direction, thus, A being the tendinous origin of a muscle, and B the tendinous insertion, the fleshy fibres c run obliquely between these two

tendons. The fibre

acting thus obliquely loses power, but gains

the property of pull

B

ing what is attached to its further extremity through a greater space, while it contracts; and consequently the velocity is increased. This mechanical arrangement is intelligible on the law, that velocity of motion through space is equal to power and weight. Here, there is a resignation of power in the muscle to gain velocity of motion. The same effect is produced by the manner in which the tendons run over the joints. If they went in a straight line to the toes or tips of the fingers, the muscles would act more powerfully; but the tendons being braced down in sheaths, they move the toes and fingers with a velocity increased in proportion to their loss of power.

Let us see how far this corresponds with other mechanical contrivances. A certain power of wind, water, or steam being obtained, the machinery is put in motion; but it is desired to give a blow, with a velocity far greater than the motion of the water or the turning of the wheels. For that purpose a flywheel is put on, the spokes of which may be considered as long levers. The wheel moves very slowly at first; but being once in motion, each impulse accelerates it with more and more facility; at length, it acquires a rapidity, and a centrifugal force, which nothing but the explosion of gunpowder can equal in its effects. The engineer, not having calculated the power of accelerated motion in a heavy wheel, has seen his machinery split and burst up, and the walls of the house blown out, as by the bursting of a bombshell. Or, a body at rest receives an impulse from another, which puts it into motion-it receives a second blow; now, this second blow has much greater effect than the first for the power of the first was exhausted in changing the body from a state of rest to that of motion-but being in motion when it receives the second blow, the whole power is bestowed on the acceleration of its motion; and so on, by the third and fourth blows, until the body moves with a velocity equal to that of the body from which the impulse is originally

given. The slight blow given to a boy's hoop is sufficient to keep it running; and just so the fly-wheel of a machine is kept in rapid action by a succession of impulses, each of which would hardly put it in motion. If we attempt to stop the wheel, it will inflict a blow in which a hundred lesser impulses are combined and multiplied.

In the machinery of the animal body, there is, in a lesser degree, the same interchange of weight with velocity and force.

When a man strikes with a hammer,* the muscle c, near the shoulder, acts upon the humerus, B, in raising the extended lever of the arm and hammer, with every possible disadvantage, seeing that it is inserted near the centre of motion in the shoulderjoint; and the same remark applies to the muscle D. But the loss of power is restored in another form. What the muscle D loses by the mode of its insertion, is made up in the velocity communicated to the hammer; for in descending through a large space, it accumulates velocity, and velocity and weight are equal to force. The advantage of the rapid descent of a heavy body is, that a smart blow is given, and an effect produced which the combined power of all the muscles, without this mechanical distribution of force, could not accomplish. It is, in truth, similar to the operation of the fly-wheel, by which the gradual motion of blade and clavicle, and inserted into the arm-bone; D, a muscle which draws the arm down, as in striking with a sword or hammer.

*

A, the scapula, or shoulderblade; B, the humerus, or armbone; c, the deltoid muscle of the shoulder, arising from the shoulder

an engine is accumulated in a point of time, and a blow struck capable of stamping a piece of gold or silver. In what respect does the mechanism of the arm differ from the engine with which the printer throws off his sheet? Here is a lever with a heavy ball at the end; in proportion to its weight it is difficult to be put in motion; the printer, therefore, takes hold of the lever near the ball, at A; were he to continue pulling at that part of the lever, he would give to the ball no more velocity than that of his hand; but having put the ball into motion, he slips his hand down the lever to B. Had he applied his hand near B at first, he could not have moved the weight; but the ball being now in motion, if

B

he direct the whole strength of his arm to the lever near the centre of motion, the velocity of the weight at the further end will be greatly accelerated. Thus the weight and velocity being combined, the impulse given to the screw is much more powerful than if he had continued to pull upon the further end of the lever at A.

