CHARACTERISTICS OF THE ANIMAL BODY 21 



members of the group are collagen, gelatin, chondrin, elastin, and 

 keratin. Collagen is the intercellular substance of connective 

 tissues, and under the name of "ossein" forms a large part of the 

 organic basis of bones. By boiling with water it is readily con- 

 verted into gelatin, which is similar to it. Chondrin is obtained 

 from the ground substance of hyaline cartilage. Elastin is ob- 

 tained from the fibers of elastic tissues. Keratin is the charac- 

 teristic constituent of epidermal structures, such as hair, nails, 

 feathers, and horn. It contains from 3 to 5 per cent, of sulphur, 

 a large portion of which is easily set free as hydrogen sulphid, 

 and will combine with other substances, particularly lead solu- 

 tions, to form the black sulphid of lead, which is used for hair 

 dyes. 



Enzymes are another class of organic substances, probably of a 

 nitrogenous nature. Their mode of action is yet a mystery. They 

 are of supreme importance to the body and will be referred to 

 more in detail when the various organs manufacturing them are 

 considered. 



Other substances which are properly classed as nitrogenous 

 compounds are various pigments of the body, for example, the bile- 

 pigments, the blood-coloring matter, the pigments of the skin, 

 hair, horn, eye, and that found in melanotic tumors so common in 

 white and gray horses. 



Nitrogenous fats, such as lecithin, need but passing mention. 



Amino-acids are simple nitrogenous bodies formed from the 

 proteins of digested foods, and are used by the animal to build 

 up its tissues. 



Urea and uric acid are a group of nitrogenous waste com- 

 pounds found in the excreta of the body. 



(2) The non-nitrogenous substances are fats and various 

 carbohydrates. 



Fats contain the same elements as the carbohydrates, but the 

 oxygen is present in smaller percentage, and not present in the pro- 

 portion to form water. The principal fats present are stearin, 

 palmitin, and olein. In the body fat is found interwoven or inter- 

 mingled with the muscles and the vital organs and in the fatty or 

 adipose tissue lying under the skin. Chemically, fats are esters of 

 glycerol and the fatty acids. The amount of fat in the body varies 

 with the age and condition of the animal. It is useful in forming 

 a covering or insulator to the body and to prevent rapid radiation 



