266 FECES AND DEFECATION. 



a male adult varies from 120 to 150 grams ; this may be increased 

 to 500 grams, if the diet is a vegetable one. About 74 per cent, 

 of feces is water. 



Color of Feces. The color of feces is very much affected 

 by substances ingested. Normally it may be called brown, due to 

 bile-coloring matter, or to hematin derived from the blood-color- 

 ing matter hemoglobin, which occurs in meat, or to both. Large 

 quantities of bread and fat give to the feces a yellowish color. 



Reaction of Feces. The statements on this point are at 

 variance, and indeed the reaction is not always the same. One 

 authority states it to be alkaline on the surface, due to contact 

 with the mucous membrane of the intestine, and acid in the inte- 

 rior; another regards the feces as having an alkaline reaction 

 ordinarily, and as being acid exceptionally. In infants they are 

 said to be acid. 



Composition of Feces. The feces contain such portions 

 of the food as are indigestible, as cellulose, keratin, and chloro- 

 phyl, and will therefore vary in composition according to the 

 composition of the food. In addition there enter into its composi- 

 tion various substances derived from the bile : stercobilin, which 

 represents all that remains of the bile-pigments, and which is the 

 same as urobilin, and hydrobilirubin ; cholesterin, ezcretin, excre- 

 toleic acid, indol, skatol, to the last of which substances the fecal 

 odor is principally due ; volatile fatty acids, calcium or magnesium 

 soaps, mucin, ammonia, sulphuretted hydrogen, carbon dioxid, 

 hydrogen, nitrogen, methan, and numbers of bacteria (Fig. 141). 

 These gases are the result of the decomposition of the proteids by 

 the action of bile. Besides these, the feces may contain the pro- 

 ducts of the excretory action of the epithelium of the gastro-intes- 

 tiual canal (p. 208). 



Meconium. The first feces which are evacuated by the 

 infant at birth are termed meconium. This is dark brownish 

 green in color, acid in reaction, and contains mucin, biliverdin, 

 bilirubin, bile-acids, cholesterin, fats, fatty acids, and the phos- 

 phates and sulphates of calcium and magnesium. 



Defecation. The act of defecation is governed by the ano- 

 spinal center. The mucous membrane and muscular coat of the 

 rectum are supplied with nerves from the several plexuses of 

 spinal nerves. Feces do not ordinarily pass into the rectum until 

 about the time of evacuation, when they are expelled from the 

 sigmoid flexure into the rectum. At this time the sphincter ani 

 is in a state of contraction, which is its usual condition, kept so 

 by the impulses that come from the spinal cord. This contraction 

 keeps the anus closed even during sleep, and is entirely inde- 

 pendent of the action of the brain ; it is an involuntary act. 

 When, however, feces enter the rectum, the nerves of its mucous 

 membrane become stimulated, and impulses are conveyed by 

 afferent nerves to the anospinal center in the lumbar enlargement 



