TEETH OF REPTILES AND FISHES.

175

like the duck, upon the water-insects, shell-fish, and aquatic plants, that it obtains from the mud, into which it is continually plunging its singular bill; and its jaws, entirely destitute of teeth, are furnished with horny ridges, by which it can in some degree masticate its food.

187. Among Birds, there is an entire absence of teeth; and the mechanical division and the reduction of food is performed in the stomach, in the manner hereafter to be mentioned (§ 200). The mouths of almost all Reptiles, excepting the Turtle tribe, are furnished with numerous teeth (fig. 104); but these are not adapted for much variety of purposes, being principally destined to prevent the escape of the prey which

the animals have secured; Fig. 104.-HEAD OF GAVIAL. (Crocodile and their shape is conse

of the Ganges.)

quently nearly uniform, being for the most part simply conical. There are some Lizards, however, which are herbivorous; and these have large rough teeth, somewhat resembling the molars of Mammalia. The Iguanodon, an animal of this tribe, attained a gigantic size in past ages of the world.

188. In Fishes, the teeth are commonly very numerous (fig. 105), but they have for their object only to separate and retain

Fig. 105.-HEAD OF SHARK.

their food; and there is little variety in their form. Frequently they have no bony attachment, being only held by the gum, as in the Shark; and they are consequently often torn away, but they are as readily replaced. Sometimes, how

176

MASTICATING INSTRUMENTS OF INVERTEBRATA.

ever, the tooth seems like a continuation of the bone of the jaw, not being in any way separated from it, and the tubular structure of the latter being continued into it without any interruption. The teeth of fishes are often set, not only upon the proper jaw-bones, but upon the surface of the palate, and even in the pharynx or swallow.

189. In the Invertebrata there are generally no proper teeth; in the Articulated and sometimes in the Molluscous series, however, we meet with firm horny jaws, which are often furnished with projections that answer the same purpose; and in most Gasteropods we find a very curious organ, commonly designated as the tongue, more correctly the palate, the surface of which is beset with innumerable toothlike points (fig. 106), by whose rasping action the food is reduced. These teeth present great varieties of form and

arrangement in the different genera and species of this group; and these varieties appear to bear some relation to the nature of the food on which the animals respectively live. It is remarkable that in an animal so low in the scale as the Echinus or Sea-Urchin (§ 119), a very complex dental apparatus should exist. This consists of five long hard teeth, which surround the mouth; and these are fixed in a framework which is worked by a powerful set of muscles, and thus serve effectually to grind down the food.

Insalivation.

190. The act of mastication is connected with another; which is also of great importance in preparing for the subsequent process of digestion. This is the blending of the saliva with the food, during its reduction between the teeth, -an act which is termed insalivation. The saliva is separated from the blood, by glands which are situated in the neigh

SECRETION OF SALIVA, AND ITS USES.

177

bourhood of the mouth; of these there are three pair in Man, two beneath the tongue (fig. 107), and one in the cheek, each pouring-in its secretion by a separate canal. The salivary fluid is principally composed of water, in which a small quantity of animal matter and some saline substances (chiefly common salt) are dissolved; the whole amount of these, however, is not more than 1 part in 100. The secretion of saliva is not constantly going on; but the fluid is formed as it is wanted. The stimulus by which the gland is set in action may be simply the motion of the jaws; thus, on first waking in the morning, the mouth is usually dry, but it is soon rendered moist by the movements which take place in speaking. The contact of solid substances with the membrane lining the mouth appears also to excite the flow; hence dryness of the mouth may often be remedied for a time, when no water is at hand, by taking a pebble into its interior, and moving this from side to side. There are certain substances, however, whose presence in the mouth has a special influence in provoking an increased secretion of saliva; and every one knows, too, that the simple idea of savoury food will excite an increased flow, making the "mouth water" as it is popularly termed. These are instances of the power of the nervous system, through which such impressions are conveyed, over the act of secretion.

191. In the case of farinaceous or starchy food, the admixture of saliva occasions the commencement of that chemical change in which its digestion consists, namely, its conversion into sugar; but in general, the benefit derived from this process of insalivation is just that which is obtained by the chemist, when he bruises in a mortar, with a small quantity of fluid, the substances he is about to dissolve in a larger amount of the same. If the preliminary operations of mastication and insalivation be neglected, the stomach has to do the whole of the work of preparation, as well as to accomplish the digestion; thus more is thrown upon it than it is adapted to bear; it becomes over-worked, and manifests its fatigue by not being able to discharge even its own proper duty. Thus the digestive function is seriously impaired, and the general health becomes deranged in consequence. A malady of this kind is very prevalent in the United States; and is almost universally attributed by medical men, in part at least, to the

N

178

DEGLUTITION OR SWALLOWING.

general habit of very rapidly eating or rather "bolting" the meals. There is another evil attendant on this practice,—that much more food is swallowed than is necessary to supply the wants of the system; for the sense of hunger is not so readily abated by food which has not been prepared for digestion; and thus the feeling of satiety is not produced, until the stomach has already received a larger supply than it is well able to dispose of. Imperfect mastication of the food is very apt to occur, in persons who are losing their teeth by old age or decay; and where these are not replaced by artificial means, the next best remedy is to cut the food into very small portions, before it is taken into the mouth, and to masticate it there as thoroughly as possible.

Deglutition.

192. In the Mammalia, the cavity of the mouth is guarded behind by a sort of moveable curtain, which is known as the veil of the palate (fig. 107); and this hangs down during

Fig. 107. PERPENDICULAR SECTION OF THE MOUTH AND THROAT.

mastication, in such a manner as to prevent any of the food from passing backwards. This partition, which does not exist

DEGLUTITION OR SWALLOWING.

179

in Birds and other animals that do not masticate their food, hangs from the arch and sides of the palate, so as to touch the tongue by its lower border; but it can be lifted in such a manner as to give the food free passage beneath it, into the top of the gullet. When mastication is completed, the food is collected on the back of the tongue into a kind of ball 11; and this, being carried backwards by the action of its muscles, presses against the partition just mentioned, and causes it to open. The food thus passes into a sort of funnel, formed by the expansion of the top of the œsophagus or gullet; this cavity, termed the pharynx, communicates above with the nostrils, and in front with the larynx, which is at the top of the trachea or windpipe. The oesophagus is a long and narrow tube, which descends from the pharynx to the stomach, lying just in front of the vertebral column, and behind the heart and lungs. It is surrounded by muscular fibres, disposed in various ways; by the action of which the food that has once passed into the pharynx is propelled downwards to the stomach.

193. But in order to reach this tube, the alimentary ball must pass over the glottis or aperture of the larynx. With a view to prevent its falling-in, the larynx is drawn, in the very act of swallowing, beneath the base of the tongue; and this action presses down a little valve-like flap, the epiglottis, upon the aperture, so as in general effectually to prevent any solid or fluid particles from entering it. But it sometimes happens that, if the breath be drawn-in at the moment of swallowing, a small particle of the food, or a drop of fluid, is drawn into the glottis ; and this action (commonly termed "passing the wrong way,") excites a violent coughing, the object of which is to drive up the particle, and to prevent it from finding its way into the lower part of the windpipe. It may also happen that a larger substance may slip backwards, by its own weight, into the glottis, when there was no intention of swallowing, and when the larynx was consequently not drawn forwards beneath the tongue. The presence of such a substance in the windpipe excites a violent and frequently almost suffocating cough (§ 342); the effect of which is sometimes to drive it up through the glottis, and thus to get rid of the source of irritation.

194. The act of swallowing is itself involuntary, and may