102 i^TiTLDN OF FARM ANIMALS 



sparingly s-X : While small amounts of soluble proteins 

 and somewhat larger quantities of soluble carbohydrates occur, 

 they ordinarily play but a subordinate role in nutrition. One 

 obvious result of the chemical changes brought about by the 

 enzyms and organized ferments of the digestive tract is to 

 convert these insoluble substances into soluble ones. Thus 

 starch yields sugar, cellulose the organic acids, fats form 

 soaps and protein yields peptones and amino acids. It was 

 natural, therefore, that digestion should be looked upon as 

 a process of solution and compared to the preparation of 

 extracts in a pharmaceutical laboratory by means of various 

 solvents. 



The solvent action of the digestive juices is important, since 

 the animal, like the plant, absorbs its real food substances 

 substantially in aqueous solution. The mere dissolving of the 

 ingredients of the feeds, however, is far from being the only or 

 even the chief function of the digestive juices, as is clearly 

 indicated, for example, by the existence of a coagulating enzym 

 like chymosin, which precipitates the soluble casein, or the pres- 

 ence of the various invertases, which attack substances already 

 soluble. 



145. Colloids converted into crystalloids. The principal 

 nutrients belong to the class of substances called colloids. 

 Gelatin is a typical colloid as are, indeed, all the proteins and 

 the more abundant carbohydrates, while the sugars, organic 

 acids, etc., are classed as crystalloids. 



As related to digestion, the most important distinction be- 

 tween colloids and crystalloids is the difference in the osmotic 

 pressures of their solutions by virtue of which crystalloids 

 diffuse readily through permeable membranes. This diffusi- 

 bility plays an important part in the resorption of the digested 

 material into the blood and lymph current, as will appear in the 

 next section, although it is by no means the only factor con- 

 cerned. 



A review of the chemical changes which take place in diges- 

 tion shows that they are all in the direction of molecular simplifi- 

 cation. They are substantially processes of cleavage by which 

 large molecules are split into two or more smaller ones. Such 

 a change, however, is in the direction from the colloid to the 

 crystalloid condition. The final products of digestion are 



