MOVEMENTS OF THE INTESTINES 731 



separated by transverse waves of constriction from the more fluid parts 

 which are being driven to and fro in the proximal and intermediate portions. 

 By means of occasional peristaltic waves these hard masses are driven into 

 the distal part of the colon. The distal colon must be regarded as a place 

 for the storage of the faeces and as the organ of defsecation. In the transverse 

 colon^in the descending and iliac-colon, the anti-peristaltic movements and 

 consequent churning of the contents are probably slight. These therefore 

 represent the intermediate colon, with propulsive peristalsis as its chief 

 activity. The descending colon is never distended, and Elliott therefore 



FIG. 348. Skiagram to show normal position of colon in man, and the position attained 

 by its contents at different periods after a meal containing bismuth. The bismuth 

 meal was taken at 8 A.M. The times of arrival at different levels are marked on the 

 colon. (HERTZ.) 



regarded it as a transferring segment of exaggerated irritability. The 

 storage of the waste matter takes place chiefly in the sigmoid flexure. 

 This with the rectum represents the distal portion of the colon. The dis- 

 tinguishing feature of the distal colon is its complete subordination to the 

 spinal centres. It probably remains inactive until an increasing distension 

 excites reflexly through the pelvic visceral nerves a complete evacuation of 

 this portion of the gut. Stimulation of these nerves in an animal, such as the 

 cat, produces a rapid shortening of the distal part of the colon, due to 

 contraction of the recto-coccygeus and longitudinal fibres of the gut, followed 

 after some seconds by a contraction of the circular coat. This originates 

 at the lower limit of the area of anti-peristalsis, i.e. probably at the upper end 

 of the sigmoid flexure, and, spreading rapidly downwards, empties the whole 

 of this segment of the gut. In- man the emptying of the rectum itself is 

 largely assisted by the contractions of the voluntary muscles of the 

 abdominal walls and pelvic floor. 



The last section of the rectum is emptied at the close of the act by a 



