XIII. 



THE ANIMAL 



273 



Carbon. 

 54-3 



Hydrogen. 

 7-2 



Nitrogen. Oxygen. 



16-7 21-8 per cent. 



(2) Collagen, an insoluble body yielding gelatine by long boiling 

 with water. It contains about 0*6 per cent sulphur. 

 Hammarsten gives the following : 



On decomposition, it yields albuminous substances and a large 

 quantity of glycocoll [amino-acetic acid, CH 2 (NH 2 ).COOH], some- 

 times called sugar of glue. 



(3) Keratin. This is the chief constituent of horns, hoofs, skin, 

 feathers, hair, wool and nails. Obtained from different sources, it shows 

 a variation in composition ; in general, it resembles collagen or gelatine, 

 but differs from those substances in containing much more sulphur (4 

 or 5 per cent), part of which is in a very loose state of combination 

 and may be removed by alkalies or even by boiling with water. 

 Keratin is insoluble in water, alcohol, or ether, but can be dissolved 

 by heating with water to 150 or 200 under pressure. 



DIGESTION. The food of an animal is rarely in a form capable 

 of direct absorption into the system. Before it can be taken into the 

 blood stream and utilised in the body, it is usually necessary that 

 certain chemical changes should be produced so as to render its con- 

 stituents soluble and diffusible. These changes are brought about by 

 the process of digestion, which consists mainly in breaking down in- 

 soluble, complex carbon compounds into simpler, soluble substances, 

 chiefly by the aid of enzymes or unorganised ferments. Digestion is 

 accomplished partly by mechanical, partly by chemical means. 



The process commences with mastication the food is submitted 

 to a comminuting action by the teeth and tongue and at the same 

 time is mixed with saliva. This is a very dilute solution, of faint 

 alkaline reaction, containing various substances, secreted by special 

 glands and poured into the mouth. It has been estimated that an ox 

 secretes 112 Ib. of saliva daily. The results of investigations as to 

 the composition of saliva show it to contain only from 0'4 to 1 per cent 

 of solid matter. In human saliva, potassium thiocyanate is usually 

 present, its average amount being probably about -01 per cent. 



Alkaline chlorides, phosphates and sulphates are also present, 

 together with mucin, a proteid body of slimy consistency. Of special 

 importance is the characteristic enzyme of the saliva ptyalin or 

 salivary diastase. This enzyme, which works most rapidly at about 

 30 and is destroyed at about 65 or 70, has powers similar to those 

 of plant diastase, i.e., it converts starch, first into soluble starch, next 

 into dextrin, and finally into maltose. The conversion of the starch 

 of food into sugar is commenced, but not completed, in the mouth. 



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