266 DIGESTION. [CHAP. xxv. 



in structure that of the stomach, and is supplied with glands like 

 the solitary glands, which pour out an abundant secretion. 



No material change takes place in the faeces as they pass 

 through the large intestine, excepting such as is produced by the 

 absorption of fluid from them by the mucous membrane. Thus the 

 faeces become drier the longer they remain in the bowels. 



Defecation. The contents of the large intestine are pushed 

 onwards by a vermicular action, essentially the same as that of 

 the small intestine. Propelled thus in successive portions, they 

 accumulate in the rectum, whence they are prevented from 

 escaping by the contraction of the sphincter. The act of expulsion 

 of the faeces from the rectum, the act of defaecation, is effected 

 partly by the contraction of the muscular fibres of the rectum, 

 excited by the stimulus of distension, and partly by the contrac- 

 tion of the abdominal muscles and of the diaphragm, which by 

 reducing the size of the abdominal cavity, and compressing the 

 intestinal canal at all points, greatly assists the detrusive efforts of 

 the rectum itself. 



Within certain limits the act of defaecation is favoured by the 

 bulk of the intestinal contents. When the rectum is moderately 

 distended, and its inner surface sufficiently lubricated by mucus, 

 defaecation is effected with but little aid from the abdominal 

 muscles, and mainly by the expulsive force of the rectum, which 

 is sufficiently strong to overcome the passive contraction of the 

 sphincter. If the contents of the rectum be too bulky, they occa- 

 sion over-distension of the gut, and diminish its contractility. 

 Under such circumstances, immense accumulations may take place ; 

 and, as small portions may continue from time to time to be ex- 

 pelled, under the influence of the abdominal muscles, the practitioner 

 may thus be deceived as to the real nature of the case. On the 

 other hand, when the faeces do not accumulate in the rectum in 

 sufficient quantity to distend the rectum, the act of defaecation is 

 rendered difficult by the imperfect actions of the rectum itself, and 

 great efforts are required on the part of the abdominal muscles, 

 which often cause the protrusion of portions of the mucous mem- 

 brane near the anus. Under these circumstances it is, that enemata 

 act so favourably, by giving the gut its natural stimulus, that of 

 distension. The action of the abdominal muscles in defaecation is 

 chiefly voluntary, but partly reflex. If the rectum be the seat of 

 irritation, as in dysentery, the reflex action is much increased, and 

 the repeated strainings which occur during the act of defaecation in 

 this disease are, in a great measure, thus caused. 



