CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 381 



occasionally do, begin at various points along the length of the 

 intestine. A movement started by artificial stimulation some way 

 down the intestine, may travel not only downwards but also up- 

 wards ; it has been disputed however, whether in the living body 

 any natural backward peristaltic movement really takes place. 

 In the living body the intestines have periods of rest, alternating 

 with periods of activity, the occurrence of the periods depending 

 on various circumstances ; the intensity of the movements also 

 varies very considerably. 



224. Movements of the Large Intestine These are funda- 

 mentally the same as those of the small intestine, but distinct in 

 so far as the latter cease at the ileo-caecal valve, at which spot the 

 former normally begin ; they are simpler, in as much as the 

 pendent loops are absent, and not so vigorous, since relatively to 

 the diameter of the tube, the amount of muscular fibre is less. 

 Along the colon where the sacculi are well developed the move- 

 ment may perhaps be described as almost intermittent from 

 sacculus to sacculus, the contents of one sacculus being driven 

 by the peristaltic contractions of its circular fibres into the next 

 sacculus, which prepares to receive them by a relaxation of its 

 circular and a contraction of its longitudinal fibres 



Since the lips of the ileo-caecal valve are placed transversely 

 across the caecum, not only does distention of the caecum, by 

 stretching the valve along the line of the lips, bring them into 

 apposition, but the pressure exerted by the peristaltic movement 

 has the same effect. In this way any return of the contents from 

 the large to the small intestine is prevented. 



Arrived at the sigmoid flexure, the contents, now more or less 

 solid faeces, are supported by the bladder and the sacrum, so that 

 they do not press on the sphincter ani. 



225. Defoecation. This is a mixed act, being superficially 

 the result of an effort of the will, and yet carried out by means of 

 an involuntary mechanism. Part of the voluntary effort consists 

 in producing a pressure-effect, by means of the abdominal muscles. 

 These are contracted forcibly as in expiration, but the glottis 

 being closed and the escape of air from the lungs prevented, the 

 whole force of the pressure is brought to bear on the abdomen 

 itself, and so drives the contents of the descending colon onward 

 towards the rectum. The sigmoid flexure is by its position shel- 

 tered from this pressure ; a body introduced per anum into the 

 empty rectum is not affected by even forcible contractions of the 

 abdominal walls. 



The anus is guarded by the sphincter ani, which is habitually 

 in a state of normal tonic contraction, capable of being increased 

 or diminished by a stimulus applied, either internally or externally, 

 to the anus. The tonic contraction is in part at least due to the 

 action of a nervous centre situated in the lumbar spinal cord. If 

 the nervous connection of the sphincter with the spinal cord be 



