There is no part of our frame over which habit has more influence than the muscles, and especially those which are directed by the will. All those actions by which we maintain a voluntary intercourse with our fellow-crea tures, and with the external world, are effected by com plicated trains of muscular contractions, which are origi nally acquired by long practice, and after many unsuc cessful attempts, but which by degrees become so through the operation of custom, that they are at length performed without one being conscious of them. The same remark applies to our associated muscular which may be traced up to the effect of habit, connecting different actions, which, in the first instance, seem to have no necessary dependence upon each other.
It is not necessary to make many observations upon the use of the muscles, as this is sufficiently obvious from the consideration of their characteristic properties. They arc the organs of motion, both of that kind of motion by which the body is moved from place to place, constituting loco• motion; that by which each of its separate parts is moved when we act upon contiguous bodies ; and that by which many of the minute actions of the animal are performed which are essential to the exercise of the vital functions All those effects are brought about by the simple act o contraction, or the shortening of the fibres, by which tin ends are approximated, and the parts moved to which the are attached. The action of the muscular coats is no les: efficient than that of the muscles, although it is less ob vious to the eye. The fibres in this case not being col lected in large masses that act simultaneously, the effec of their contraction is not the motion of any large organ but as the different fibres appear to act more independent ly of each other, they produce in the organ to which the belong a peculiar kind of undulatory, or, as it is termed vermicular motion, which serves to keep their surface ii a state of continued agitation, and to communicate it ti their contents.
There is one question connected with this part of th subject, which has often been discussed, whether all motions that occur in the body are to be re ferred to contractility, or whether every motion whicl cannot be accounted for by the laws of elasticity, or o gravity, or any of the general physical powers of matter may be explained upon the principles of muscular con traction. A difficulty has arisen in certain cases, becaus we observe very obvious motions to take place in some organs, and yet we are not able to detect any muscular fibres in them. One of the most remarkable cases of this description, which has been appealed to as sufficient to decide the controversy, is the motion of the iris. In this organ, which is so large as to be quite open to our inspec tion, and in which the motions are considerably extensive, no muscular fibres have yet been satisfactorily demon strated, nor, if they were so, is it very clear in what way they could be disposed, so as to produce the desired effect.
To account, therefore, for the motion of the iris, it has been thought necessary to have recourse to some other principle.* Blumenbach has attempted to solve this problem, by ascribing to this and some other parts, in which a similar kind of anamoly exists, a peculiar property or power, which he terms vita prol2ria, and supposing that this vita profiria causes the contractions of these organs. But this hypothesis of Blumenbach cannot be regarded in any other point of view than as a mere form of speech, which throws no light upon either the phenomenon itself or its causes, and which neither tends to generalize analogous facts, nor to show how they bear upon each other. No thing, however, can be more injurious to the progress of knowledge, than to form hypotheses Which are merely verbal, and which really give us no insight into the nature of the subject. Every one must be conscious that there are many parts of physiology which are-yet unexplained, and we conceive the motion of the iris to be one of these, nor are we to regard it as in any respect humiliating, that, on so abstruse a subject, we are frequently under the ne cessity of confessing the insufficiency of the present state of our knowledge.
IN considering the mechanism of muscles, we must re fer to the description which was given of these organs, as consisting of bundles of fibres lying parallel to each other, which possess the power of shortening themselves, and, in this way, bring their extremities nearer together. The ultimate object of this shortening of the fibres is, to move the ends of the bones to which the fibres are attach ed; but, in order to promote the symmetry of the form, or to facilitate motion, it frequently happens that the flesh of the muscle itself is not inserted into the part which is to be moved, but that a membranous body intervenes, which, according to its situation, or the use of the organ, is either condensed into a strong cord, constituting a ten don, or is spread out into a membranous expansion. The motions of the bones, depending upon the shortening of the muscular fibres, are generally performed by means of joints ; and it will be found that the bones are acted upon by the muscles in the manner of levers ; the part where the muscle or tendon is inserted into the bone represent ing the power, the joint the fulcrum, and the part that is moved constituting the weight. With a few exceptions only, the bones form levers of that description- which are styled by mechanicians of the third kind ; in which the power is placed between the fulcrum and the weight. The motion of the fore-arm may be taken as an example of the effect of muscular contraction, and the manlier in which it is produced. When we wish to raise a weight by bending j the elbow joint, it is effected by muscles situated below the shoulder, which have tendons inserted into the top of the bone of the fore-arm near the elbow.