The action of muscles is never intermitted, and only diminished in the sleeping state. This action arises from a less degree of that power which moves the limbs, and is styled, by Haller, the vis insita; by later authors, irritability, or tonic power. When muscles are not exercised, this power is very slightly exerted; but, by the position which the limbs assume, we discover the relative strength of the antagonizing muscles. Thus we find the flexors stronger than the extensors; for during sleep the head falls forward, the body, legs, arms, and fingers are lightly bent. We see the cause of this strength, when we find that the flexors have stronger and more numerous fibres; that their insertion is farther from the centre of their motions, and under a larger angle, which, as we have remarked, must increase when flexion has begun. This superiority of the flexors bends the fetus in utero into a round ball. When the infant is born, the same superior power of the flexors continues, though in a less degree, and frequent pandiculations are required, to give activity and energy to the extensors, which they again lose in advanced age. When we awake from a sound sleep the same yawning pandiculations occur; and Barthez fancifully refers the crowing of the cock, and the fluttering of his wings, to a similar purpose. It is always useful to examine, in diseases, the position of the limbs during sleep, particularly of children. If they deviate from the bent to a more straight position, there is generally some irregularity in the state of tone, and, of course, in the vital influx.

It is supposed by some authors, that the vital influx is not necessary to the contraction of muscles, but that they possess irritability as a quality, and this is the strict meaning of the vis insita of Haller; but we do not, in any instance, find this, for any time, unconnected with life. Irritability is, indeed, lost at different periods in different organs; for some, particularly the heart, soon lose it, while the voluntary muscles retain it; and apparently those which derive their nerves from the intercostal system lose it sooner than those whose nerves originate from the base of the cerebrum. Irritability is also exhausted in the agonies of death, and retained for a longer time when the death has been sudden and violent, except it happen from lightning. On this principle fish are crimped; for the muscles are cut across, while the irritability remains, and the muscular portions contract so as to give greater firmness to the flesh. To preserve this irritability, the fish receives a blow on the head as soon as it is taken from the water, when the operation is performed, is washed in hard water, whose astringency, from the selenite it contains, assists the contraction. Irritability is also soon destroyed by narcotic poisons, either if partially applied, or, more generally, if life is extinguished by their means. The power of contraction, by volition, by association, by a stimulus on the brain, or on the nerves in their course, as it acts through the medium of the nerves, is called the nervous flower; though when volition only is the cause, Dr. Cullen chooses to call it the animal flower. The tonic flower, as we have said, is that state of constant contraction owing to life, or perhaps to the action of antagonizing muscles.

The state of muscles differs according to their vigour and their mobility. The first attends firm and robust, the second weak, constitutions. In the latter, muscular contraction frequently alternates with relaxation; and the greatest degree of this state is Convulsions, vide in verbo. Vigorous contraction is less subject to alternate with relaxation; but such alternation is common to muscles in general, and found in every long muscle but among the round muscles in the heart, the intestines, probably also the stomach only. Spasm is not a disease of vigour, but of a high degree of irritation, and owing to an irregular influx of the vital power. We have enumerated association among the stimuli, rather from a loose analogy than with strict pretensions to accuracy, including, in this way, each source of muscular action.

Muscular irritability is exhausted not only by exercise, by narcotic poisons, and every kind of excitement, but by mental exertions also: and the student, constantly at the midnight lamp, finds it greatly diminished, not only by his mental exercise, but by disuse. Violent emotions have a similar and more sudden effect. At the time we observe that violent muscular action diminishes, and occasionally destroys, sensibility. Thus, in battle, the soldier does not feel his wound; and the juggler violently contracts the muscles of his leg when he runs the pin into them. These facts, which might be supported by many analogous ones, seem to show that the sentient and moving powers are more closely connected than physiologists have supposed.

The bulk of a muscle is chiefly made up of cellular substance and blood vessels. When each is separated, the true muscular fibres occupy a very small space, and the muscles are found equally, often more, distinct in the emaciated invalid than in the robustest subject. We may conclude from hence, that muscular fibres are not destroyed nor produced; and we think the opinion is supported by observation. If a muscle is destroyed, the limb remains useless, or at least partially useful, by the assistance of those which remain. If cut through, the fibres are united by a compact cellular substance. Yet late authors have been fond of employing the analogy between the fibrin and the muscular fibre, particularly when they found the fibrin contract by the galvanic stimulus; and, in support of it, they quote an observation of Haller, that the generality of muscular arteries are curved on themselves, in a remarkable manner, when entering into the muscles. This structure, they think, must retard the blood, and facilitate the separation and deposition of the fibrin. The muscular flesh, they add triumphantly, is the most azotic fluid of the whole system, and the fibrin the most animalized portion of the blood. If, however, the fact be as just stated, that muscular fibres are not reproduced, this necessity no longer appears, and we know not that contraction is connected with azot. Irritability and sensibility are both, however, apparently peculiar to a fibrous structure; and we know so little of the means by which the nervous influence is connected with the nerve as a simple solid, that we may suppose this union in part depending on fibres, and, when these are formed, that they may share a portion of this incomprehensible power. Thus, then, the fibrin may enjoy a certain portion of life; thus the muscles may be more directly a part of the nervous system. Yet we have disclaimed both opinions; and, though the course of our argument has led us to this view of the subject, we must add that such reasoning appears loose and precarious. Sec Blood, vol. i. p. 357, & c. and Nervus.