Obrázky na stránke

that not less than a hundred are employed every time we breathe : yet we draw our breath every moment, without considering, or even being sensible of the vast and complicated apparatus that is necessary to effect it, The least impediment to our breathing throws us into the greatest distress ; but how little do we value the inestimable blessing, till disease or accident makes us sensible of its enjoyment.

The exquisite and delicate mechanism of different parts of the frame claims our highest admiration ; but our wonder is greatly increased, when we consider, that it performs its different functions for fifty or sixty years together, with very little diminution of its power. What hinge could the most skilful workman contrive, that might be used as often as our elbow-joint is, for so long a term, without being disordered or worn out ? Have we not here a strong proof of the vast superiority of the works of God, to the most ingenious contrivances of man.

Those important faculties of sight and hearing, which are of so much use, and which procure us so many enjoyments, depend upon muscles so extremely small, that they must be magnified to be visible. In the tongue the muscles are very numerous, and so implicated with one another, that the nicest dissectors cannot trace them ; yet they are so arranged, that they never interfere with each other, nor interrupt the various offices of speaking and swallowing. In the other parts of the body, the same admirable economy is preserved. The muscles are every where diffused ; they lie close to each other, in layers, as it were, over one another, after crossing, sometimes passing through, and even imbedded in one another, yet each at perfect liberty to perform its peculiar office, without interrupting the power of its neighbour.

The action of muscles is often required where their situation would be inconvenient. In such a case, the body of the muscle is placed in some commodious position at a distance, and communicates with the point of action by means of slender tendons, or strings, resembling wires. If the muscles, which move the fingers,

had been placed in the palm or back of the hand, they would have enlarged it to a clumsy and very inconvenient thickness. They are, therefore, disposed in the arm, and even up to the elbow; from this position they act by long tendons, strapped down at the wrist . by ligaments, beneath which they pass to the fingers. The same artful

arrangement is observed in the muscles that give motion to the toes, and many of the joints of the foot. Instead of swelling and distorting the foot, they form a graceful enlargement of the calf of the leg. The variety in the figure of the muscles, according to their situation and office, is likewise beautifully contrived : some have double, some triple tendons ; others none: in some places, one tendon belongs to several muscles ; in other places, one muscle to several tendons.

One set of muscles enables us to move a certain part one way, and a different set enables us to move it another way. That we have the power to frown, smile, cough, breathe, to lift up or close our eye-lids, raise or bend our heads, stoop, incline to one side or the other, move our fingers or toes, raise or depress our limbs, walk or sit down, speak, or sing, swallow, open or shut our mouths, or perform any action whatever, we owe to particular muscles, which are appointed to set that part in motion.

Surely no one can be acquainted with the art and wisdom so wonderfully displayed in the structure of the human body, without acknowledging that there is a God, and that the work is his : for nothing short of infinite intelligence, could have produced any thing so complicated and so perfect.


The functions of circulation and of respiration are carried on by means of organs situated in a cavity, which is called the chest, or thorax. The organs which are concerned in the preparation of the food, and in nutrition, lie in a cavity beneath, called the cavity of

the abdomen. The chest is occupied chiefly by the heart and the lungs; the abdomen by the stomach, the intestines, the liver, the spleen, and the pancreas or sweet-bread. These two cavities are separated by a partition, called the diaphragm, or medriff, which is partly of a fleshy, and partly of a membraneous nature, and readily gives way, by its laxity, to the alternate expansion and contraction of the chest in the action of breathing, to which its muscular power eminently contributes. The stomach is connected with the mouth, by means of a long tube, which is called the esophagus, or gullet, by means of which, it receives the food from the mouth.

The first action, to which the food is subjected, is mastication, or chewing, and for this purpose, most animals are provided with teeth. When there are no teeth, other resources are provided in the stomach itself, for that sort of preparation which it is necessary that the food should undergo, previous to digestion. Birds have no teeth; and with various other animals, as fish, and serpents, the teeth seem to be adapted only to prevent the escape of that prey which is swallowed whole.

