408 DIGESTION. 



are very fluid, or when the call for an evacuation has not been 

 regarded and has become imperative, the immediate dis- 

 charge of matters when the sphincter is relaxed shows that 

 the rectum has been more or less distended. In many per- 

 sons of constipated habit, and particularly in old subjects, the 

 rectum may become the seat of large accumulations of hard- 

 ened and impacted faeces ; but this is a pathological condition. 



The sensation which ordinarily precedes and gives rise to 

 evacuation of faecal matter is peculiar, and very variable in 

 intensity. When this sensation is well marked but not ex- 

 cessive, it is probably due to the presence of fascal matter in 

 the rectum, not in sufficient quantity, however, to press for- 

 cibly on the sphincter. Pressure upon the rectum from any 

 cause, or irritation of its mucous membrane, is apt to give 

 rise to this peculiar sensation to a very marked degree. In 

 some diseases, the exaggeration of this sensation, then called 

 tenesmus, is very distressing. 



In the process of defecation, the first act is the passage, 

 by peristaltic contractions, of the contents of the sigmoid 

 flexure of the colon through the slightly constricted opening 

 of the rectum into its dilated portion below. The fsecal mat- 

 ter, however, is not allowed to remain in this situation, but 

 passes into the lower portion of the rectum, in obedience to 

 the contractions of its muscular coat, assisted by the action 

 of the abdominal muscles and diaphragm. The circular fibres 

 of the rectum undergo the ordinary peristaltic contraction ; 

 and the action of the longitudinal fibres is to render the rec- 

 tum shorter and more nearly straight. The internal and the 

 external sphincter present a certain amount of resistance to 

 the discharge of the faeces, more particularly the external 

 sphincter, which is a striated muscle of considerable power. 

 There is always, however, a voluntary relaxation of this mus- 

 cle, or rather a cessation of its semi-voluntary contraction, 

 which immediately precedes the expulsive act. The dila- 

 tation of the anus is also facilitated by the action of the 

 levator ani, which arises from the posterior surface of the 



