566 SPECIAL PHYSIOLOGY. 



salts formed bj such organic acids with soda or potash, are either 

 absorbed into the blood, and there converted into carbonates, or they 

 are thus changed in the intestinal canal. 



Water remains undecomposed, and is absorbed freely during the 

 digestive process, constituting the natural menstruum, in which the 

 different soluble substances are dissolved, and in which the fatty mat- 

 ters are suspended. 



Of substances the digestion of which is doubtful, may be mentioned 

 vegetable mucus, gums, pectin, and cellulose. The three former, 

 though soft and tender substances, miscible but probably not actually 

 soluble in water, are said by some, indeed, neither to be capable of 

 being absorbed, nor yet to be so chemically changed as to become so. 

 The softer kinds of cellulose, such as that contained in the growing 

 tissues of green vegetables, in the tuber of the potato, and in the pulp 

 of fruits, are supposed to be dissolved in small quantity, if not for 

 nutrient purposes, yet in order to set free their starchy, gummy, sac- 

 charine, and albuminoid contents. Herbivorous animals, however, 

 certainly digest large quantities of cellulose and vegetable pectin, by 

 changing them into sugar. Chlorophyll, speaking generally, is indi- 

 gestible. Though putrescent meat, such as high game, may be first 

 sweetened, and then digested by the gastric juice, yet certain decom- 

 posing substances, like poisonous or fermenting sausages, cannot be 

 corrected by the juices of the stomach, but excite vomiting and diar- 

 rhoea, and, when absorbed, often prove fatal. 



Circumstances which Modify Digestion. 



The rate of gastric digestion of certain articles of diet has already 

 been mentioned (p. 552). It partly depends on the relative solubility 

 of the various proximate constituents of the food ; but it may also be 

 greatly modified by other circumstances, such as the quantity, consist- 

 ence, and peculiar mixtures of the food, its condition of subdivision, 

 its absolute quantity, the relative quantity of its different constituents, 

 the absence or presence of stimulating substances, the conditions of 

 the nervous system, the state of sleeping or waking, the condition of 

 the body as regards health, habit, individual peculiarities, bodily 

 fatigue, and even the condition of the mind. Rest arid exercise also 

 affect the digestive process. 



The more rapidly and perfectly the constituents of any given kind 

 of food are capable of being dissolved, the more easily such food is 

 digested, and vice versa. As a rule, bread, not too new, nearly all 

 kinds of meat, poultry, and white fish, eggs, milk, jelly, and the gelatin- 

 forming tissues, and well-boiled potatoes, are easy of digestion ; whilst 

 new bread and potatoes, fatty meats, fat, tendons, cartilage, cheese, 

 and green vegetables, are more difficult of digestion. Hard-boiled eggs 

 are, of course, more difficult to digest than the fine coagulum of albu- 

 men formed in a custard, or in the gravy of meat, owing obviously 

 to the difference of consistence and degree of subdivision in the two 

 cases. Mashed potatoes and finely grated cheese, and soft cream- and 

 milk-cheeses, are more easily and rapidly digested than plain boiled 



