward 

 the left, to the anus which is situated upon the median line. At first 

 forming the blind extremity of the large intestine, it subsequently com- 

 municates with the exterior by a perforation which becomes the anus. 

 In the chick-embryo, according to Burdach, 1 the perforation of the anus 

 appears on the fifth day of incubation ; in the human embryo it is 

 formed during the seventh week. In certain instances, this opening 

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

 the malformation known as imperforate anus. If the rectum be other- 

 wise fully developed, it may sometimes be felt, distended with meconium, 

 immediately under the integument ; and an artificial opening may be 

 successfully made by an incision at the anal region. In other cases, 

 there is also a deficiency, more or less extensive, of the rectum itself, 

 the large intestine terminating in the upper portion of the pelvic cavity. 



At the point of junction between the small and the large intestine, a 

 lateral diverticulum of the latter shows itself, and increases in extent, 

 until the ileum seems at last to be inserted obliquely into the side of the 

 colon. The diverticulum of the colon is at first conical in shape ; but 

 afterward that portion which forms its free extremity becomes narrow, 

 elongated, and sometimes twisted upon itself, forming the appendix 

 vermiformis ; while the remaining portion, which is continuous with the 

 intestine, becomes exceedingly enlarged, and forms the caput coli. 



The caput coli and the 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 ap- 

 pear, after which they increase slowly in size, but are still comparatively 



1 Trait6 de Physiologie, traduit par Jourdan. Paris, 1838, tome iii. pp. 274, 468. 



