ANIMALS. 115 



from plants, are water, oxygen from the air, and small 

 quantities of mineral matter, such as salt, lime, pot- 

 ash, etc., with which animals must be supplied when 

 these are not furnished in sufficient quantity in the regu' 

 lar food. 



The process of animal life consists in converting the 

 substances of plants into the substances of the animal 

 body, or in making them serve the different wants of the 

 animal. 



The substances forming the animal body may be divided 

 into four classes : 



1. Water. Water is generally the largest ingredient, 

 comprising from one third to two thirds of the entire 

 weight. It is essential to plants, furnishing a medium 

 through which the elements of food may be distributed. 

 In animals, it fills a similar office. It forms four fifths 

 of the blood, and exists in the juices throughout all parts 

 of the body. 



2. Nitrogenous Substances. Nitrogenous substances, 

 or substances containing nitrogen, are of first impor- 

 tance in the animal body. They make up the muscular 

 tissue, or lean meat, the nerves, the skin, hair, wool, 

 feathers, horns, etc. They also form a large part of the 

 solid matter in the blood. 



3. Fat. Fat does not fill so important an office as 

 the nitrogenous substances. It does not make up the 

 tissues and other essential parts of the body. It is al- 

 ways present, however, in greater or less quantity. Par- 

 ticles of fat are to be found scattered between the fibers 

 of the muscles. In well-fed animals fat is also stored up 

 in large quantities beneath the skin, and about the bones 

 and internal organs. 



There are several varieties of animal fats, as stearin, 



