244 HEALTH AND DISEASE 



in 100 parts is very considerable, but when it is dried the total quan- 

 tity of nitrogen is tolerably uniform, being in peas, 4'69; vetches, 5 '57; 

 turnips, 4"76; carrots, 2. It must be remembered that a considerable 

 proportion of nitrogen is contained in other constituents of plants which 

 are not proteids, and that they are not all capable of undergoing digestion. 

 Thus, for example, more than half the nitrogen of lettuces, water-cress, and 

 spinach exists in the form of nitrates, which are useless as flesh-formers; and 

 the same holds with the amides of beet-root, potatoes, and unripe pulse. 



In giving a connected account of the process of digestion in the horse 

 of its ordinary food, hay and oats, the first point to be noted is, that in 

 the mouth these substances are subjected to mastication and insalivation. 

 The large and uneven surfaces of the molar teeth bruise and break down the 

 stems and fruit of cereals, and the seeds of leguminous plants, enabling them 

 to be more readily acted on by the several fluids of the alimentary canal. 

 Simultaneously with this crushing process, the contraction of the muscles 

 moving the jaws, and the stimulus of the sense of taste, causes an abundant 

 secretion of saliva, the quantity having been ascertained by experiment to 

 amount to between 80 and 90 lbs. per diem, most of which is reabsorbed 

 in its further course down the alimentary canal. The saliva softens the 

 food, enables it to be rolled into a coherent mass, and facilitates its 

 deglutition. In addition, it exerts a powerful chemical action on the 

 starchy compounds, for it contains a ferment named ptyalin which has the 

 power of converting starch, through several intermediate stages, first into 

 dextrin and then into a form of sugar named maltose. This action is 

 termed diastatic or amylolytic, and has for its result the chemical union 

 of water with starch, thus changing it from an insoluble substance into a 

 soluble one; from starch, that is to say, which will not pass through an 

 animal membrane and cannot therefore be absorbed, into sugar, which 

 readily permeates the walls of the blood-vessels and Lacteals. The ferment 

 is most abundant, or most active, in the saliva which flows from the parotid 

 gland. It has not indeed been isolated, but it is known to act far more 

 energetically on boiled than on raw starch. The presence of dextrin, mal- 

 tose and a little grape-sugar, after the addition of saliva to boiled starch, is 

 easily demonstrable after a minute or two; whilst with raw starch, especially 

 in the case of wheat starch and potato starch, a much longer time is re- 

 quired. The proteids and oils of the food undergo no change in the mouth. 



Gastric digestion. — Having traversed the (esophagus or gullet, the 

 crushed and moistened food enters the stomach and is immediately sub- 

 jected to the action of the acid gastric juice. This is a clear fluid which is 

 secreted by the innumerable minute glands in response to the stimulus 

 imparted to the mucous membrane by the presence of food. The acidity of 



