88 PHYSIOLOGY OF THE DOMESTIC AKIMALS. 



nitrogenous group. Water and various salts belong to the second, or 

 inorganic group. These will be taken up in turn : — 



CONSTITUENTS OF CELLS. 



I. Organic. II- Inorganic. 



"Water and Salts. 



A. Nitrogenous. B. Non-nitrogenous. 



Proteids and 

 their Derivatives. i 



1. Carbo-hydrates. 2. Hydro-carbons. 



Starches and Sugars and Eats and Oils, 



their Derivatives. 



A. NITROGENOUS ORGANIC CELL-CONSTITUENTS — PROTEIDS AND 



THEIR DERIVATIVES. 



General Characteristics of Proteids. — Proteid, or albuminous 

 bodies, is the name given to a number of neutral, nitrogenous products 

 of complex nature widely distributed throughout the animal and vege- 

 table kingdoms, and agreeing more or less in chemical composition and 

 properties with the white of an egg. They are found dissolved in the 

 fluid media of the animal body, as constituents of the digestive juices, 

 and in different degrees of solidity in the various tissues. They are 

 never, during health, eliminated from the body in excretions. They are 

 present during all periods of life. The higher plane of organization of 

 man and the higher animals depends mainly upon the abundance and 

 variety of the albuminous constituents of their tissues; for, while in 

 plants the cell-walls are largely composed of non-nitrogenous matter, 

 such as cellulose, in animals analogous parts are formed of various 

 complex albuminoids. 



Proteids are organic, colloidal bodies, composed of carbon, hydrogen, 

 oxygen, nitrogen, sulphur, and occasionally phosphorus. * They are 

 absolutely essential to life, whether animal or vegetable, but are exclu- 

 sively of vegetable origin ; that is, although they may be assimilated 

 and modified by the vital processes occurring in animal cells, they 

 must first have been preformed by the chemical processes occurring in 

 vegetable cells. "When found as constituents of the tissues of car- 

 nivorous animals they have been derived directly, with but slight 

 modification, from the herbivora which have served for their food, 

 while the herbivorous animals find them invariably ready formed in 

 the tissues of vegetables which serve as their food, and which require 

 but slight modification to be converted into the constituents of the 



