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 i 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 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 arc 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.1
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 arc 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 manlier. 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 &esophagus, while another set of mils cks causes the epiglottis to close the passage into the larynx. The muscular fibres of the (esophagus itself are now brought into play, and by their successive contraction, propel the food from the upper to the lower part of the tube, and thus convey it to its final destination. These three stages, which altogether constitute a very complicated train of actions, are so connected with each other, that the operation appears to be of the most simple kind ; it is one of the first that is performed by the newly born animal, and is exercised during the whole period of existence with the most perfect facility.* The food, after having thus experienced the action of the first order of parts, which, as we have seen above, is principally, if not entirely, of a mechanical nature, is finally deposited in the stomach. The stomach is a bag of an irregular oval form, which lies obliquely across the upper part of the abdomen, in what is termed, from the presence of this organ, the epigastric region. The structure of the sto mach, considered in its physiological relation, is threefold. A large portion of it is composed of membranous matter, which gives it its ge neral form, determines its bulk, and connects it with the neighbouring parts, constituting its external coat. To the interior surface of this coat are attached a number of muscular fibres, by which the various contractile actions of the stomach are performed ; these, although not capable of being exhibited as a connected or continuous structure, are considered, accord ing to the custom of the anatomists, as com posing the muscular coat, while its internal coat consists of a mucous membrane, which appears to be the immediate seat of the se creting glands, from which the stomach de rives its appropriate fluids. But besides this, which may be regarded as the physiological structure of the stomach, by which its parts are so arranged as to give the organ its form and position, its contractile power, and its chemical action, the anatomists have resolved it into a greater number of mechanical divisions, depending principally upon the minuteness to which they have carried their dissections. In this way no less than six or even eight distinct strata or coats have been assigned to the sto mach. First, the peritoneal covering, which it has iu common with all the other abdominal viscera, the dense membrane which more especially gives the stomach its form, called in the language of the older writers the ner vous coat, two muscular coats,l- one composed of longitudinal and the other of circular fibres, and the innermost, or, as it has been termed, the villous coat, together with three cellular coats, which are situated between the former and connect them with each other. The ner vous coat is usually described as being the seat of the glands, as well as of the bloodvessels, nerves, and absorbents which belong stomach ; but although they cannot perhaps be actually traced beyond this part, there is some reason to suppose that their ultimate destination is on the innermost or villous coat.