THE COMPOSITION OP ANIMALS 21 



is over two-thirds fat and less than one-fourth water, and protein 

 makes up only about 7 per cent of the increase. It is easy to under- 

 stand,, in view of these figures, why fat and older animals can com- 

 mand higher prices than young or lean animals, and why the per- 

 centage dressed weight of cattle, for example, is higher in the case 

 of the former kind of animals than with the latter. 1 



Components of the Animal Body. We shall now briefly con- 

 sider the chemical composition and main characteristics of the 

 various groups of components found in the animal body. 



Protein. Protein substances are found in all parts of the ani- 

 mal body, in the blood, lymph, muscles, connective tissues, milk, etc. 



The blood is the vehicle by which the digested and absorbed 

 nutrients are distributed throughout the body, and which supplies 

 its different parts with the substances necessary for growth and 

 the exercise of vital functions. Blood makes up about 8 per cent 

 of the body weight of horses, cattle, and sheep, and less than 

 5 per cent of that of the pig. It is composed of a liquid portion 

 called plasma, in which the blood-cells or corpuscles are suspended. 

 The plasma makes up about two-thirds of the blood; it contains 

 three protein substances in solution, viz., fibrinogen, serum globulin, 

 and serum albumen. On clotting of the blood or when it is whipped, 

 the fibrinogen is changed, through a special ferment called thrombin, 

 into fibrin, which entangles the blood-corpuscles and holds them in 

 a solid clot. The liquid that separates from clotted blood on stand- 

 ing is called blood-serum. 



There are two kinds of blood-corpuscles, red and white. The 

 red corpuscles are minute, round discs, that vary in shape and size 

 in different animals. They are composed of a spongy albuminoid 

 substance which holds in its meshes the red coloring matter called 

 haemoglobin. This is a very complex protein substance and con- 

 tains about one-half of one per cent of iron, in addition to the ordi- 

 nary components of protein. Haemoglobin is a dark, purplish red, 

 crystalline substance which has great affinity for oxygen. It absorbs 

 oxygen in the lungs, forming oxyhaemoglobin ; this again readily 

 gives up its oxygen in the cells of the different body tissues when 

 the oxidation (combustion) of nutrients takes place. The chemical 

 changes that occur in the cells and are necessary for the continu- 

 ance of life and for growth are dependent on this supply of oxygen 

 and on the nutrients which are carried to the different parts of the 

 body by the blood. 



1 See live weight and dressed weight of steers of different breeds and 

 ages, Woll, " Handbook for Farmers and Dairymen," 6th ed., p. 206. 



