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he takes vibrate to the interior of the head, and jar on the brain.
SCIENTIFIC USE OF COMPARATIVE ANATOMY.
Suppose a man ignorant of anatomy to pick up a bone in an unexplored country, he learns nothing except that some animal has lived and died there; but the anatomist can, by that single bone, estimate, not merely the size of the animal, as well as if he saw the print of its foot, but the form and joints of the skeleton, the structure of its jaws and teeth, the nature of its food, and its internal economy. This, to one ignorant of the subject, must appear wonderful, but it is after this manner that the anatomist proceeds. Let us suppose that he has taken up that portion of bone in the limb of the quadruped which corresponds to the human wrist; and that he finds that the form of the bone does not admit of free motion in various directions, like the paw of the carnivorous creature. It is obvious, by the structure of the part, that the limb must have been merely for supporting the animal, and for progression, and not for seizing prey. This leads him to the fact that there were no bones resembling those of the hand and fingers, or those of the claws of the tiger ; for the motions which that conformation of bones permits in the paw would be useless without the rotation of the wrist-he concludes that these bones were formed in one mass, like the cannon-bone, pastern-bone, and coffin-bones of the horse's foot.1
The motion limited to flexion and extension of the foot of a hoofed animal implies the absence of a collar-bone and a restrained motion in the shoulder-joint; and thus the naturalist, from the specimen in his hand, has got a perfect notion of all the bones of the anterior extremity. The motions of the extremities imply a condition of the spine which unites them. Each bone of the spine will have that form which permits the bounding of the stag, or the galloping of the horse, but it will not have that form of joining which admits the turning or writhing of the spine, as in the leopard or the tiger.
And now he comes to the head :-The teeth of a carnivor1 For these are solid bones, where it is difficult to recognise any resemblance to the carpus, metacarpus, and bones of the fingers ; and yet comparative anatomy proves that these moveable bones are of the same class with those in the solid hoof of the belluce of
ous animal, he says, would be useless to rend prey, unless there were claws to hold it, and a mobility of the extremities like the hand to grasp it. He considers, therefore, that the teeth must have been for bruising herbs, and the back teeth for grinding. The socketing of these teeth in the jaw gives a peculiar form to these bones, and the muscles which move them are also peculiar; in short, he forms a conception of the shape of the skull. From this point he may set out anew, for, by the form of the teeth, he ascertains the nature of the stomach, the length of the intestines, and all the peculiarities which mark a vegetable feeder.
Thus the whole parts of the animal system are so connected with one another, that from one single bone or fragment of bone, be it of the jaw, or of the spine, or of the extremity, a really accurate conception of the shape, motions, and habits of the animal may be formed.
It will readily be understood that the same process of reasoning will ascertain, from a small portion of a skeleton, the existence of a carnivorous animal, or of a fowl, or of a bat, or of a lizard, or of a fish ; and what a conviction is here brought home to us of the extent of that plan which adapts the members of every creature to its proper office, and yet exhibits a system extending through the whole range of animated beings, whose motions are conducted by the operations of muscles and bones!
After all, this is but a part of the wonders disclosed through the knowledge of a thing so despised as a fragment of bone. It carries us into another science; since the knowledge of the skeleton not only teaches us the classification of creatures now alive, but affords proofs of the former existence of animated beings which are not now to be found on the surface of the earth. We are thus led to an unexpected conclusion from such premises : not merely the existence of an individual animal, or race of animals, but even the changes which the globe itself has undergone in times before all existing records, and before the creation of human beings to inhabit the earth, are opened to our contemplation.
A joint consists of the union of two bones, of such a form as to permit the necessary motion ; but they are not in contact : each articulating surface is covered with cartilage, to prevent the jar which would result from the contact of the bones. This cartilage is elastic, and the celebrated Dr. Hunter discovered that the elasticity was in consequence of a number of filaments closely compacted, and extending from the surface of the bone, so that each filament is perpendicular to the pressure made upon it. The surface of the articulating cartilage is perfectly smooth, and is lubricated by a fluid called synovia, signifying a mucilage, a viscous or thick liquor. This is vulgarly called joint oil, but it has no property of oil, although it is better calculated than any oil to lubricate the interior of the joint.
When inflammation comes upon a joint, this fluid is not supplied, and the joint iš stiff, and the surfaces creak upon one another like a hinge without oil. A delicate membrane extends from bone to bone, confining this lubricating fluid, and forming the boundary of what is termed the cavity of the joint, although, in fact, there is no unoccupied space. External to this capsule of the joint, there are strong ligaments going from point to point of the bones, and so ordered as to bind them together without preventing their proper motions. From this description of a single joint, we can easily conceive what a spring or elasticity is given to the foot, where thirty-six bones are jointed together.
