CONSTITUENTS OF ANIMALS 106 



These contain carbon, hydrogen and oxygen ; the 

 nitrogenous substances contain nitrogen, and generally 

 a little sulphur, in addition. 



The nitrogenous substances constituting the animal 

 frame may be generally classed as — (1) albuminoids 

 (proteids) ; (2) gelatinoids ; and (3) horny matter. 

 These three groups are related in composition, though 

 differing a good deal in their properties. The albu- 

 minoids form the substance of animal muscle and 

 nerve, and the greater part of the solid matter of 

 blood ; they are, undoubtedly of the first importance 

 in the animal economy. The gelatinoids form the 

 substance of skin and sinew, of all connective tissue, 

 and also the combustible matter of cartilage and bone. 

 Plorny matter, named by chemists keratin, is the 

 material of which horn, hair, wool, and feathers are 

 constituted. The whole of these nitrogenous bodies 

 contain very similar amounts of nitrogen — namely, 

 15 — 18 per cent. Besides the nitrogenous matters 

 constituting tissue, the animal juices contain a variety 

 of nitrogenous substances, as sarcine, creatine, &c., 

 with which we are not immediately concerned. 



The fats occurring in the animal body are princi- 

 pally stearin, palmitin, and olein. Stearin prepon- 

 derates in hard fats, and olein in fluid fats. 



Of the incomhustihle constituents by far the largest 

 part is contained in the bones. In fat animals 75— 85 

 per cent, of the total ash constituents are found in 

 the bones. Bone ash chiefly consists of phosphate of 

 calcium, with a small quantity of cai-bonate of calcium 

 and phosphate of magnesium. In muscle by far the 

 most abundant ash constituent is phosphate of potas- 

 sium. Potassium salts are also abundant in the 