The nature of the teeth depends on the nature of the food which the animal is designed to use ; namely, whether it is animal, vegetable, or of a mixed nature. By the inspection of the teeth, therefore, we are able to form an opinion as to some of the most material habits of an animal. The teeth which first exhibit themselves, are called milk, deciduous, or temporary teeth, from their being intended to continue only a few years. Those which supply their places when they are shed, are, from their never being shed, called permanent.

The teeth in man are composed of two parts; a bony, which constitutes the body of the tooth, and is very similar to real bone, and a bright, smooth, thin external covering, called the enamel. The part which is out of the jaw, is called the crown and neck : while the fangs, or roots, are planted deep in the jaw. There is a small cavity in the body of the tooth, which descends in the form of a small tube into the fangs, and contains the vessels and nerves, which were employed

in the original formation, and subsequently in the nutrition of the tooth. This is the structure of the teeth in the omnivorous and carnivorous animals ;, but in the graminivorous, the enamel descends into the body of the tooth, and by forming several perpendicular layers, enables the tooth to resist the attrition necessary in mastication ; if there were only one layer of enamel, it would be soon worn off. Between the teeth of the omnivorous, and carnivorous animals there is also a difference. In the carnivorous, the teeth fit into each other very nicely; whereas in the omnivorous, there is a certain latitude of motion permitted, for the operation of grinding the food.

The temporary teeth, in the human race, are twenty in number, and are divided into three kinds ; the front, called also incisors, or cutting teeth, of which there are eight, namely, four in each jaw; the canine teeth, called dog teeth, or cuspidate, which are four in number, one on each side of the incisors, and are of a pointed or conical form; and the grinders, or molares, which amount to eight, being two back teeth, above and below, on each side. The permanent teeth are thirtytwo in number. These are, as in the temporary, eight incisors, and four cuspidate ; two bicuspidate, or twopointed, next to the cuspidate on each side, amounting to eight; and three molares on each side, above and below, making twelve, of which the four hindermost are denominated dentes sapientiæ, or teeth of wisdom, from their not appearing till adult age. The cause of this increase of teeth, is, that there is very great disproportion between the magnitude of the jaw, in the young and adult; and as the teeth, from their nature and mode of growth, do not admit of any increase of size, it was necessary, when the jaw became larger, that not only a supply of larger teeth, but additional teeth should be given.

Many of the carnivorous animals are beasts of prey, and their teeth are part of their natural weapons of attack. The tusks, or canine teeth, are, in such animals, and indeed in some others, as the hog, very formidable instruments of offence.


Cattle and 'sheep, whose front teeth are confined to biting the grass, have them sharp, and the enamel of these teeth covers their outside only, as in man; but neither cattle nor sheep have incisors, in the upper jaw. In horses, where both the front teeth and the molares are employed as grinders, the enamel is distributed through the body of the tooth, in both descriptions of teeth, in the same way as in graminivorous animals.

There is a very curious difference in the disposition of the enamel in the African and Asiatic elephant, which is worth notice. In the African, it is always in the form of transverse lozenges, which touch each other in the middle of the tooth; in the Asiatic, it is in the form of transverse flattened ovals; and this difference is so constant, that it may be always known, by a slight inspection, whether the tooth has belonged to the one or the other of these species.

In the shark, whose teeth are spear-shaped, and very sharp, notched at the edges, and covered with enamel, several ranges of them are formed and continually forming in the jaw, to supply such as are broken or torn away. The same is the case in a species of skate, which has teeth of a similar kind, and is apt to have them injured, by breaking the shells of lobsters, crabs, &c. which are its chief food. There is also, a singular power of renewal in the teeth of venemous serpents. These animals are distinguished by having a sharp, hollow tooth, or fang, in the upper jaw, on each side, the base of which communicates with a poison gland situated below the eye. This tooth, in ordinary eircumstances, lies flat: but it is capable of being erected ; and then, either on biting, or by the action of the same muscles which erect it, the poison gland is pressed upon, and a minute portion of the poison forced through the hole of the tooth into the wound. The poison fang is very apt to get entangled and broken ; but there is a provision for its supply, in the germs of future fangs, which exist as pulp, in little bags in the jaw: the new fangs become ossified, and assume the office of the old


« PredošláPokračovať »