452 PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



inhibits the tonic contraction of the sphincter. By the action of the will 

 the tonic action of the sphincter may be increased, or the action of the 

 sphincter may be completely inhibited. While the abdominal contrac- 

 tions above alluded to aid in defecation, the sigmoid flexure serves to 

 ward off the pressure of the abdominal walls ; therefore, the contraction 

 of the abdominal muscles is not sufficient alone to produce defsecation, 

 but must be accompanied by the peristaltic action of the large intestine 

 and sigmoid flexure. 



As a rule, the contraction of the abdominal muscles is voluntary, 

 but the contact of the fecal mass with the mucous membrane of the sig- 

 moid flexure is itself sufficient to inaugurate and normally complete the 

 act of defecation. Therefore, defecation may be produced in states of 

 entire unconsciousness. 



Evacuation of feces occurs at various intervals in different animals— 

 in the horse, usually six to seven times in the twenty-four hours ; in the 

 sheep, four to five times ; and in the hog, once or twice. 



In the act of defecation in the horse, the loosely attached raucous 

 membrane of the lower part of the rectum is also extruded, forming the 

 so-called "rose of the anus," and is again retracted after the act is com- 

 plete ; the object of this is, perhaps, to reduce friction between the mu- 

 cous surface and the more or less hard fecal masses. The horse is able 

 to evacuate its rectum while in motion ; animals, as a rule, adopt a more 

 or less squatting position, with the back highly arched, so as to increase 

 the expulsive action of the abdominal muscles. 



