The muscular tissue in these animals is very tough, but where it is more fragile, as in the Frog, it may give way in the intervals between spots of contraction, and become ruptured and disorganized in various degrees.* In fishes I have seen a succession of phenomena similar to what has been described in the Crab ; waves of contraction advancing and receding, but gradu ally augmenting in bulk, till the whole fibre was finally contracted. ( Fig. 303.) In all these examples, as long as the ends of the fragment are fixed, and will not yield to the convellent force, that force is seen to be exerted in a momentary manner in successive portions of the mass. In proportion as they yield to it, the resistance which enabled the contraction of new parts to stretch those from which it was receding is removed, and the ap pearances of contraction remain. A distinction is required between the contractile force and the contraction resulting from its exercise. The latter will be permanent, if no force from with out be exerted to obliterate it by stretching, for a contracted muscle has no power of extending itself; there is no repellent force between its molecules. From these phenomena, therefore, it is possible to eliminate the appearances re sulting from a subsided force, and to judge of the mode and duration of action of the force itself. Thus sifted, they prove that, even when directly stimulated by water after removal from the body, a muscle contracts in successive por tions, never in its totality at once, and that no particle of it is capable of exhibiting an active contraction for more than an instant of time.
The appearances presented by muscle that has been ruptured by its own inordinate con traction in fatal tetanus in the human subject will supply the link wanting to connect the foregoing phenomena with those occurring in healthy contraction during life: for tetanic spasm differs from sustained voluntary contrac tion only in its amount and protracted duration, and in its being independent of the will ; none of which circumstances are of essential import ance in regard to the nature of the act of con traction itself.
The muscles are so arranged in the body that no amount of contraction which the me chanism of the bony and ligamentous frame work will permit one of them to undergo, can by possibility occasion the rupture of a relaxed antagonist: to be ruptured the antagonist must be itself contracted. But a muscle, if contract ing beyond its natural amount, may be so re sisted by mechanical powers, in or out of the body, as to rapture itself. Hence, the contrac tion of a muscle is a necessary condition, and generally the essential cause of its own rupture: the other condition being a force greater than the tenacity of the ruptured part, holding its ends asunder ; this latter may be either the active or passive contraction of antagonists, or mere mechanical resistance. But it is evident that for a muscle to be ruptured by its own contraction, that contraction must be partial, as is shewn in the case of the Frog s muscle already mentioned. An examination of muscle ruptured in tetanus is found to bear out these observations in the fullest manner.* The ele mentary fibres present numerous bulges of a fusiform shape, in which the transverse stripes are very close. These swellings or contracted
parts are separated from one another by inter vals of various lengths, in which the fibre has either entirely given way or is more or less stretched and disorganized. These appearances are met with after all contractility has departed ; they are the vestiges of the spasm during life. Yet in other muscles, which have been likewise convulsed, but not ruptured, they are not found. Their presence is, therefore, the result of the rupture. They admit only of the follow ing explanation : the contractile force has ope rated at the points contracted, and by its excess the intermediate portions have been stretched to laceration. Having once given way, the con tracted parts have become isolated, and can no longer have been extended after the subsidence of their contractile force; they consequently retain the form and appearances they possessed, when surprised, as it were, by the rupture they have themselves produced of the intervening parts.
Supposing, for a moment, that active con traction were a universal and equable act, and that by the superior power of an antagonist a weak muscle had been ruptured, the appear ances resulting would manifestly be entirely different from those now detailed. The fibres beyond their ruptured point would have their transverse stripes uniformly approximated.
From the preceding facts I conclude, 1st, that active contraction never occurs in the whole mass of a muscle at once, nor in the whole of any one elementary fibre, but is always partial at any one instant of time; 2dly, that no active contraction of a muscle, however apparently prolonged, is more than instantaneous in any one of its parts or particles; and therefore, 3dly, that the sustained active contraction of a muscle is an act compounded of an infinite number of partial and momentary contractions, incessantly changing their place, and engaging new parts in succession ; for every the of the tissue must take its due share in the act.
Two phenomena yet remain to be mentioned, which, by admitting of a satisfactory explana tion on this view of the subject, give strong testimony to its correctness. The first is the muscular sound heard on applying the ear to a muscle in action. It resembles, according to Dr. ‘Vollaston's apt simile,* the distant rumb ling of carriage wheels, or an exceedingly rapid and faint tremulous vibration, which, when well marked, has a metallic tone. It is the sound of friction, and appears to be occasioned by those movements of the neighbouring fibres upon one another, with which the partial contractions must be attended in their incessant oscillations. The other phenomenon is one, the existence of which has been recently ascertained by MM. Becquerel and Breschet,f viz. that a muscle during contraction augments in temperature. They have found this increase to be usually more than 1° Fahr., but sometimes, when the exertion has been continued for five minutes, as in sawing a piece of wood, it has been double that amount. This development of heat seems to be in a great measure attributable to, and even a necessary consequence of, the friction just alluded to.