An elegant author has this very natural remark on the joints :— In considering the joints, there is nothing, perhaps, which ought to move our gratitude more than the reflection, how well they wear.
A limb shall swing upon its hinge, or play in its socket, many hundred times in an hour, for sixty years together, without diminution of its agility, which is a long time for anything to last, for anything so much worked and exercised as the joints are. This durability I should attribute, in part, to the provision which is made for the preventing of wear and tear : first, by the polish of cartilaginous surfaces ; secondly, by the healing lubrication of the mucilage; and, in part, to that astonishing property of animal constitutions, assimilation, by which, in every portion of the body, let it consist of what it will, substance is restored, and waste repaired.”—(Paley.)
If the ingenious author's mind had been professionally called to contemplate this subject, he would have found another explanation. There is no resemblance betwixt the provision against the wear and tear of machinery and those
for the preservation of a living part. As the structure of the parts is originally perfected by the action of the vessels, the function or operation of the part is made the stimulus to those vessels. The cuticle on the hands wears away like a glove; but the pressure stimulates the living surface to force successive layers of skin under that which is wearing, or, as the anatomists call it, disquamating; by which they mean, that the cuticle does not change at once, but comes off in squamae, or scales. The teeth are subject to pressure in chewing or masticating; and they would, by this action, have been driven deeper in the jaw, and rendered useless, had there not been a provision against this mechanical effect. This provision is a disposition to grow, or rather to shoot out of their sockets ; and this disposition to project balances the pressure which they sustain ; and when one tooth is lost, its opposite rises, and is in danger of being lost also for want of that very opposition.
MECHANICS OF MUSCULAR POWER.
We may compare the muscular power to the weight which impels a machine. In studying machinery, it is manifest that weight and velocity are equivalent. The handle of the winch in a crane is a lever, and the space through which it moves, in comparison with the slow motion of the weight, is the measure of its power. If the weight raised by the crane be permitted to go down, the wheels revolve, and the handle moves with the velocity of a cannon-ball, and will be as destructive if it hit the workman. The weight here is the power, but it operates with so much disadvantage, that the band upon
the handle of the winch can stop it; but give it way, let the accelerated motion take place, and the band would be shattered which touched it. Just so the fly-wheel, moving at first slowly, and an impediment to the working of a machine, at length acquires momentum, so as to concentrate the power of the machine, and enable it to cut bars of iron with a stroke.
The principle holds in the animal machinery. The elbow is bent with a certain loss of mechanical power; but by that very means, when the loss is supplied by the living muscular power, the band descends through a greater space, moves quicker, with a velocity which enables us to strike or to cut.
Without this acquired velocity we could not drive a nail : the mere muscular power would be insufficient for many actions quite necessary to our existence.
CORRESPONDENCE BETWEEN LIVING PROPERTIES AND
MECHANICAL PROPERTIES OF ORGANS.
We shall take an instance to illustrate the difference betwixt the mechanical connexion of parts, and their relations through the living properties. And it will, at the same time, shew how curiously the living properties and the mechanical properties are made to correspond with each other.
A stream of water is converted into a mechanical power ; it fills a cistern, which is attached to a lever; the cistern descends by the weight of water; by its descent a valve is pushed open; the water escapes, and the cistern ascends, and remains so, till the stream flowing into it again, depresses it. Thus the regularity of the supply of water gives regularity of motion to the machine. Compare this with the heart.
may describe the heart as consisting of two cavities, the one called the Auricle, and the other the Ventricle. The one sinus receives the blood returning by the veins, and gradually filling, like a cistern, it becomes so distended, that its muscular power is excited ; it contracts, and delivers the blood with a sudden impetus into the second cavity, or the ventricle; which, in its turn, excited by the distention, contracts, and propels the blood into the artery. Here the action of the heart is accounted for, by its mechanical distention with the blood : and the regularity of its motions necessarily corresponds with the regularity of the supply. The distention produces action, and the propulsion of the blood from the cavity allows a momentary state of rest, until another volume of the blood excites another pulse.
But we have now to observe, that when this irritability or muscular power was bestowed upon the heart, it was directed by a law entirely different from the irritability as possessed by other muscles. A property of alternate activity and rest was given to it, quite unlike the contractility of other parts; and, accordingly, when the heart is empty, when there is no distention of blood at all, the two cavities will continue their alternate action. Nay, if the heart be taken