CHARACTEB1STICS OF ANIMALS 21 



Proteins comprise a large part of the solid matter of the muscles, 

 tendons, blood, skin, etc. The most plentiful body protein is 

 myosin, which is found in the muscles. Proteins are the sub- 

 stances used by the animal for growth. They are mostly colloidal 

 in character, only a very few having been obtained in crystalline 

 form, so do not diffuse through an animal membrane. Chemists 

 have not been able to synthetize any but the very simple pro- 

 teins, This undoubtedly is because the protein molecule is of 

 such great size and complextiy that during life it is capable of a 

 wide variety of combinations and readjustments. Carbonic 

 acid, water, urea, and uric acid are the end-products of protein 

 decomposition, but many intermediate substances, such as the 

 cleavage products, glycin and leucin, occur during the breaking- 

 down process. 



Albuminoids are closely allied to the proteins, but differ from 

 them by being insoluble in all neutral solvents. The best-known 

 •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, collagen is readily 

 converted into gelatin, the familiar jelly-like substance used so 

 generally. Chondrin is obtained from the ground substance 

 of hyaline cartilage. Elastin is obtained from the fibers of elastic 

 tissues. Keratin is the characteristic 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 sub- 

 stances, particularly lead solutions, to form the black sulphid of 

 lead, which is used for hair dyes. When these keratin-containing 

 structures are burning, their sulphur is liberated and produces 

 the characteristic disagreeable odor. 



Enzymes are complex organic compounds capable of producing, 

 by catalytic action, the transformation of some other compound. 

 They are peculiar in that they remain unchanged though used 

 over and over again, act best at body temperature, are inhibited 

 by cold, are the product of glands, cannot be isolated. The 

 specific reactions of the various enzymes will be mentioned when 

 the organs manufacturing them are considered. Well known 

 enzymes are the ptyalin of saliva and the pepsin of gastric 

 juice. 



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