Although we should not be tempted to examine any more of the hypotheses which have been formed on this subject, it may be proper to consider in what degree the cause of muscular contraction is a legitimate object of in quiry, and towards what points we ought particularly to direct our attention. The simple act of contraction must consist in the diminution of the length of the fibres, of which the muscle is composed ; but we have no proof, either from the evidence of our senses, or from any cor rect deduction of reasoning, of any specific structure or composition of the fii.re, which can, in any degree, ex plain the manner in which this diminution is effected. It is not likely that any farther discoveries can be made on this subject by the aid of microscopes, for it appears that there is a limit to the employment of high magnifiers, beyond which the liability to ocular deception is so great, as to counteract any supposed advantage of the increased size of the object. IF we are unable to account for the shortening of the fibres, still less are we able to explain why the various things which we call stimuli, so extremely heterogeneous in their nature, and which have no common property, should coincide in this single point of acting in the same manner upon the fibre. This is so unlike the operation of any other physical cause with which we are acquainted, that we must, at least for the present, consider it as an ultimate fact, one of those mysteries in nature, which daily present themselves to our observation, but which elude all our attempts to refer to any more general principle.
The attempts that have been made to explain the cause of contractility are no less numerous than those respecting the mode in which the contraction is effected, but they have hitherto been equally unfounded ; nor indeed do we possess the least insight into this part of the subject. Be cause it was observed that there is an intimate connexion between the temperature of an animal and the degree of its contractility, some physiologists have conceived that contractility depends immediately upon ca;oric interposed between the fibres. Others, perceiving how remarkably the muscles are affected by the electric fluid, supposed that this was the immediate source of muscular contrac tility, while other writers have contented themselves with saying, that contractility depends upon structure. The fibre, they say, does not differ in its general properties from many other forms of matter, but it possesses a pecu liar structure ; this must therefore be the immediate cause of its contractile power.
An hypothesis respecting contractility, which at one time excited considerable attention, was, that this property depends upon the chemical composition of the fibre ; whenever the different elements are combined in such pro portion as to form the fibre, contractility is one of the pro perties which necessarily results from the combination. Humboldt published a train of experiments, which were directed to this point, and which, if correct, would seem to prove, that apparently slight changes in the chemical composition of the muscle entirely destroy its contrac tility, and that by restoring the original composition of the muscle, the contractility is also restored.
A modification, or peculiar form of the chemical hypo thesis, was brought forward a few years ago, according to which oxygen was regarded as the principle of contrac tility, or was that substance which, by being imparted to the system, or removed from it, gave the muscles their contractile power. The source of the oxygen thus ob tained was conceived to be the air inspired by the lungs, which was united with the arterial blood, and in this way carried to the muscles. For some time this opinion ac quired a considerable degree of popularity, and it appeared to be supported by many pathological facts, but we appre hend that it is now generally admitted to be entirely with out foundation. Nor indeed can any of the above men tioned speculations afford us a satisfactory reply to the proposed question. They teach us that contractility can only exist under a certain range of temperature, and as attached to a certain mechanical structure and chemical composition, and they likewise prove that the electric fluid is one of the most active and powerful stimulants, but they do not establish that kind of connexion between the sup posed cause and its effect, which can induce us to regard them as bearing this kind of relation to each other. In deed there are many well known facts, which seem to lead to an opposite conclusion. Many circumstances affect the contractility of a muscle, which cannot be supposed to alter either its chemical or its mechanical properties. A muscle, immediately after death, as far as we can judge, is similar in its structure and composition to a living muscle, yet, upon the application of stimulants, we find it to be no longer contractile. If it be argued that the che mical or mechanical composition are different, but that the difference is too minute to be detected, we should reply, that where the difference is too minute to be observed, we call have no proof of its existence.
In tracing the history of opinions, we must not over look a doctrine which has been sanctioned by some dis tinguished physiologists, especially by Haller and Cullen, that contractitiy is nothing more than a peculiar mode of attraction which subsists between all kinds of matter ; nor that of Fordyce and Hunter, who ascribe it to what they term the attraction of life. But these, when stripped of the peculiar language in which they are conveyed, amount to nothing more than the expression of the fact in new terms, for still the fundamental difficulty remains, what is it that determines the attraction between the particles of matter to exert their force in this peculiar manner, and under these peculiar circumstances.