respond with the bone before, and the tendon behind: and it is suspended by an appropriate muscle, which raises it like a bolt, after it has served the office of throwing off the tendons from the centre of motion. In addition, the sketch shows, that where these tendons pass behind the joints, they are thickened and hardened into cartilages, so that the bolt operates more effectually in directing them backwards, and producing the projection, equivalent to that of the heel or the hock.* These are the means by which "she lifteth up herself on high, and scorneth the horse and his rider."

After the many illustrations which we have adduced from mechanics, the muscular power itself must be a subject of surprise and admiration. Gravity, the expansion and condensation of steam, the evolution of gases, the spring or elasticity of material, or all these combined, could not have answered the various offices performed by this one property of life-muscular contractility. The irritable and contractile fibre, of which muscle is composed, when chemically considered, does not differ from the fibrine of the blood; but from being endowed with this property of contraction, and adapted with "mechanical ingenuity," it fulfils a thousand distinct purposes, in volition, breathing, speaking, digestion, circulation; and it is modified in all these functions according to the wants and condition of every class of animals.

From what the reader already understands of the conformity subsisting among all parts of an animal body, he will readily comprehend that a perfect relation must be established between the bones and the muscles that as the bones of different animals exhibit a variety in their size, relative position, and articulations, so must there be an adaptation of the muscles. Accordingly, we sometimes find the muscles separated into smaller, and sometimes consolidated into more powerful masses. To the anatomical student, the mode of demonstrating the muscles of the human hand and arm becomes the test of his master's perfection as a teacher. When they are taken successively, just as they present themselves in the arm, nothing can be more uninteresting, tedious, and difficult to attend to, than such a demonstration; but when they are taught with lucid

* I am indebted to Mr Shaw for these interesting demonstrations of the ostrich's foot.

arrangement, according to the motions performed by the distinct groups of muscles, it is positively agreeable to find how much interest may be communicated to the subject.

It would be foreign to the object of this work to introduce such demonstrations here. Yet it is remarkable how closely the muscles of the arm and hand resemble the muscles of the fore extremity of certain animals-the lion, for example. The flexors, extensors, pronators, and supinators, in the brute, are exactly in the same relative place which the student of anatomy is taught to observe with so much interest in the human

arm.

This example shows how accurately the arrangement of the muscles conforms to the structure of the bones; and that in proportion as the bones of the extremity of any animal resemble, in shape and power of motion, those of the human arm, so will the muscles-another proof of the extent of the system of analogies established in the animal frame.

There is one circumstance more which should not be omitted in the comparative anatomy of these muscles, as it exhibits another instance of conformity in the structure of parts, to the offices they have to perform. We have just stated that the power of contraction is a vital property. The continued action of a muscle, therefore, exhausts its vitality. Now, to support that action, when inordinate, there must be a more than usual provision for the supply of the living power to the musclethere must be a means of increasing or maintaining the circulation of the blood within it, that being the source of all vital power.

In the loris tardigradus* it has been observed that the axillary and femoral arteries, the great arteries of the anterior and posterior extremities, present this peculiarity-the main vessel is subdivided into a number of equal-sized and tortuous cylinders, which, previous to the distribution of the proper branches to the muscles, again unite to form a single trunk. As this subdivision of the trunk of the vessel produces a retardation of the blood, it has been argued that it is adapted to the slow motion of the animal. On the contrary, I believe it to be a provision for long-continued action. The animals which possess this peculiarity in their circulation are not more remark

There is some doubt as to the reunion of the vessels.

able for the slowness of their progression than for the tenacity of their hold; their extremities are long and their muscles powerful, either for sustaining the animal by grasping the branches of trees, or for digging. But surely the strength of the muscles cannot be maintained by retarding the circulation of the blood: it is a principle universally admitted, that the expenditure of arterial blood always bears a proportion to the vital force employed.

Buffon tried to make a dog amphibious, by immersing the puppy, before it had breathed, in tepid water. One of our own physiologists thought it possible, by putting ligatures upon the arteries which go to the limbs, and forcing the blood to take a circuitous course, and by numerous channels, to the muscles, to make a tardigrade animal, like the loris, out of a vivacious spaniel. We need hardly say that these experiments failed. They were undertaken in a misconception of the nature of the living properties of muscles, which are more finely adjusted than anything in the mere mechanism of the body. Every muscle has its prescribed mode of action, from the unwearied irritability causing the incessant motion of the heart, to the simple effort of the muscle which guides the pen. Some muscles are ever in action, with but short intervals of rest; others act in regular succession: some are under the will, others withdrawn from it: some act quickly, as the heart; others slowly, as the stomach: but these are original endowments, and do not result from the force or languor of the circulation of the part.

