state are related to each other, that the mode to which the term irritability is applied, where the contraction is sudden and transitory, and the mode called tonicity, where the contraction is gradual and its duration indeterminate, are but varieties of the same state. This opinion has been gaining many supporters of late, and it is advocated in one of the most recent and important text-books of physiology.*

The second and more important of these considerations is, that the state opposed to muscular contraction is something more than mere passive relaxation.

This view has been advocated by more than one writer, and it is distinctly enunciated in the works of Bichât. In regard to the voluntary muscles, however, the opinion of this physiologist is somewhat obscure, for he speaks of these muscles being restored to the state in which they were previous to contraction by the action of the antagonist muscles. But at the same time, he cites some facts which show the presence of a positive power of extension:-thus, he speaks of the contraction and subsequent dilatation of a muscle after its removal from the body, when there can be no extraneous traction, and of the convulsive movements of a similar character which may be seen in divided muscles when these have been

Physiological Anatomy and Physiology of Man: Todd and Bowman, vol. ii. p. 172-1845.

exposed during surgical operations; and he further instances, as of similar significance, the existence of certain hysteric convulsions in which all the muscles are hard and expanded, so that those which (from the position of the part) ought to be relaxed, are as firm and unyielding as those which ought to be contracted.

In relation to the involuntary muscles the existence of a twofold mode of action is very apparent, and it is found that there is a power of expansion in these structures altogether independent of extraneous force. In some degree the contents of the viscera-in whose walls these muscles are seated-may oppose and partly counteract the state of contraction, (and in the absence of all antagonist fibres in the muscles themselves, this is the only extraneous power which could be productive of this result,) but any counteraction of this character is more than doubtful. Be this as it may, however, Bichât's arguments show that there is something more than this in the relaxation of these muscles.

In the first place, this conclusion is necessary from the fact, that the walls of the alimentary canal alternately contract and dilate, when the contents are artificially removed, or when the passage is emptied. In the second place, a similar conclusion is necessary from the fact, that alternate and rhythmical changes of an analogous character may be observed in the heart and

neighbouring great vessels when completely evacuated of their contents. It is also observed, that on irritating a heart with the point of a knife, or any other instrument, the first change was the production of dilatation; and further, as showing the activity of this state, it is said that the force of the diastole as felt by grasping the heart, and the force of the systolic contraction as felt by placing the fingers in the ventricular cavities, are equally great so far as may be judged by this coarse experiment. This fact had been observed before Bichât's day, and it has been since corroborated by Cruvelhier, and other observers of considerable authority.

In each system of muscular structures it would appear therefore that there are two modes of motion, one of contraction, and another of an opposite character. And to show the definitiveness of the idea which Bichât possessed in relation to the latter mode, he concludes his remarks on this important subject by hinting at some analogy between this quasi-state of expansion, and that more conspicuous movement of this character which is seen in the iris and in the several forms of erectile tissue.

The actual nature of this twofold change, however, is only to be understood after a careful and particular examination of the individual phenomena of muscular action; but before we proceed to this there

are still two preliminary considerations which deserve attention as well calculated to facilitate our future

inquiries.

It may be supposed, in the first place, that the mystery of muscular action will receive some explanation from the history of the fibrine dissolved in the blood, for this substance is analogous, in essential particulars, to that which occurs in muscular tissue. Now the history of the two is indeed parallel in some respects. As to the fibrine of the blood, it is found to remain in a fluid condition during life, and to solidify after death, so that we may properly argue that the latter in relation to the former is the passive state, and that the influence of life-be this what it may-is exerted in counteracting the state of solidity. In the fibrine of muscular tissue there is also a twofold condition, and from the solidification which takes place after death we may argue that there is during life, if not a condition of fluidity, a quasi-softness approaching to this state. In addition to the solidification which supervenes upon death, there is also another form which occurs during life, and the question is as to what is the meaning of this. Is it, we may ask, in any way related to the phenomenon which occurs in the fibrine of blood and muscle after death? If it is, then the inference will be that muscular contraction in relation to the opposite condition is a phenomenon which, like the coagulation of

the blood, is determined by the withdrawal and not by the communication of a stimulus.

It may be presumed, in the next place, that the dartos will contribute towards the knowledge of muscular action, for this tunic is closely allied to true muscle both in structure and function. In the dartos, moreover, there are few nerves and vessels, and therefore the phenomena of irritability may be supposed to be simple and intelligible, and not complicated by those agencies which obscure the secret action of true muscle. Here, indeed, it seems as if the problem of mobility were stated in the simplest terms, and the result made known as clearly as possible. What these are we may learn in the first chapter, where it is shown that the state of contraction is associated with the operation of cold, and the state of relaxation with the presence of warmth. Contraction, indeed, in relation to the antagonistic phase of relaxation, is seen to be a phenomenon connected with the withdrawal, and not with the communication of a stimulus. The true connexion of contraction in the dartos with the analogous phase of muscular activity cannot now be determined, but if it is such as may be presumed from the anatomical relation of the two structures, the result is very significant,-and the more so as it harmonises with what is deduced from the changes in fibrine.

Under any circumstances the histories of the dartos