CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 489 



movement 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-csecal valve are placed transversely 

 across the csecum, not only does distension 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. 



275. Defalcation. 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 and the rectum are by 

 their position sheltered from this pressure ; a body introduced per 

 anum into these parts only is not affected by even forcible contrac- 

 tions 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 

 broken, relaxation takes place. If the spinal cord be divided 

 somewhat higher up, for instance in the thoracic region, the 

 sphincter, after the depressing effect of the operation, which may 

 last several days, has passed off, regains and subsequently main- 

 tains its tonicity, shewing that the centre is not placed higher up 

 than the lumbar region of the cord. The increased or diminished 

 contraction following on local stimulation is probably due to reflex 

 augmentation or inhibition of the action of this centre. The 

 centre is also subject to influences proceeding from higher regions 

 of the cord, and from the brain. By the action of the will, 

 by emotions, or by other nervous events, the lumbar sphincter 

 centre may be inhibited, and thus the sphincter itself relaxed ; or 

 augmented, and thus the sphincter tightened. A second item 

 therefore of the voluntary process in defsecation is the inhibition of 

 the lumbar sphincter centre, and consequent relaxation of the 

 sphincter muscle. Since the lumbar centre, in animals at least, 



F. ii. 32 



