The muscular vessels permit under certain circumstances the escape of the blood they contain; hence different kinds of remarkable hemorrhages, especially in scorbutic patients, sometimes in putrid fevers, rarely or never in those diseases that are characterized by an increase of vitality. Infiltrated with blood in preternatural hemorrhages, particularly in false aneurisms, the muscles lose in part their motion; this happens also in contusions, in which similar infiltrations are observed.
The veins everywhere follow the arteries in the muscles; they have the same distributions, and receive from the contractions of these organs an essential assistance to their action. The throw of blood is more powerful when the patient we have bled contracts his muscles, than when he relaxes them; the fluid is as it were expressed, as from a wet sponge which is squeezed. The arterial circulation does not exhibit this phenomenon. I have observed that if we open the artery of the foot of an animal, and by the irritation of the nerves, make the muscles of the leg and thigh, through which this artery passes before reaching the foot, contract powerfully, the throw of blood is not stronger than during the relaxation.
I have many times injected the veins of the muscles of animal life with ease, from the trunks towards the branches, which makes me believe, notwithstanding what Haller has said, that in these organs, as in the heart, the valves are less numerous than in many others. No doubt the assistance the veins derive from their surrounding organs supplies the place of these folds, or rather renders them useless, the weight of the column of blood not making a great effort against the venous parietes. The varices of the muscular veins are, as we know, extremely rare. These veins are of two orders; one accompanies the arteries and follows the same course, the others are spread superficially on the surface of the organ, without having corresponding arteries.
There are absorbents and exhalants in the muscles; but we can with difficulty trace the first, and the second cannot be perceived.
Nerves.
Almost all the nerves of the muscles of animal life come from the brain; the ganglions furnish a few of them; when this happens, as in the neck, the pelvis, &c. besides the filaments coming from these nervous centres, there are always filaments of cerebral nerves; without this, these muscles would be involuntary. Few organs receive more nerves in proportion to their size than the muscles. In general the extensors appear to have rather fewer than the flexors; but the difference is trifling. It is true that all the great nervous trunks are in the direction of flexion, that in that of extension there are only branches, as we see in the posterior part of the arm, of the fore-arm, the vertebral column, &c. It is true also that this remark is likewise applicable to the existence of the vessels, which are larger and more numerous in the first than in the second direction; but this greater number of vessels and nerves arises from this, that there are many more flexors than extensors, that the first are stronger and have more numerous fibres; so that each of these fibres hardly receives more nervous or vascular filaments in one kind of muscles than in the other. I think that there is but little foundation for what has been said upon the difference of the strength of the fibres of the flexors and of the extensors, upon the predominance of the first, &c. If these are superior, it is either because they are more numerous, as in the foot, the hand, &c. or more advantageously arranged, as in the trunk on which the abdominal muscles act very far from the point of attachment to bend the spine, whilst to extend it, the dorsal muscles exert their action immediately at the side of this point of attachment, as also in the neck, where the muscles that draw down the lower jaw and head when this bone is fixed, are much further from the occipital condyles, than the muscles which produce extension. Whatever may be the cause of the superiority of the flexors, the fact cannot be doubted. 1st. In hysterical convulsions, in those of infants, in all the spasmodic motions in which the will has no part, the contractions take place much more in the direction of flexion than in that of extension. 2d. In old people the flexors finally become superior to the extensors; for example, the fingers and toes are almost uniformly bent. 3d. In all the motions, the power is always on the side of flexion.
The nerves enter the muscles of the extremities at a very acute angle, because the nervous trunks are in the natural direction of these organs. In the trunk, on the contrary, the nerves going from the spine, the cervical especially, enter their muscles at almost a right angle or one less evidently acute; this circumstance is of no importance. Each branch in the fleshy fibres, is at first divided and then sub-divided in their interstices, and afterwards lost in their texture. Does each fibre receive a small nervous filament? We should be led to believe so from this observation, that the principal branch being irritated, all the fibres are put into action, no one remains inert. But on the other hand, if we irritate one of them, all move also, which is certainly a sympathetic phenomenon or one arising from the communication of the cells.
Are the nerves deprived of their cellular coverings, and do they become pulpy when they enter the muscles? Dissection has shewn me nothing like it.