488 SPECIAL PHYSIOLOGY. 



nitrogenous food, and possibly of the lime salts, but they can scarcely 

 be regarded as possessing positive nutrient properties. Other con- 

 diments, and spices, serve to stimulate the secretion of the digestive 

 fluids, and excite the movements of the alimentary canal. 



In the artificial preparation of food, so as to render it soluble, or 

 more easy of solution, we assist the digestive function itself, which, in 

 adapting nutrient substances, by a series of processes, for absorption 

 into the tissues of the body, has, for its immediate aim, the minute 

 subdivision and the solution of these substances. 



The process of digestion, accordingly, includes certain mechanical 

 and chemical acts. The former have for their object, to triturate and 

 comminute the food, to mix it with fluids and with the various secre- 

 tions in the alimentary canal, to move it within and onwards through 

 the several portions of that canal, and lastly, to expel from the body 

 the unabsorbed residue. The latter are accomplished by the aid of 

 the various digestive fluids poured into the alimentary canal. Con- 

 sidered in the order in which they take place within the body, the 

 several processes necessary to digestion, are mastication, or the chew- 

 ing of the food, and insalivation, or the mixing it with saliva, which 

 occur simultaneously in the mouth ; deglutition or swallowing, in 

 which the food is conveyed through the pharynx arid oesophagus, into 

 the stomach ; gastric digestion, which takes place in the stomach, by 

 aid of the gastric juice, also called chymification, and sometimes, 

 though erroneously, digestion proper, for further true digestive pro- 

 cesses occur in the intestine ; and, lastly, intestinal digestion itself, 

 accomplished by aid of the bile, pancreatic juice, and intestinal juice, 

 immediately preparatory to the proper act of absorption of the di- 

 gested materials, by the lacteals, in which they appear as chyle. 

 Absorption of certain constituents of the food, however, likewise 

 occurs, more or less, through the capillaries of every part of the ali- 

 mentary canal. The residue of the food, or ingesta, together with 

 the unabsorbed secretions, form the egesta, the expulsion of which, 

 constitutes the function of defecation. 



The mechanical and chemical processes of digestion, require sepa- 

 rate and lengthened consideration. 



MECHANICAL PROCESSES OF DIGESTION. 



Mastication and Insalivation. 



The parts concerned in mastication, are the teeth and jaws, the 

 muscles which move the lower jaw upon the upper one, the muscles of 

 the cheeks, the lips, the tongue, and palate. 



The teeth in Man, as in all Mammalia, are developed in two sets ; 

 a first, less numerous, and smaller set, known as the milk, temporary, 

 or deciduous teeth, and a second set, larger and more numerous, called 

 the permanent teeth. 



The milk teeth are twenty in number, ten in each jaw. The five 

 teeth, in either half of each jaw, commencing at the middle line, con- 



