Active contraction.—This is the form of con traction which is attended with those manifes tations of power or force that specially charac terize muscle. It is always excited by a sti mulus, and is always exerted in opposition to another force within the body, which it is able more or less completely to master. The oppo sing force is generally the passive contractility of antagonist muscles, but it may be the elasti city of parts, or, in the case of hollow muscles, the resistance of their own contents. Active contraction is partial in extent and duration. It requires intervals of rest, being attended with exhaustion of the power which produces it, which exhaustion in the voluntary muscles is attended with the sensation termed muscular fatigue.
3. Of the differences between the minute movements of muscle in passive and active con traction. in passive contraction.—It is, per haps, impossible in the higher animals to ob serve the nature of the microscopic movements occurring during the passive shortening of a muscle ; but in the lower and smaller forms of life this may sometimes be accomplished. It may always appear doubtful, however, whether, any contraction that may be here witnessed be entirely of the passive kind, and consequently the movements here noticed are not worthy of implicit reliance. But it is more easy and quite as satisfactory to bring a muscle under inspec tion, which is still in situ and in equilibrium with its antagonists ; in such, contractile force is still present, though its effects are neutralized. This may be done in various small animals; per haps the tail of small fish or of the tadpole of the common frog is the best adapted for the purpose. In the latter, when deprived of its thick tegu ment, I have succeeded in gaining such a view, and have found the contraction to be quite uni form throughout, the transverse stripes being sta tionary and equidistant. This is nothing more than might have been expected on a priori grounds. The contraction being the effect of the passive exercise of the property shared equally by all parts of the tissue, would be equal in equal masses, and as the elementary fibres are of pre cisely equal width and substance from end to end, no part of them could predominate in ac tion, as long as no special stimulus was applied. It may be concluded, therefore, that passive contraction is attended by a movement abso lutely uniform throughout the whole mass of an elementary fibre or of a muscle.
In active contraction.—The case is far other wise in active contraction, as may now be con sidered proved by a considerable body of evi dence.
It might be argued, prior to direct proof; that active contraction, being the answer to a stimulus, must be partial, at least at its com mencement, since no stimulus can be applied at the same instant to every particle of a muscle.
Certain features of the contractions witnessed in fragments removed from the body, and ex amined in water under the microscope, have a close bearing on the present question. It has been already said that such contractions are uniformly partial ; but they present two further varieties, either 'remaining in the part where they first occur, or leaving it as they engage others in the neighbourhood. The accidental circumstances under which the fragments are placed explain these varieties. In the former
case the fragments are free to move ; their ends approach in proportion to the amount of con traction, and as there is no force to extend them again when the contractile force ceases to be manifested in them, and advances to fresh parts, the contraction has the appearance of being permanent. In the latter case, certain parts of the fibre (as its broken extremities) are fixed more or less firmly, so as to offer a resist ance to the contraction that takes place, this resistance enabling the contractile force ad vancing to new parts to obliterate the traces of contraction in the parts in which it is sub siding, by stretching them. The ends usually become fixed in consequence of their being the first to thicken from contraction and from their thus receiving the pressure of the la mina of mica or glass with which it is requi site to cover the object, and they are the first to contract, because irritated both by being broken and also by the water, which is absorbed soonest where the sheath is deficient. This fixing of the ends brings the fibres in question nearly into the condition under which they exist in the living body, where it has already been explained that there is always a resistance to be overcome in active contraction. This particular variety of the phenomenon, therefore, deserves special study. Those animals whose muscles are most tenacious of their contractility are the best suited for examination, and among these the young crab or lobster may be most easily obtained. In an elementary fibre from the claw, laid out on glass, and then covered with a wet lamina of mica, the following phenomena are always to be observed. The ends become first contracted and fixed. Then contractions commence at isolated spots along the margin of the fibre, which they cause to bulge. At first they only engage a very limited amount of the mass, spreading into its interior equally in all directions, and being marked by a close approx imation of the transverse stripes. These contrac tions pull upon the remainder of the fibre only in the direction of its length, so that along its edge the transverse stripes in the intervals are very much widened and distorted. These contrac tions are never stationary, but oscillate from end to end, relinquishing on the one hand what they gain on the other. When they are nume rous along the same margin they interfere most irregularly with one another, dragging one ano ther as though striving for the mastery, the larger ones continually overcoming the smaller, then subsiding as though spent, stretched again by new spots of contraction, and again, after a short period of repose, engaged in their turn by some advancing wave : this is the first stage of the phenomenon. ( Fig. 302.) The con tractions increase in number and extent, and gradually engage the whole substance of the fibre. There is still the same struggle, the same alternate action and repose in individual parts, but as the contractions by degrees predominate, the ends of the fibre are drawn more and more near, (intermediate portions by their contrac tion receiving some of the pressure,) until at last the whole fragment is reduced to a third of its original length, and stiffened with the rigor mortis.