To return to the subdivided and tortuous artery-were the blood-vessels of the living body like rigid tubes, and the laws of the circulation the same as those of hydraulics, such a form of the artery would certainly be the means of retarding the course of the blood. But it is impossible to believe that the circulation of the blood can be performed according to the same laws which govern the flow of water in dead tubes. The artery is dilatable; it contracts with a vital force; and both the dilatability and the contractility of arteries are subject to the influence of the living principle. When, therefore, the artery of a limb is divided into four or five vessels, which are tortuous, as in the sloth, the result will be a greater capacity of dilatation, and a greater power of contraction; and these, being vital

operations, will be subject to be influenced and adjusted according to the necessity for the increase or diminution of the circulation. If such a peculiarity in the form of the vessels in the extremities of these animals retard the blood, it can only be during repose; for, on excitement, so far from retarding, it must bestow a remarkable power of acceleration. I conclude, therefore, that this variety of distribution in the arteries is a provision for an occasional increase of activity in the muscles of the limb, and for forcing the blood into contact with the fibres, notwithstanding their long-continued action and rigidity. We have seen, in the preceding chapter, that the same animal which at one time moves out its paw as slowly as the hand of a watch, at another, when seizing its prey, acts with extreme rapidity consequently, we cannot admit the inference that the tortuous and subdivided artery is a provision for languid movements.

:

OF THE RIGHT AND LEFT HAND.

In speaking of the arteries which go to the hand, it may be expected that we should touch on a subject, formerly a good deal discussed, whether the properties of the right hand, compared with the left, depend on the course of the arteries for it has been affirmed that the superiority of the right arm is owing to the trunk of the artery which supplies it, passing off from the heart more directly, so as to admit of the blood being propelled more forcibly into the small vessels of that arm, than the left. This, however, is assigning a cause altogether unequal to the effect, and presenting too confined a view of the subject: it partakes of the common error of seeking in the mechanism, the explanation of phenomena which have a deeper origin.

Among all nations, there is a universal consent to give the preference to the right hand over the left. It cannot, therefore, be a conventional agreement: it must have a natural source. For the conveniences of life, and to make us prompt and dexterous, it is pretty evident that there ought to be no hesitation which hand should be used, or which foot should be put forward; nor is there, in fact, any such indecision. Is this readiness taught, or is it given to us by nature?

Sir Thomas Browne says, that if the right side were originally

the most powerful in man, we might expect to find it the same in other animals. He affirms that squirrels, monkeys, and parrots feed themselves with the left leg rather than with the right. But the parrot may be said to use the strongest foot where most strength is required; that is in grasping the perch and standing, not in feeding itself.

That the preference for the right hand is not the result of education, we may learn from those who by constitution have a superiority in the left. They find a difficulty in accommodating themselves to the modes of society: and although not only the precepts of parents, but every thing they see and handle, conduce to make them choose the right hand, yet will they rather use the left.

It must be observed, at the same time, that there is a distinction in the whole right side of the body, as well as in the arm : and that the left side is not only the weaker, in regard to muscular strength, but in its vital or constitutional properties. The development of the organs of motion is greatest upon the right side; as may at any time be ascertained by measurement, or the testimony of the tailor or shoemaker. Certainly, the superiority may be said to result from the more frequent exertion of this side; but the peculiarity extends to the constitution also; and disease attacks the left extremities more frequently than the right. We see that opera dancers execute their more difficult feats on the right foot: but their preparatory exercises better evince the natural weakness of the left limb; in order to avoid awkwardness in the public exhibitions, they are obliged to give double practice to the left leg; and if they neglect to do so, an ungraceful preference to the right side will be remarked. In walking behind a person, we seldom see an equalised motion of the body; the tread is not so firm upon the left foot, the toe is not so much turned out, and a greater push is made with the right. From the peculiar form of woman, and from the elasticity of her step, resulting from the motion of the ankle rather than of the haunches, the defect of the left foot, when it exists, is more apparent in her gait. No boy hops upon his left foot, unless he be left-handed. The horseman puts the left foot in the stirrup and springs from the right. We think, therefore, we may conclude, that the adaptation of the form of everything in the conveniences of life, to the right hand-as for example,