If we now turn back to the diagram (page 84), we shall understand how much is gained by the muscle c being inserted near the centre of motion, although, in one sense, at a mechanical disadvantage. First, that mode of insertion is in correspondence with the principle already adverted to, that the living endowment of muscle is never spared, but is bestowed liberally in proportion to the necessities of the part. But it will also be perceived, that the arm being put in motion by the force operating near the centre of motion, the velocity will be rapidly increased by each successive impulse from the muscle; and, of course, the motion at the further extremity will be more rapid than at the insertion of the muscle. Again, in the action of pulling down the arm, as in giving a back stroke with the sword, we perceive that when the hammer descends, the rapidity is increased by the mere effect of gravity; but when the action

of the muscle is conjoined, the two forces, progressively increasing, greatly augment the velocity of the descent.

The same interchange of power for velocity, which takes place in the arm, adapts a man's hand and fingers to a thousand arts, requiring quick or lively motions. The fingers of a lady playing on the pianoforte, or of the compositor with his types, are instances of the advantage gained by this sacrifice of force for velocity of movement. The spring of the foot and toe is bestowed in the same manner, and gives elasticity and rapidity in running, dancing, and leaping.

The motions of the fingers do not result merely from the action of the large muscles which lie on the fore-arm: these are for the more powerful efforts; in the palm of the hand, and between the metacarpal bones, are numerous small muscles, (lumbricales and interossei,) which perform the finer movements, -expanding the fingers, and moving them in every direction with quickness and delicacy. These small muscles, attached to the extremities of the bones of the fingers where they form the first joint, being inserted near the centre of motion, move the ends of the fingers with great velocity. They are the organs which give the hand the power of spinning, weaving, engraving, &c.; and as they produce the quick motions of the musician's fingers, they are called by anatomists fidicinales.

But there is another use which the small muscles in the hand serve. In grasping with the hand, the strength with which it closes, when all the muscles are combined in action, must be very great; the amount of power is exhibited when we see a sailor hanging by a rope, and raising his whole body with one arm. What must be the pressure upon the hand? If the palms, and inside of the fingers, and their tips, were not guarded, by cushions beneath the skin, it would be too much for the texture even of bones and tendons, and certainly for the blood-vessels and nerves, to sustain. The elastic pad in the foot of the horse, camel, or ostrich, is not a whit more appropriate than the fine elastic texture beneath the skin of the hand. To add to the efficacy of this yielding but strong padding, a muscle is provided, which, arising in the centre, runs across half the palm to the cushion, on the inner edge, opposed to the ball of the thumb: it acts powerfully as we grasp; and it is this muscle which, by

raising the edge of the palm, hollows it, and adapts it to lave water, forming the cup of Diogenes.

Whilst the cushions on the ends of the fingers protect them in the powerful actions of the hand, we shall presently see that they are useful also in subservience to the organ of touch; that they provide a power of receiving impressions, without which the utmost delicacy of the nerves would be unavailing.

The projection of the heel in the human foot, and the prominence of the knee-pan, are provisions for increasing, by mechanical adjustment, the power of the muscles. By such means the point of insertion of the muscle is removed to a distance from the centre of motion in the joint, and the lever power thus obtained is added to the force of the muscle. The principle is maintained, and the demonstration more easy, in the joints of some animals, as in the hock of the horse; and we have a beautiful instance of it in the foot of the ostrich. Where the flexor tendons pass behind the several joints of the foot, the heads of the bones are enlarged; which throws the tendons off from the centre of motion. But there is an additional provision still. A loose pendulous body, A, hangs between the tendons and bone, at each of these joints; and it plays upon the bones in such a manner, that at the utmost degree of extension of the foot, when the bird requires to use all its power of muscular exertion to bend it again, this body is introduced to throw the tendons further

Ostrich's Foot.

backwards, and to add remarkably to the lever power. This body, A, is shaped like a wedge, with grooved surfaces to cor