THE DIGESTION OF THE FOODSTUFFS 137 



cellulose, the food inclosed by it can be acted upon by the 

 digestive fluid. 



Concerning the putrefactive decomposition of the constituents 

 of the digestive fluids, the following may be said: dyslysin is 

 formed from cholalic acid; stercobilin, the coloring matter of 

 faeces, from bile pigments. 



5. THE FORMATION OF FAECES AND THE UTILIZA- 

 TION OF FOODSTUFFS 



i. Composition of faeces. The undigested, unabsorbed 

 residue of the food and the useless constituents of the ali- 

 mentary secretion are called faeces and are discharged 

 through the anus. 



The fseces contain : 



1 . Undigested and unabsorbed constituents of the food, 

 such as remains of plants, keratin, nuclein, muscle-fibres, 

 connective tissue, lumps of casein, starch granules, fat, 

 haematin. 



2. Residue of digestive juices, e.g. cholalic acid, dyslysin 

 formed from bile acids, bile pigments, cholesterin, mucin. 



3. Cast-off epithelial cells and their decomposition prod- 

 ucts. 



4. Products of putrefaction: skatol, indol, iron sulphide, 

 fatty acids. Along with the faeces, the gases formed by 

 putrefaction (marsh gas, sulphuretted hydrogen) are dis- 

 charged. 



5 . Mineral matter of the food and of the intestinal secre- 

 tions. 



Finally, there are present in the fseces parasites of various 

 kinds. 



The reaction of the faeces may be neutral, acid, or akaline. 

 Its odor is due to the skatol, indol, and other volatile sub- 

 stances. The color is generally light or dark brown ; the 

 pigment is the stercobilin produced by putrefaction from 

 bilirubin. 



In the large intestine the faxes become solid by the 

 absorption of water, and here it is formed into balls. 



