TOPOGRAPHY OF THE THORAX AND ABDOMEN 47 



direction as the descending colon from the iliac crest to the 

 level of the first sacral vertebra where it turns horizontally- 

 inward and backward to pass over the brim of the pelvis 

 (Plates XXVIII, XXIX, XXXII and XXXIII). The pelvic 

 portion of the sigmoid is coiled upon itself and lies upon 

 the anterior surface of the rectum in the region of the first 

 four sacral vertebrae. The iliac portion of the sigmoid is 

 contracted but soon after crossing the brim of the pelvis 

 the colon becomes dilated. It turns downward for a short 

 distance, separated from the anterior wall by the jejunoil- 

 eum and its mesentery. Opposite the fourth sacral verte- 

 bra the colon turns upon itself in the anteroposterior direc- 

 tion and passes upward between -the descending loop and 

 the first part of the rectum. At the level of the second 

 sacral vertebra the intestine makes another turn in the an- 

 teroposterior direction and joins the rectum. Thus there 

 are two loops of the sigmoid and the first part of the 

 rectum in the same anteroposterior plane anterior to the 

 middle portion of the sacrum. The iliac portion of the 

 sigmoid rests upon the iliopsoas. Its mesocolon is very 

 short. The pelvic portion has a longer mesocolon which 

 allows it to swing free in the pelvic cavity where it is in 

 contact anteriorly and laterally with the jejunoileum. 



The rectum, the remainder of the large intestine may 

 be divided into two parts nearly equal in length. The 

 upper part is covered upon its anterior surface with, 

 peritonaeum and extends from near the base of the sacrum 

 to about 1 cm. above the tip of the coccyx. The lower 

 part lies below the peritonaeal cavity. The first por- 

 tion is greatly dilated and nearly fills the true pelvic 

 cavity. It is covered with peritonaeum on its anterior sur- 

 face and a part of each lateral surface. The posterior sur- 

 face is attached to the concave anterior surface of the 



