DIGESTION. 



following pages our main object will be to give 

 an account of the function of digestion as it is 

 exercised in man and in those animals which 

 the most nearly resemble him, referring to 

 other animals only so far as it may contribute 

 to illustrate or explain the nature of the ope- 

 ration in the human species. 



In the various divisions of the Mammalia 

 the first order of parts may be arranged under 

 the five heads of the mouth with its muscular 

 appendages, the teeth, the salivary glands, 

 the pharynx, and the oesophagus. With the 

 exception of the salivary glands, the effect 

 of these organs is entirely mechanical ; it con- 

 sists in the prehension, the mastication, and 

 the deglutition of the aliment. The first of 

 these organs may be again subdivided into 

 three parts, the lips, the cheeks, and the 

 tongue; the lips being more immediately 

 adapted for seizing and retaining the food, 

 and the others for conveying it, in the first 

 instance, to the teeth, for the purpose of mas- 

 tication, and afterwards to the pharynx, in 

 order that it may be swallowed. In this, as 

 in every other part of the animal frame, we 

 perceive that adaptation of the structure of 

 each individual organ to the general habits of 

 the animal, which 'forms a constant subject of 

 delight and admiration to the anatomist and 

 the physiologist. In animals that feed upon 

 succulent and luxuriant herbage the lips are 

 capacious, strong, and pendulous, for the pur- 

 pose of grasping and detaching their food, 

 while in those that employ an animal diet, 

 where their prey is to be seized and divided 

 principally by means of the teeth, the lips are 

 thin, membranous, and retractile. Again in 

 the muscles that are connected with the cheeks, 

 we find the same adaptation, although perhaps 

 not in so obvious a degree. We observe that 

 animals who receive large quantities of food, 

 either in consequence of its being of a less 

 nutritive nature, or from any other peculiarity 

 in their habits and organization, as well as 

 those whose food is of a harder consistence 

 and firmer texture, have larger and more 

 powerful muscles, both for the purpose of 

 moving the jaws with greater force, and for 

 acting upon the larger mass of matter which is 

 taken into the mouth. 



The principle of adaptation is still more 

 remarkable in the teeth. Among the different 

 orders which compose the Mammalia, we ob- 

 serve a general analogy and resemblance be- 

 tween the teeth, both as to their number, form, 

 and relative position, while, at the same time, 

 there is so great a diversity in the different 

 tribes of animals, that some of the most dis- 

 tinguished naturalists have regarded these 

 organs as the parts the best adapted for form- 

 ing the basis of their systematic arrangements, 

 inasmuch as they afford the most characteristic 

 marks of the habits of the animals, and of the 

 peculiarities of their other functions.* Thus, 

 by an inspection of the teeth we can at once 

 discover whether the individual is intended to 



* Linnaeus, Sys. Nat, t. i. p. 16 et alibi j 

 Shaw's Zool. v. i. Introd, p. vii et alibi. 



employ animal or vegetable food, some of them 

 being obviously adapted for seizing and lace- 

 rating the animals which they acquire in the 

 chace or by combat, while the teeth of others 

 are obviously formed for the cropping of vege- 

 tables, and for breaking down and triturating 

 the tough and rigid parts of which they prin- 

 cipally consist. It is with a view to this dou- 

 ble purpose of prehension and mastication that 

 the great division of the teeth into the incisors 

 and the molares, the cutting and the grinding 

 teeth, depends, the former being of course 

 situated in the front of the mouth, the latter 

 in the sides of the jaws. The chemical com- 

 position and mechanical texture of the teeth is 

 no less adapted to their office of dividing and 

 comminuting the food than their figure and 

 position. They are composed of nearly the 

 same materials with the bones generally, but 

 their texture is considerably more dense and 

 compact, while they are covered with an ena- 

 mel of so peculiarly firm a consistence, as to 

 enable them, in many kinds of animals, to 

 break down and pulverize even the hardest 

 bones of other animals, and to reduce them to 

 a state in which they may be swallowed, and 

 received by the stomach, in the condition the 

 best adapted for being acted upon by the gastric 

 juice.* 



At the same time that the alimentary matter 

 is subjected to the mechanical action of the 

 teeth, it is mixed with the fluids that are dis- 

 charged from the salivary and mucous glands, 

 which are situated in various parts of the 

 mouth. The use of the saliva is to soften the 

 food, and thus render it more easily masti- 

 cated, to facilitate its passage along the pha- 

 rynx and oesophagus, and perhaps, by a certain 

 chemical action, to prepare it for the change 

 which it is afterwards to experience, when it is 

 received into the stomach .f 



The food, after it has been sufficiently di- 

 vided by the teeth, and incorporated with the 

 saliva, is transmitted, by the act of deglutition, 

 into the stomach. There is perhaps no part 

 of the system, which exhibits a more perfect 

 specimen of animal mechanism than the pro- 

 cess of deglutition. It consists in the succes- 

 sive contraction of various muscles, that are 

 connected with the contiguous parts, each of 

 which contributes to form a series of mecha- 

 nical actions, which, when connected with 

 each other, effect the ultimate object in the 

 most complete manner. The muscles of the 

 mouth and the tongue first mould the mas- 

 ticated aliment into the proper form, and trans- 

 mit it to the pharynx ; this part is, at the same 

 time, by the cooperation of other muscles, 

 placed in the most suitable position for re- 

 ceiving the alimentary mass, and transmitting 

 it to the oesophagus, while another set of mus- 



* Hatchett, in Phil. Trans, for 1799, p. 328-9; 

 JJerzelius, View of Animal Chemistry, p. 78; 

 Pepys, in Fox on the Teeth, p. 92 et seq ; Turner's 

 Chemistry, p. 1012. 



t For the opinions that were entertained by the 

 older physiologists on this point the reader is re- 

 ferred to Baglivi, Diss. 2, circa salivam, op. 

 p. 412 et seq.; also to Mailer, El. Phys. 18. 2. 13. 



