THE DIGESTIVE SYSTEM. 335 



cholesterol. There may be a small quantity of neutral fat. When 

 vegetable food has been taken, the faecal matter will include un- 

 decomposed cellulose, but, for the most part, the faeces consist of 

 substances derived from the digestive tract itself. 



THE MOVEMENTS OF THE LARGE INTESTINE. 



The movements of the large intestine have been most satisfactorily 

 studied with the aid of X-rays (fig. 126). 1 The caecum and ascending 

 colon are filled, in the manner which has already been described, by 

 the peristaltic contractions of the ileurn, and are entirely passive 

 during the process. Later, segmental contractions occur in this part 

 of the bowel, similar in nature to those which take place in the small 

 intestine. These movements tend to mix the contents and promote 

 the absorption of water. The transference of the faeces from the 

 caecum and ascending colon to the transverse and descending colon 

 takes place at long intervals, usually three or four times in twenty- 

 four hours, by means of peristaltic contractions. These movements 

 generally follow the entry of food into the stomach, and are ascribed 

 to^a gastro-colic reflex. Scattered masses may remain in the trans- 

 verse colon for a time, and these are suddenly transferred to the 

 descending colon by a peristaltic wave. 



The faeces remain for a time in the sigmoid flexure, until, usually 

 after a meal, a certain amount passes into the rectum and gives rise 

 to the desire for defaecation. By relaxation of the sphincter ani, 

 accompanied by contraction of the walls of the sigmoid flexure and 

 rectum assisted by contraction of the voluntary muscles of the abdominal 

 wall and pelvic floor, the lower end of the bowel is evacuated. The 

 act is a reflex one, but in the adult it is under the control of the 

 higher centres. 



The Nerve Supply of the Large Intestine. The large intestine 

 receives its nerve supply from the sympathetic system and from the 

 pelvic visceral nerves or nervi erigentes. The sympathetic fibres form 

 the inferior mesenteric nerves, running from the inferior mesenteric 

 ganglion to the ascending, middle, and transverse colon, and the hypo- 

 gastric nerves pass from the same ganglion to the rectum. The pre- 

 ganglionic fibres emerge from the spinal cord by the second, third, and 

 fourth anterior lumbar nerve roots. 



The nervi erigentes emerge from the cord by the second and third 

 sacral nerve roots, and are distributed to the whole length of the large 



1 We are indebted for this diagram to the kindness of the x Oxford University 

 Press. 



