DIGESTION. 295 



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

 passage through the large bowel. 



The contents of the large intestine, as they proceed towards the rec- 

 tum, become more and more solid, and losing their more liquid and nu- 

 trient parts, gradually acquire the odor and consistence characteristic of 

 fwces. After a sojourn of uncertain duration in the sigmoid flexure of 

 the colon, or in the rectum, they are finally expelled by the act of defae- 

 cation. 



The average quantity of solid faecal matter evacuated by the human 

 adult in twenty-four hours is about six or eight ounces. 



Composition of Faeces. 



Water, 733.00 



Solids : 



Special excrementitious constituents : Excretin, ex- 

 cretoleic acid (Marcet), and stercorin (Austin 

 Flint). 



Salts : Chiefly phosphate of magnesium and phos- 

 phate of calcium, with small quantities of iron, 

 soda, lime, and silica. 



Insoluble residue of the food (chiefly starch grains, 

 woody tissue, particles of cartilage and fibrous 

 tissue, undigested muscular fibres or fat, and the 

 like, with insoluble substances accidentally in- 

 troduced with the food. 



Mucus, epithelium, altered coloring matter of bile, 



fatty acids, etc. 

 Varying quantities of other constituents of bile, and 

 derivatives from them. 



267.00 

 1000 



Defalcation. The act of the expulsion of faeces is in part due to an 

 increased reflex peristaltic action of the lower part of the large intes- 

 tine, namely of the sigrnoid flexure and rectum, and in part to the more 

 or less voluntary action of the abdominal muscles. In the case of active 

 voluntary eiforts, there is usually, first an inspiration, as in the case of 

 coughing, sneezing, and 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 pres- 

 sure is relieved only at the orifice, the sphincter of which instinctively 

 or involuntarily yields. 



Nervous Mechanism. The anal sphincter muscle is normally in a 



