288 HAND-BOOK OF PHYSIOLOGY. 



through the small intestine, and twenty-four to thirty-six hours by the 

 passage through the large bowel. (Brinton.) 



Defalcation. Immediately before the act of voluntary expulsion of 

 faeces (defcecation) there is usually, first an inspiration, as in the case of 

 coughing, sneezing, ancl vomiting; the glottis is then closed, and the 

 diaphragm fixed. The abdominal muscles are contracted as in expira- 

 tion; but as the glottis is closed, the whole of their pressure is exercised 

 on the abdominal contents. The sphincter of the rectum being relaxed, 

 the evacuation of its contents takes place accordingly; the effect being, 

 of course, increased by the peristaltic action of the intestine. As in the 

 other actions just referred to, there is as much tendency to the escape of 

 the contents of the lungs or stomach as of the rectum; but the pressure 

 is relieved only at the orifice, the sphincter of which instinctively or in- 

 voluntarily yields (see Fig. 144). 



Nervous Mechanism of Defaecation. The anal sphincter muscle 

 is normally in a state of tonic contraction. The nervous centre which 

 governs this contraction is probably situated in the lumbar region of the 

 spinal cord, inasmuch as in cases of division of the cord above this region 

 the sphincter regains, after a time, to some extent the tonicity which is 

 lost immediately after the operation. By an effort of the will, acting 

 through the centre, the contraction may be relaxed or increased. In ordi- 

 nary cases the apparatus is set in action by the gradual accumulation of 

 faeces in the sigmoid flexure and rectum pressing against the sphincter 

 and causing its relaxation; this sensory impulse acting through the brain 

 and reflexly through the spinal centre. Peristaltic action, especially of the 

 sigmoid flexure in pressing onward the faeces against the sphincter, is a 

 very important part of the act. 



The Gases contained in the Stomach and Intestines. Under 

 ordinary circumstances, the alimentary canal contains a considerable 

 quantity of gaseous matter. Any one who has had occasion, in a post- 

 mortem examination, either to lay open the intestines, or to let out the 

 gas which they contain, must have been struck by the small space after- 

 ward occupied by the bowels, and by the large degree, therefore, in which 

 the gas, which naturally distends them, contributes to fill the cavity of 

 the abdomen. Indeed, the presence of air in the intestines is so constant, 

 and, within certain limits, the amount in health so uniform, that there 

 can be no doubt that its existence here is not a mere accident, but in- 

 tended to serve a definite and important purpose, although, probably, a 

 mechanical one. 



Sources. The sources of the gas contained in the stomach and 

 bowels may be thus enumerated: 



1. Air introduced in the act of swallowing either food or saliva; 2. 

 Gases developed by the decomposition of alimentary matter or of the 



