MUSCULAR MOTION.— Under this head it is intended to consider the contractility of muscle, its source, the stimuli that excite it, and the nature of the minute movements occurring during the act of contraction.
a. Of the contractility muscle.—This subject having already been ably discussed in this work (see CONTRACTILITY), I shall here confine myself to such a brief statement as may appear to be required by the advance of knowledge since the publication of the article in question.
1. Is it a property inherent in the muscular fibre? Are we to believe in the vis insita' of Huller?—The supporters of this opinion have always been exposed to the objection that in the cases of contraction adduced by them as the effect of a topical or immediate stimulus, the isolation of the muscle from all connexion with nervous fibrils has not been demonstrated. Moreover, what has generally been regarded as their strongest ground, viz: the statement that involuntary muscles are not capable of being excited to contraction by irritation of their nerves, has lately been shown by the numerous experiments of Midler( 1'aletitin, and others, to be erroneous and unworthy of credit. But I have elsewhere• adduced the evidence of direct microscopical observations made on living fragments of the elementary fibre of voluntary muscle, entirely isolated from every extraneous tissue, whether nerve or vessel, to spew that this is a property inherent in this tissue, and that, whatever be its source, it is capable of being brought into action by a stimulus topi cally applied. Thus, when such a fragniest is examined, contraction is found to occur first at its broken extremities, and if water (which has long been known to be a rapid exhauster of muscular irritability) be brought into con tact with it, it is seen to absorb the fluid and excited to contractions, which com mence at the surface. The same thing is fre quently to be met with under a different form. A particle of foreign matter, as a hair or piece of dust, may be included by design or accident in the field so as to touch the side of the fibre at a single point. When this happens, the fibre
will often exhibit a contraction so plain and so limited to the point touched, as to give un equivocalproof of its being the result of the irritation of pressure.
Theseinteresting, phenomena may be observed more or less satisfactorily in all animals whose fibres retain their irritability for a sufficient length of time after removal from the body, and the crab and lobster will be found the most favourably adapted for the purpose. In many reptiles and fishes, also, the steps occur slowly enough to be adequately scrutinized.
The facts in question can admit only of one explanation if it be conceded that the mus cular element has been here separated from the nervous; and certainly that separation has been effected unless the nervous tubules send off from their terminal loops a set of fibrils which penetrate the sarcolemma and diffuse themselves through the contractile ma terial within ; a supposition for which there exists at present no foundation in the obser vations of the most diligent investigators of this subject.
They will, therefore, probably, be regarded as conclusive proof that contractility is a pro perty inherent in the very structure of muscle, and capable of being excited to action inde pendently of the immediate instrumentality of nerves.
The determination of this point must have a very important bearing on the question of the nature and cause of contraction, into which no small confusion has been introduced by the attempts to account for that phenomenon by various hypotheses of electrical action. That one, especially, which aims at establishing an attraction between distant points of the fibres where the nerve crosses them, (the zig-zag hypothesis' of Prevost and Dumas,)and which, with the wrongly interpreted facts on which it. principally rests, has had an immense, though sometimes unperceived influence, ever since it was broached, on the whole question of con traction, is entirely pofuted by the facts above mentioned. There are some others, sprung out of this, which do not here require mare than a passing allusion.