1252 THE DIGESTIVE SYSTEM. 



of two folds of peritoneum, one on the front, the other on the back, between the two 

 parts, profoundly modifies the subsequent growth of the caecum, and determines very 

 largely its adult form. For, when the caecum begins to expand, the medial aspect is 

 prevented, by its connexion with the termination of the ileum, from enlarging as freely 

 as the rest of the wall ; in consequence of this the lateral part grows and expands much 

 more rapidly, producing the lop-sided appearance already referred to, and soon comes to 

 form the lowest part or f undus of the caecum, and the greater part of its sac ; whilst the 

 original apex, with the vermiform process springing from it, anchored, as it were, to the 

 end of the ileum, is thrust to one side, and finally lies on the medial and posterior aspect 

 of the caecum, a little way below, and usually posterior to, the end of the ileum. 



The position of the caecum varies at different periods of foetal life. About the eleventh 

 or twelfth week it lies immediately beneath the liver, and to the left of the median plane ; 

 it then gradually travels to the right, crossing the descending part of the duodenum, and 

 is found lying on the right side, just beneath the liver, at the fourth month. From there 

 it descends slowly to its adult position, which it usually approaches towards the end of 

 foetal life, but it may not actually reach it until some time after birth. An imperfect 

 descent gives rise to the lumbar position of the caecum, or an excess in this direction to 

 the pelvic position (referred to on p. 1213). 



Rectum. The rectum and anal canal are formed from the posterior portion of the 

 hindgut, and from the proctodeum. 



The primitive closed cloacal portion of the hindgut becomes divided by a vertical 

 septum into ventral and dorsal portions. The ventral, with the allantois growing from 

 it, forms the sinus urogenitalis, the dorsal forms the rectum. 



The proctodeum is separated from the rectum by the proctodeal membrane, but that 

 membrane disappears, and thus the rectum comes to open on the surface. 



The rectum and anal canal at first form a single continuous straight tube, which 

 passes downwards in front of the comparatively straight anterior surface of the sacrum 

 to the anal orifice. 



That is the condition which the parts present at birth. After birth, the bony pelvis 

 undergoes great enlargement. The sacrum and coccyx become curved, and the antero- 

 posterior diameter of the pelvis minor increases very considerably. 



The urinary bladder and, in the female, the uterus both organs at birth lying mainly 

 in the abdomen descend into the pelvis minor. The anal orifice appears to be moved 

 further forwards in the perineum, through the bending of the sacrum and coccyx, and 

 the rectum is pushed back into the hollow of the sacrum. Hence the " flexura sacralis " 

 is formed. 



The "flexura perinealis" is formed by the junction of the curved rectum with the 

 straight vertical or backwardly directed passage formed by the intestine as it passes 

 through the tissues of the pelvic floor. 



The increase in the thickness of the pelvic floor gives to the anal canal the length 

 which it attains in the adult. 



DEVELOPMENT OF THE PERITONEUM. 



At first the primitive alimentary canal is suspended from the dorsal wall of the 

 embryo, along the median plane, by a simple dorsal mesentery, which extends along the 

 whole length of the tube, and is common to all its divisions a condition found in the 

 adult stage of many reptiles. There is also present, in the upper part of the cavity, after 

 the stomach and liver descend into the abdomen, a ventral mesentery (Fig. 972), which 

 connects the stomach and duodenum to the back of the liver, and, passing on, connects 

 the front of the liver to the anterior abdominal wall and diaphragm. The portion of this 

 ventral mesentery, between the stomach and liver, becomes the lesser omentum; its 

 anterior portion, between the liver and the abdominal wall, forms the falciform ligament 

 (Fig. 972) ; and, in its inferior margin, the umbilical vein runs from the umbilicus to 

 the liver. 



The portion of the dorsal mesentery lying behind the stomach is known as the 

 mesogastrium. At first it is relatively short; but with the growth of the posterior 

 wall of the stomach, and the turning of that organ over on its right side, the meso- 

 gastrium becomes elongated, and is folded on itself, forming more or less of a pouch, 

 directed downwards and to the left. The wall of this pouch becomes in part the greater 

 omentum, and the cavity enclosed by it forms the greater part of the omental bursa. 

 In the rotation of the stomach and the accompanying passage of the lesser omentum from 

 an antero-posterior to a more or less transverse direction, a portion of the cavity of the 



