42 DIGESTION AND FOOD. 



deoxidation of sugar, which thus supplies the oxygen wanted ; 

 and thirdly, it is also formed by a species of fermentation, 

 along with carbonic acid and hydrogen, the latter being con- 

 verted into water by the oxygen of the blood." (GREGORY.) 



Both in animals and vegetables do the oils exist in vesicles, 

 from which they may be expressed, and appear in the shape 

 of minute globules of various sizes in the field of the 

 microscope. 



Animal fats are divided into two groups those that are 

 saponifiable, and those that are not saponifiable. The first 

 undergo decomposition when in the presence of an alkali, so 

 that the fatty acid set free combines with the latter to form 

 soap, and a base, glycerine, is deposited. It is this change 

 that the saponifiable fats undergo when acted upon by the 

 intestinal juices, which are capable of forming a fatty emul- 

 sion. Pats being themselves insoluble, cannot be absorbed 

 until thus modified by the pancreatic and other secretions. 



Glycerine is a dense, sweet liquid, which does not readily 

 decompose, and, when introduced into the system, acts much 

 like cod-liver oil. 



The crystalline fats met with in animals such as cholesterine 

 and serolin, are developed there by chemical changes, and do 

 not exist in vegetable matter. With regard to cholesterine, 

 there would appear to be some relation between it and the 

 acids of bile. 



NITROGENOUS PRINCIPLES. The third great group of prin- 

 ciples essential in food, because existing as constituents of 

 animal tissue, are the nitrogenous or albuminoid. They 

 exist in both animal and vegetable matter albumen in the 

 first, and gluten in the second, being typical of the class. 



Albumen, fibrine, and caseine are the principal albuminoid 

 substances in animals, and they all contain a definite propor- 

 tion of nitrogen, oxygen, hydrogen, and carbon, so that they 



