DIGESTION 2o; 



usually consists of the denser portions of the connective tissue of meats and 

 the cellulose of vegetables and cereals. When the latter are eaten in large 

 amounts the cellulose residue is increased and by its mechanic stimulation 

 increases the peristalsis and hastens the transfer of the feces through the 

 intestine. The decomposition products are derived from protein, fat, and 

 carbohydrate food by bacterial action and include skatol, indol, fat acids, 

 soaps, xanthin, ammonia, sulphuretted hydrogen, etc. The excretion from 

 the intestine itself contributes a considerable portion to the fecal mass. The 

 inorganic salts include phosphates of calcium and magnesium together with 

 various sodium and potassium compounds. 



Defecation. Defecation is the final act of the digestive process and 

 consists in the expulsion of the indigestible residue of the food and its asso- 

 ciated compounds from the intestine. This act usually takes place in the 

 human being but once in twenty-four hours, as the diet contains but a 

 minimum quantity of indigestible matter. Previous to their expulsion the 

 feces which have accumulated in the sigmoid flexure must pass downward 

 into the rectum. In so doing they develop the sensation which leads to the 

 act of defecation. The descent of the feces is accomplished by the peristaltic 

 contraction of the intestinal wall. Coincident with the passage of the feces 

 into the rectum there is a relaxation of the sphincter muscles and a contrac- 

 tion of the longitudinal and circular muscle-fibers, in consequence of which 

 the feces are expelled. These complex muscle actions are also aided by the 

 voluntary contractions of the diaphragm and abdominal muscles. 



Nerve Mechanism of Defecation. The act of defecation is primarily 

 reflex though somewhat influenced by voluntary efforts. The reflex charac- 

 ter of the act is especially noticeable in young children in whom by reason of 

 the imperfect development of the brain there is a lack of volitional control. 

 During the intervals of defecation the anal orifice is tightly closed by the 

 tonic contraction of the internal non-striated sphincter and the external 

 striated sphincter muscles, thus preventing the escape of gases or semi-liquid 

 material. The tonic contraction of both muscles is maintained by the 

 activity of nerve-centers located in the lumbar region of the spinal cord. 

 The circular and longitudinal fibers of the rectum proper are at the same 

 time in a relaxed or inhibited condition, the result of an inhibition, or a want 

 of stimulation, of their governing nerve-center or centers in the lumbar region 

 of the spinal cord. When the desire to evacuate the bowel is experienced, 

 impressions are being made by the feces on the afferent nerve endings in the 

 mucous membrane of the sigmoid flexure and of the rectum. The nerve 

 impulses thus developed are transmitted to the defecation or rectal nerve- 

 centers in the spinal cord and to the cerebrum and influence in one direction 

 or another their activities. 



In the young child the arrival of the transmitted impulses in the spinal 

 cord is immediately followed by an inhibition of the sphincter centers and a 

 stimulation of the rectal muscle centers, as a consequence of which, the 

 sphincter muscles relax and the expulsive muscles contract thus discharging 

 the feces. 



In the adult if the- act of defecation is to be permitted the same mechan- 

 ism is brought into action. In their expulsive efforts, these latter muscles 

 are assisted by the contraction of the diaphragm, abdominal, and other 

 muscles in response to volitional efforts. After the expulsion of the feces, 



