The distinguishing characteristic of muscular fibres is their irritability, the quality by which they contract in obedi ence to the will, or on the application of stimuli. This is an endowment residing in all muscular organs, but not in equal degree. The hollow muscles, which are subservient to the vital functions, hold the first rank ; these are followed by the muscles of respiration, and the other vo luntary muscles close the enumeration. It is doubted whether the arteries or the large venous trunks be irritable.
The contraction of a muscle consists in a shortening of its fibres, which are mark ed by transverse rugre, and feel indurat ed. But although its length is thus di minished, its circumference is propor tionally enlarged. These circumstances produce an approximation of the move able points to which the muscle is attached, and in this way all the motions of the bo dy are perfbrmed.
An uninterrupted supply of blood, and connection with the brain by the nerves, is essential to the voluntary action of muscles : ligature of the arteries or nerves destroys this power. But these organs still retain the faculty of contract ing on the application of stimuli, even after the connection with the brain be cut off, and the animal be in other re spects dead : this power is the irritabili tas of Haller, the vis insita, or muscularis; which, as that great physiologist and his folloWers contend, is peculiar to the mus cular fibres exclusively. That this pro perty does not depend on the nerves is clear, from the fact of several parts sup plied with nerves not possessing it ; and f'rom its remaining after the nerves of a part have been divided.
The nerves may perhaps be regarded as the more remote or exciting causes of muscular motion, of which irritability is the proximate or efficient cause. The passions of the mind act on the sensori um, which reacts on the nerves of the heart, and thus heightens the irritability of that organ, exciting palpitation and other irregular motions. The operations of the will on our organs of motion may be explained in the same way.
This distinction of the causes of mus cular motion may be supported by the experiments, in which the irritability of the muscles has remained after para lysing a part, by tying or cutting its nerves : and by cases of naprilvsi, in which sensation has remained in-a limb after its power of motion has ceased, or vice versa.
As it would be a fruitless labour -to enumerate and consider all the hypothe ses have been framed concerning muscular motion, we shall pass over that part of the subject, and refer the reader to the article GALVAIsTISM for an account of the effects of that principle on the mus cles. - The real power of muscles is immense. In the human body they are generally in serted near the centre of motion, and consequently with a mechanical disad vantage ; so that much of their force is expended in overcoming this obstacle. Hence it has been calculated, that the deltoid exerts a force equal to 2568 pounds to surmount a resistance of 50 pounds. The force with which a mus cle contracts is in a direct ratio with the number of its fibres ; but the degree of its contraction, and consequently the ex-‘ tent of motions that it can effect on the limb, is relative to the length of the fi bres. The precise limits of contraction in each fibre cannot be assigned ; fbr though the long muscles of the extremi ties „ are supposed to diminish only a third of their length in contraction, the circu lar fibres of the stomach, which, in the state of extreme dilatation of this organ, from circles of nearly a foot in diameter, can contract to a ring of one inch in cir cumference.
Our body contains about four hundred and fifty muscles, which, when we consi der their wonderful and artificial con struction and collocation, and the united advantages of firmness and mobility in the instruments of motion to which they are fixed, bestow on us two endowments of the highest utility and consequence ; the greatest agility of the whole body and of individual parts, combined with a won derful strength and power of enduring continued exertions. Both these prero gatives arise partly from the perfection in the fabric of the muscles themselves; which, as well as the perfect state of the bones and joints, is most conspicuous in the adult stage of life ; and partly froM exercise and habit, the influence of which in augmenting the extent and Celerity of muscular motion is most con spicuous in the feats of the opera and rope dancer, the runner, the boxer, the _rtcr, &c.