494 OF FOOD, AND THE DIGESTIVE PROCESS. 



which prepares it for the important changes it is subsequently to undergo. 

 The simplest conditions requisite for the accomplishment of these purposes 

 are the following: a fluid capable of performing the solution and of effecting 

 the required chemical changes ; a fluid capable of separating the unorganiz- 

 able matter, by a process analogous to chemical precipitation ; and a cavity 

 or sac, in which these operations may be performed. In the lowest Animals, 

 we find this cavity formed on a very simple plan ; being evidently nothing 

 else than an inversion of the external integument, communicating with the 

 exterior by one orifice only, through which the food is drawn in and the ex- 

 crementitious matter rejected. The fliiid necessary to dissolve the food, which 

 is known by the name of gastric fluid or juice, and that required to separate 

 the portion which is to be thrown off, which is known as the bile, are secreted 

 in the walls of the stomach. In the Sea-Anemone, which affords a very charac- 

 teristic example of this type of structure, it cannot be ascertained that the very 

 rapid solution of food, which takes place in the digestive cavity, is assisted 

 by any movement of its walls. In Polypes of a higher conformation, how- 

 ever, the digestive cavity is provided with a second orifice ; the stomach opens 

 into an intestinal tube, through which the excrement is rejected in little pellets ; 

 and the food, before entering the true digestive cavity, is submitted to a pow- 

 erful gizzard or triturating apparatus. Still, the bile, like the gastric juice, is 

 secreted in the walls of the stomach; as may be distinctly perceived in many 

 of these animals, on account of their transparency, and the bright yellow co- 

 lour of the fluid. As we ascend the animal series, we find no essential change 

 in the character of the digestive apparatus. The biliary follicles are gradu- 

 ally collected into a glandular mass, which is altogether removed from the 

 walls of the stomach, and which pours its secretion into the intestinal tube, 

 at a short distance from its commencement; the gastric juice, however, is still 

 secreted in minute sacs imbedded in the substance of the membrane. Several 

 accessory glands are added, the uses of which are not accurately known; and 

 particular modifications of the apparatus are adapted to peculiarities in the 

 nature of the food, or in the mode of its ingestion. As a general rule it may 

 be stated, that the digestive apparatus is most simple in Carnivorous animals, 

 in which it has to effect little change upon the aliment except solution, in or- 

 der to bring it to the state fit for absorption ; whilst it is most complex in those 

 that feed upon Vegetable matter, which needs to undergo a greater change, 

 both in its chemical composition and in the mechanical arrangement of its 

 components, before it can be rendered subservient to animal nutrition. 



656. Mastication and Deglutition. The first step in the process of reduc- 

 tion, is the Mastication of the food, and the impregnation of its comminuted 

 particles with the Salivary secretion. Mastication is evidently of great im- 

 portance, in preparing the substances to be afterwards operated on, for the 

 action of their solvent; and it exactly corresponds with tire trituration to 

 which the Chemist would submit any solid matter, that he might present it in 

 the most advantageous form to a digestive menstruum. The complete disin- 

 tegration of the alimentary matter, therefore, is of great consequence ; and, if 

 imperfectly effected, the subsequent processes are liable to derangement. This 

 derangement we continually meet with: for there is not, perhaps, a more fre- 

 quent source of Dyspepsia than imperfect mastication, whether resulting from 

 the haste with which the food is swallowed, or from the want of the proper 

 instruments. The disintegration of the food by mechanical reduction, is mani- 

 festly aided by Insalivation : and the admixture of Saliva appears further to 

 have the effect of commencing the transformation of the amylaceous or starchy 

 particles into sugar. From recent experiments it would seem that Saliva, if 

 acidulated, possesses the same power of acting on azotized compounds, as that 

 which characterizes the gastric juice; and consequently, when introduced into 



