4. When by any stimulus, applied to mus cular fibres, the filaments directly stimulated are thrown into action, the contractions very generally and rapidly extend to many others in their neighbourhood, frequently even to the whole muscle of which they form a part ; but the contractions of single fibres appear to be of short duration, and the more enduring efforts to be made by many short successive contractions and relaxations or vibrations of the individual fibres.
From these facts it would appear, that each exertion of this property of Irritability essen tially consists in a greatly increased attraction among the particles or globules constituting the muscular fibres, and alteration of the di rection in which this attraction acts,—rapidly communicated from one particle to another, both along the same fibre, and among adjacent fibres,—and rapidly succeeded by repulsion, or return to the previous state of the cohesive attraction existing among these particles.
When a muscular mass, consisting of many -such fibres, is thrown into this kind of action, it is easy to understand that its breadth and thickness, and its rigidity or resistance to com pression will be increased; that its extremities will be approximated ; and that if it be dis posed around a cavity, containing a fluid and provided with an outlet, that fluid will be expelled. It is by such contractions, that all the more conspicuous movements, even of the organic life, of the higher animals, are per formed, and that their locomotive and vocal powers are exerted ; and it is worth while to pause for a moment to consider the almost in conceivable amount of moving power, and ra pidity of motion, which various facts indicate in muscles thus contracting, whether under the influence of the will, or from other stimuli.
It seems well ascertained that the contrac tions of the left ventricle of the human heart, in its ordinary unexcited state, are sufficient to expel its fluid contents, in free space, a dis tance of 71 feet, and to balance a weight of above 50 lbs.; and this power is exerted regu larly more than once in every second, and often, even independently af disease, twice in every second, during the whole of human life. The ordinary action of the left ventricle of the Whale's heart suffices to egpel, according to Dr. Hunter's statement, at each pulsation, above ten gallons of blood, with a great velo city, through a tube of a foot in diameter.
Two instances given by Haller, and quite within the limits of ordinary experience, suf ficiently exemplify the great power occasion ally exerted by voluntary muscles ; and which will appear the more extraordinary when it is remembered, that the direction of muscular fibres, as regards the line in which they are to act,—the points of their insertion into the bones they are to move,—and the line in which they act, as regards the motion which they give to these bones,—are all, very ge nerally, such as to render their action dis advantageous, and require a greater amount of moving power than might otherwise have been necessary. The instances recorded are,
the case of a man, who could raise a weight of 300 lbs. by the action of the elevator mus cles of his jaw ; and that of a slender girl, afflicted with tetanic spasm, in whom the ex tensor muscles of the back, in the state of tonic contraction or opisthotonos, resisted a weight of above 800 lbs., laid on the abdomen with the absurd intention of straightening the body. In some of the lowest classes of ani mals the intensity of the muscular power ap pears to be greater than in any of the largest. Thus, a flea has been known to leap sixty time3 its ONVII length, and to move as many times its own weight. • Again, the rapidity of the changes of po sition of the component particles of muscular fibres may be estimated, although it can hardly be conceived, from various well-known facts. The pulsations of the heart can some times be distinctly numbered in children at more than 200 in the minute ; and as each pulsation of the ventricles occupies only one third of the time from the commencement of one pulsation to the commencement of the next, this implies that each contraction takes place in utluth part of a minute, or that ten times in each second, for many hours toge ther, the whole of the convoluted muscular fibres of the ventricles must be thrown into folds, and again smoothed out. Again, it is certain that by the movements of the tongue and other organs of' speech, 1500 letters can be distinctly pronounced by some persons in a minute. Each of these distinguishable sounds must require a separate contraction of muscular fibres; and the production and ces sation of each of these sounds must imply, that each separate contraction must be followed by a relaxation of equal length. Each con traction must, therefore, have been effected in aolooth part of a minute, or in the 50th part of a second. Haller calculated that in the limbs of a dog at full speed, muscular contractions Must take place in less than the 200th part of a second, for at least many minutes in suc cession.* But the property of Irritability, which acts throughout so great a portion of the animal creation, as a moving power of this extra ordinary efficiency, is not the only contractile power, which certain organic textures possess, or which the conditions of 'their existence re quire them to exert during the living state. Even in muscular fibres themselves, in certain °mans and still more in other textures of animal bodies, contractions are often observed, peculiar to the living state, but differing essen tially from those which come under the defi nition of Irritability already given.