When the stimulus ceases to act upon the muscle, or when the muscle ceases to obey the action of the stimulus, contraction likewise ceases and relaxation ensues ; the fibres no longer exercise their power its shortening them selves, and the muscle is reduced to ts former state. The replacement of the parts is brought about in various ways ; sometimes by the elasticity of the membranes contained in, or immediately connected with, the muscle, by that of the ligaments and cartilages attached to it, by the action of antagonist muscles, or by the weight or reaction of the parts that were moved, being now left at liberty to exercise their powers. We arc therefore to consider relaxation as altogether a passive effect, or a negative quality, merely consisting in the absence of contraction.
There is a circumstance respecting muscular contrac tion, which it is important to notice, that the power of contraction is limited in its duration. When a stimulus is applied to a muscle, although the application be con tinued as at first, or even increased, after a certain time it ceases to act, and relaxation necessarily follows ; the mus cle is then said to be exhausted, and it requires some time to intervene before the part is recruited, or can so far re gain its power as to be again capable of obeying the action of stimuli. In general, the degree of exhaustion cor responds with the previous degree of stimulation, but they are very far from being in an exact ratio to each other. It is not perhaps very easy to ascertain the exact amount of contraction of which muscles are capable; in ordinary cases, we should consider it a powerful exertion which could reduce a muscle to two-thirds of its natural length, but there are instances where it would seem to go far beyond this limit.
Now that we are become familiar with the conception of contractility, as a distinct property inherent in muscle, and peculiar to it, we can scarcely imagine any physical quality which is of a more specific character, yet it was only of late years that its nature was properly understood; for although Baglivi, Glisson, and others, entertained some correct notions upon the subject, Haller was the first who clearly pointed it out as a distinct quality, and announced it as the exclusive property of the muscular fibre. Before his time contractility had been more or less confounded with elasticity, yet the difference between them is suffi ciently obvious. Elasticity is the mere effect of reaction; it is never the origin or source of power, but only restores, in a contrary direction, the force which had been previous ly impressed. Even when acting to the greatest advan
tage, it cannot afford any real increase of power, nor can 'it produce an effect, in the smallest degree greater than the cause ; nor can reaction take place as long as the force continues to be applied. But the contrary to all this occurs with respect to muscular contractility. The me chanical effect bears no proportion to the cause producing it, and at the very time that the cause is still acting, the effect takes place, and infinitely surpasses the force of the agent. Although no facts occur more frequently to our observation than those here alluded to, yet so difficult is it to detach the mind from a wrong direction, into which it may have once swerved, that, until the last century, all the attempts to account for muscular contraction were de rived from the laws of mechanical impulse. The most learned men of the age, profound mathematicians, and i acute rcasoners, invented various hypotheses of this de scription, which were brought forward with all the parade of geometrical demonstration, and supported by a series of problems, theorems, corollaries, and lemmas. To all such misapplied learning, it is sufficient to reply, that a me chanical force of an indefinite extent is frequently pro duced without the intervention of any mechanical cause whatever, and must therefore be referred to a power of a totally different nature.
We have mentioned above, that one of the most fre quent causes of muscular contraction is the exercise of volition ; but it is observed that different muscles bear a different relation to this faculty. Some muscles are almost entirely under the power of the will, while there are others over which the will has no control. Hence arises a divi sion of muscles Into voluntary and involuntary, the first in cluding the muscles of locomotion, and those immediately concerned in the action of the external senses; the second including the muscles employed in the internal functions, such as the heart, the parts about the lungs, and the sto mach. The law of contractility, by which relaxation al ways succeeds to contraction, is the most observable in those muscles that are affected by the stimulants that act independently of the will ; when the stimulating power is applied to them in a regular and uniform manner, the alternations of contraction and relaxation proceed with perfect regularity, and give rise to some of the most im portant operations of the animal economy.