148 HUMAN PHYSIOLOGY 



The products of putrefaction, phenol, aromatic oxy-acids, indol 

 and skatol are also absorbed in part, but are excreted by the 

 kidneys (see pages 51, 104 and 136). 



Besides the products of digestion of foods, some of the con- 

 stituents of the digestive fluids are absorbed, e.g. the bile 

 salts, which, arriving at the liver, stimulate the bile secre- 

 tion. The digestive ferments, pepsin and ptyalin, when 

 absorbed, are excreted by the kidneys, while trypsin and 

 steapsin are destroyed in the blood. 



According to another view, the ferments excreted by the 

 kidneys are not derived from the intestine,' but are absorbed, 

 in the form of zymogens, by the blood from the glands 

 between the periods of digestive activity. 



Other mucous membranes, besides those of the stomach 

 and intestine, are able to absorb dissolved substances, but 

 such absorption is of no physiological importance. 



The skin can absorb small quantities of certain substances. 



Subcutaneous injection of foodstuffs. Many foods can 

 be used by the animal economy when, in proper form, they 

 are directly introduced into the tissue fluids. This is true in 

 case of subcutaneous injection of native albumin, fat, or 

 dextrose. 



