824 PHYSIOLOGY 



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 distinguishing 

 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 evacua- 

 tion 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 longi- 



FIG. 346. 'Skiagram to show normal position of colon in man, and the 

 position attained by its contents at different periods after a meal contain- 

 ing bismuth. The bismuth meal was taken at 8 A.M. The times of 

 arrival at different levels are marked on the colon. (HERTZ.) 



tudinal 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 

 of course 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 forcible contraction of the levator ani and the other perinseal 

 muscles, and this contraction also serves to restore the everted mucous 

 membrane. 



The carrying out of this reflex act is dependent on the integrity 

 of a certain part of the lumbar spinal cord. If this ' centre ' be 

 destroyed, the tonic contraction of the sphincter muscles disappears. 



