The strong mineral acids, and the caustic alkalies, rea dily dissolve the muscular fibre ; but the only circumstan ces that are deserving of any particular attention, are the effects of the nitric acid. The same gases are extricated during the solution, as from many other animal substances, but with this difference, that the muscular fibre yields a larger proportion of azote. As this is the element which predominates in animal substances, and which particularly distinguishes them from vegetables, muscles are said to be the most completely animalized part of the system, thus affording an example of what has been already alluded to, that the chemical composition of the body bears a certain relation to its specific powers. Diluted nitric acid has the power of producing a very peculiar change in the muscular fibre, which is also capable of being brought about by other means. When it is digested either in an acid of this description, or is kept for a length of time at a certain temperature, instead of undergoing the usual pro cess of putrefaction, it nearly preserves its original form, but loses its colour and consistence, and is converted into a substance very similar to spermaceti.
A well known and very remarkable example of this kind of change occurred between thirty and forty years ago in Paris, where, in consequence of an order from the police, it was determined to remove the contents of an immense burial ground, when, to the surprise of those engaged in the undertaking, the bodies were found to be, for the most part, converted into this peculiar substance. Fourcroy examined it with much minuteness, and from its appear ing to possess properties intermediate between those of fat and wax, named it adipocire. This substance is very similar in its properties to spermaceti, differing from it principally in melting at rather a lower temperature, and not exhibiting any degree of the crystalline or laminated texture. By observing what takes place when we employ nitric acid in the formation of adipocire, it would seem that the change consists in the abstraction of part of the azote, and the addition of a quantity of oxygen. When the acid is employed in a concentrated state, and its action is promoted by heat, the muscle is dissolved and decom posed; carbonic acid, nitrous, and azotic gases, are disen gaged, and several new compounds are produced.
We must now proceed to give an account of the proper ties and uses of muscles. As a large quantity of mem brane always enters into their composition, we find that they partake of the qualities of this substance, as well as of those which belong more exclusively to the proper mus cular fibre. Accordingly, muscles are always, in some degree, elastic ; but it is impossible to ascertain whether this elasticity belongs to the fibre itself, because the fibre cannot be detached from the membrane which encloses it, without having its structure much injured ; but, from its consistence and texture, as described by Sir A. Carlisle, we may conjecture that it is not so. The properties, how ever, which the muscular fibre may have in common with membrane, or with any other form of matter, are little worthy of our attention, compared with that power which it exclusively possesses, and from which all its actions are immediately derived, what has been designated under the name of contractility. Contractility may be defined, the
power which the fibre possesses of shortening itself upon the application of a stimulus. Under the appellation of stimulus are included a variety of things, which seem to have scarcely any common property, except that of acting upon the muscular fibre. It has been asserted, and is in deed literally true, that every body in nature is a stimulus to the muscular fibre, because, independently of any other quality, the mere contact of a material substance produces this effect. Stimuli have been arranged in various ways ; but perhaps the most convenient and comprehensive is into the three classes, of mechanical, chemical, and what may be termed physiological. 'Mechanical impulse of all kinds, beginning with the slightest touch that is capa ble of being perceived, and proceeding to any force short of what absolutely destroy the texture of the part, are of the first class ; a great variety of chemical substances, alcohol, acids, alkalies, metallic salts, and many vegetable acids, that have few properties in common, are of the second class, while, in the third class, we may place those agents that seem to operate through the intervention of the nervous system, as the electric fluid, and particularly that modification of it which is styled galvanism. Inde pendent of any external agents, the muscles are thrown into the strongest contractions by a variety of affections, which arise from internal causes, and, above all, from the act of volition. By a process which has hitherto proved inexplicable, we no sooner will the motion of any muscle, than the desired effect is immediately produced.
When, by means of any of these stimulating powers, a muscle is thrown into action, the middle or belly swells out and becomes hardened, the ends are brought nearer together, and the surface, which was before smooth and shining, now becomes furrowed and wrinkled. In conse quence of this swelling out of the middle, and approxima tion of the extremities, the more moveable of the two ends is drawn towards that which is the most fixed, and the bones or other parts which are attached to it, are moved in the same direction. It has been a question, whether the contraction of a muscle increases its specific gravity, or whether they do not exactly gain in thickness what they lose in length. On this point Sir G. Blanc made the following experiment : he enclosed a living eel in a glass vessel, filled with water, the neck of which was drawn out into a tube ; then, by a wire introduced into the vessel, he irritated the animal, so as to produce strong contraction, but he found that the water in the tube remained stationary during the operation. An experiment of a similar kind was performed by Sir A. Carlisle on a man's arm, but with a different result, for here the volume of the muscle appeared to be increased by the contraction, indicating a consequent dimunition of specific gravity. But such ex periments probably do not admit of a very decisive result, for it may be supposed, that while one muscle, or set of muscles, is contracting, the effect may he counter-ba- lanced by the relaxation of a corresponding muscle or set of muscles.