In a word, an attentive study of these inte resting phenomena has convinced me that in the bare fact of contraction the build of the fibre is an item of no importance whatever : the exquisite symmetry displayed in the apposition of its component particles is, as it were, dis regarded and overlooked, while the whole pro cess is to be referred to the material itself, the ultimate tissue, whose property is contractility. This property appears to reside both in the par ticles and the substance connecting them.
The ultimate movements, therefore, on which contraction depends, whatever they may con sist in, are molecular, and far be3ond the reach of sense.
It will be perceived that this view of the subject is the only one which can harmonize the fact of contraction in voluntary muscle with the same phenomenon in structures which have no complicated internal arrangement of particles. In regarding contractility, therefore, as a property of the living muscular fibre in general, it is meant that it resides in it as a property without which it would not be muscle, and in such a manner that no particle, how ever microscopic, can be detached from muscle which does not of itself, and independently of the rest, possess this property, as long as it possesses vitality.
It follows from what has been advanced that those hypotheses which refer contraction to a force exerted between determinate but distant points of the fibre, as where the nervous fibrilla: cross it, or at intervals such as has sometimes seen in insects, must fall to the ground. They are so entirely incompatible with the facts above stated that it can scarcely be necessary to dwell at length on the other reasons for rejecting them, or on the explana tion of the phenomena adduced in their support. The main fact on which they have been built is that long ago mentioned by Hales, and more recently studied with minute care by Prevost and Dumas, viz. that in the abdominal muscles of the frog detached and excited by galvanism, the elementary fibres are seen to be thrown more or less into a zig-zag form. It is evident that in interpreting what they saw these eminent physiologists mistook the relaxed fibres for contracted ones. In conducting such experi ments many precautions are required, and at the best, nothing of the real process of contrac tion can be witnessed. As Miller correctly remarks, the muscle is too thick to be seen under a high power. Besides, the shock of galvanism causes only an instantaneous con traction, during which the muscle is so agitated that it is in vain to attempt to.examine its con dition. It gets out of the focus of the instru ment. What is seen afterwards is the contraction but its result, viz. an approximation of the extremities of the fibres. If the galvanic shock has acted uniformly on all the fibres (which is rare), they all remain straight; but if on a part only, those which have escaped con traction are thrown into zig-zags by having their ends brought nearer through their cellular con nexion with neighbouring contracted fibres. It is most natural that the precise point of such flexures should often be determined by the passage of nerves or vessels across the fibres.
This is corroborated by the circumstance that relaxed fibres fall at once into zig-zags when their ends are made to approach by mechanical means.
MM. Prevost and Dumas have themselves drawn attention to an example of shortening without zig-zags in the case of the distended abdominal muscles of the female frog before spawning. They found that the fibres of those muscles when cut across remained straight, after shortening from 145 to 107 millimetres.
Independently of the immense disadvantage at which the hypothesis in question supposes the force to act, (viz. either" between the par ticles at the retiring angles only of the zigzags, or between the distant angles themselves,) it seems quite inconsistent with the able experi ments of Schwann, which show that the power of a muscle diminishes in a direct ratio with the degree of its contraction. With these experiments, indeed, any hypothesis is at variance which is based on the idea of an attraction between isolated and separate points or particles, as, for example, the sarcous ele ments, for it cannot be conceived but that such an attractive force would augment in a multi plying ratio with the proximity of the points attracted.
2. On passive and active contraction. Pas sive contraction.—Passive contraction is that which every muscle is continually prone to undergo, independently of stimuli, and by the mere quality of its tissue. The muscles are ever kept on the stretch by the nature of their position and attachments, and cannot have their ends so approximated by attitude or other wise, as that their tendency to shorten them selves shall cease. If, for example, the rectus muscle of the thigh have its extremities brought as near together as can be effected artificially by posture, they would yet be found to ap proach still nearer on being freed from their attachment to the bones. This tendency to contract has been distinguished by the term retractility, from its being manifested by the retraction that occurs when the belly of a mus cle is cut across. But, in this instance, the retraction would appear to be in part caused by an active contraction excited by the stimulus of the injury. It has also been styled tonicity. The passive contraction of muscles continually opposes their elongation by the action of anta gonists, and restores them when that action ceases. It is that which accommodates them to an attitude artificially given, when no mus cular effort is required to maintain it. When no active contraction is present in a limb, the passive contraction remains, and being brought to a state of equilibrium in all the muscles by their mutual antagonism, the limb is said to be at rest. This is the general condition during sleep. The passive contractility of muscles, therefore, is being ever exerted, without being attended by fatigue ; there seems no good rea son for supposing it to be a property different from active contractility ; it is rather the neces sary condition of that property, in its passive or unstimulated state. Passive contraction is a vital act, for it ceases with the rigor mortis.