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board; each of these projections is sheathed with enamel, a smooth, dense, glossy substance, altogether insensible, and capable of resisting ordinary violence.
Let us look more narrowly at the teeth, and we shall find they differ from each other in their figure and use; those of the semicircle in the front of the mouth terminate in sharp ridges of bone, and operate like knives to cut substances subjected to their action. At the extremity of these we have, on each side, above and below, a tooth with one powerful point, more resembling a screw, or an awl, than a knife; the remaining teeth are furnished each with several points on their upper surface, and as the jaws are not immediately nor immoveably opposite each other, the lower jaw only is capable of motion, and the upper being immoveable, we perceive the action of the teeth to be that of the grinders of a mill: the mouthful of bread, for instance, is cut by the first teeth; the crust is pierced by the second description of teeth; and the whole is reduced to a uniform mass, of the smallest particles, by the remaining teeth. But how comes it to pass that, when thus triturated, we are not choked by the small particles escaping into the windpipe? To prevent this, and to assist us to swallow the morsel, Providence has furnished the mouth with reservoirs of saliva. Each cheek has a gland, whose office it is to secrete the liquor; and there is a large gland of the same description under the tongue. Now it does so happen that the very same action of the muscles which brings the jaws into play and grinds the food compresses these glands, and forces thence a regular and adequate supply of saliva. This action of the muscles is necessary to the production of this effect: the channel communicating between the gland and the mouth is an oblique or slanting one; and hence it is
that we are not troubled with saliva except when we have occasion for it. The saliva mixes fully with the pounded mass, and the whole is now ready to be thrown into the throat; but still there are many difficulties in the way, and many dangers to be provided against.
How is it possible to urge the morsel forward step by step till it shall arrive at the stomach? and how is its passage effected, so as to escape the orifice of the windpipe, through which the momentary supplies of air are alone to be obtained? To answer the latter of these questions first:
In the passage of the morsel into the gullet, if it fell by accident into the windpipe, the most serious consequences, even death, might ensue; for if that were blocked up the result is obvious. It is wisely provided that the outlet of the windpipe is furnished with a valve, which opens backward on the mouth; the morsel of food by the raising of the tongue is urged into the throat, and on its way presses down this valve or lid, which opens not till the danger has past. It has now arrived in the gullet, a tube composed of circular and longitudinal muscular fibres, which, like all muscles, have the power of contracting: the fibres which run lengthways shorten the gullet, and those which surround it contract it, and the contents fall down, just as when we thrust any substance into a sack by lifting it up and contracting it.
The morsel arrives at its receptacle, the stomach, and unless when appetite is wanting, it begins immediately to be acted upon. Appetite is probably created by a liquid, shed by the coats of the stomach on its inner surface, which is called the gastric juice. So powerful is its action that hardly any thing can resist dissolution; as has been proved by the experiments made upon the
living bodies of different animals. This is peculiarly the case in some birds, and in beasts of prey. An ingenious physician of Italy found that even iron was dissolved in the stomach of a bird; and gold itself cannot altogether escape solution. It is impossible to institute experiments out of the body, for not to mention that it ceases to be effused upon the death of the animal, it is always diluted and mixed with the pancreatic juice— a something resembling saliva, furnished by a gland lying in contact with the stomach. Though the gastric juice ceases with life, yet in the case of sudden death, and especially in children, it has been known that it has eaten holes in the stomach; and these corrosions are distinguished from the effect of inflammation by the absence of all turgescence in the vessels, by the previous history of the case, as well as by their always occurring in that part of the stomach which happens to have been lowermost in consequence of the position of the deceased. By the heat and the action of the stomach the food is changed into a substance called chyme, of a pulpy consistence. Reduced to this state, it is urged onward, by a motion peculiar to the stomach and the whole intestinal canal, called the peristaltic motion. This is a worm-like undulation of the whole tube, which slowly urges the chyme into the duodenum, the first portion of the intestine; there it is mixed with the pancreatic juice, and with the bile, the whole of the former and the thinner part of the latter mixing thoroughly with it, and now it is changed into a white fluid and called chyle. It flows onward with a very slow and languid current, and now mark the wonderworking hand of God; there are innumerable vessels whose mouths open on the inner surface of the intestine, and these imbibe this precious stream; they unite
together into one vessel which lies defended by the column of bone which sustains the body-the back bone-and instantly the vessel begins to ascend upward. The motion of its contents is assisted by the serpent-like twistings of the course of the vessel. Rising in a zigzag curve, and defended by its remote situation from the front of the body, and sheltered by the vertebræ behind, it proceeds upward and upward till it arrives at the neck, it there bends horizontally and empties itself into the jugular vein of the left side, as that vessel is carrying back its blood to the heart.*
The veins are those vessels which convey the blood back from all parts of the body to the heart. The arteries are vessels which contain a brighter kind of blood, and which convey it from the heart to all parts of the body. The vein, which has received the milklike fluid, carries it to the right side of the heart, where it and the blood enter at once: from the opening which receives them they are urged forward by a sudden contraction of the heart, which is in fact a forcing pump, into the inner cavity. This sudden contraction gives way in an instant, a new gush of blood flows in and is urged out, and this is called the beating of the heart, which all the arteries in every part of the body have in a less or greater degree, and which in them is called the pulse. From the inner cavity of the right side of the heart a large artery goes out, which carries in succession all the blood of the body into the lungs. The lungs are a large quantity of air-cells, and may be
*The following part of this essay was left in an unfinished state by its lamented author.
compared to a honeycomb. But how does the air enter the lungs ?-Through the windpipe from the
The artery of the lungs branches into a thousand minute vessels-and the windpipe, conveying the air, branches into almost an equal number; the blood in the small vessels is only separated from the air by their thin covering like net-work or silk; the air passes through into the blood, and the corrupted air is pressed out of the blood into the windpipe, and by it is conveyed 'out of the body. Great changes are effected in the blood by means of this apparatus :
1st. The colour is altered from what it was before. It came back to the heart dark-coloured and corrupted, deprived in a measure of its vital principle. It becomes in the lungs of a bright red colour; it is a pure stream, and ready to be sent again in its purified state into all parts of the body.
2. The milk-like fluid, or chyle, supplies the waste which the mass of the blood had sustained; pure air or oxygen is received from the windpipe, or air-cells proceeding from it, and it no longer retains its white colour or separate character, but is intimately and inseparably mixed with the rest of the blood, and constitutes one mass with it. Smaller vessels unite into larger, and the whole of the purple life-stream, now purified, is brought back from the steam engine to the forcing pump -the heart; or, as Solomon calls it, "the fountain" and "wheel at the cistern." The blood enters the left side, and from the receptacle which receives it, it is urged into the inner part of the left side of the heart. A curious mechanism is to be found in the heart, to prevent the blood from flowing in any but its right direction. This consists of valves of flesh, which only open