OBJECTS OF DIGESTION. 57 



attained. Digestion is not, therefore, to vitalize the food, as the ancients 

 supposed, nor to communicate to it any new or obscure properties ; it is 

 for the purpose of comminuting, subdividing, dissolving, or bringing it 

 into that minutely suspended state that it can without difficulty submit 

 to the absorbing action of the lacteals and veins. There is a complete 

 analogy between this operation and the artificial processes to which the 

 chemist resorts in his laboratory for the solution of various bodies. He, 

 too, uses mechanical implements the mortar and pestle to grind, the ham- 

 mer to crush, the rasp to abrade. When these have carried the subdi- 

 vision sufficiently far, he resorts to acids or other solvents, and thus 

 breaks down the compactness of the hardest minerals, and brings them 

 into the dissolved state. The animal world presents us with a thousand 

 illustrations of the principles here set forth, mechanical contrivances curi- 

 ously arranged. For instance, birds, whose plan of organization is such 

 as to meet the case of locomotion through the air, could not have the an- 

 terior part of their bodies loaded with teeth, accompanied as they must 

 have been with a powerful muscular apparatus. Such a mechanism 

 would have rendered the animal top-heavy, and would have been totally 

 inconsistent with flying. But, to avoid this difficulty, that which might 

 truly be regarded as the mouth is lodged in the interior of the body, nearer 

 the centre of gravity. It is the gizzard. Instinct teaches the bird to 

 swallow small angular stones, and the food, rasped between powerful mus- 

 cular surfaces, is soon brought into a fit condition for the action of the 

 stomach. The chemist, too, puts fragments of glass or of quartz into the 



mortar in which he is conducting the reduction of a tough or resisting 



o o o 



substance. 



The first object of digestion is, therefore, the subdivision of the food. 

 The operation begins in the mouth by a resort to mechanical implements, 

 and when these have carried the process as far as they can, the stomach 

 continues the duty. In its cavity, when in full activity, the temperature 

 is 100 ; a periodically increasing and relaxing motion of revolution is 

 kept up, gastric juice exudes in definite quantity, the hydrochloric and 

 lactic acids exert their action, and in the course of three or four hours 

 a complete reduction is accomplished. 



Allusion has been made to the probability that different portions of 

 the mucous membrane of the stomaeh discharge functions Regional divis- 

 which are wholly distinct, one portion being devoted to the ^iSaSS^' 

 elaboration of pepsin, another to the secretion of hydrochlo- ent functions. 

 ric acid, another to the preparation of a special mucus. This view de- 

 rives considerable support from many facts in comparative physiology. 

 In those cases in which the food approaches, in its mechanical and chem- 

 ical condition, to the form which it is destined to assume as a part of the 

 body of the animal receiving it, the stomach is simple in construction, 



