Whenever abroad muscle arises and terminates on one of the great cavities, it preserves everywhere nearly its breadth, because it has a large surface for its insertion. But if from a cavity it goes to a long bone, or to a small apophysis, then its fibres gradually approximate; it loses its breadth, increases in thickness, and terminates in an angle which is succeeded by a tendon, which concentrates into a very small space fibres widely scattered on the side of the cavity. The great dorsal and pectoral muscles present us an example of this arrangement, which is met with also in the iliacus, in the glutæi medius and minimus. The broad muscles of the pectoral cavity have a peculiar arrangement which the ribs require; their origin takes place by serrated points fixed to these bones, and separated by spaces between them.
The broad muscles are most often simple; many rarely unite to form compound ones. Different cellular layers separate them, like the long muscles; but they are hardly ever like them covered by aponeuroses; the greatest number is merely subjacent to the integuments; the reason is that their form naturally protects them from these displacements, of which we have spoken in the article on aponeuroses, and which, without these membranes, would be so frequent in the long muscles. I do not know that the cramp has ever been observed in those of which we are treating. When the abdominal muscles are laid bare by incisions made through the integuments of a living animal, I have observed, that in contracting, the bulk of each preserves the same place.
III. Forms of the Short Muscles.
The short muscles are those in which the three dimensions are nearly equal, having a thickness in proportion to their width and their length. They are found in general in the places, where is required on the one hand much power, and on the other small extent of motion; thus around the temporo-maxillary articulation the masseter and pterygoids, around the ischio-femoral the quadratus the gemini the obturators, &c. around the scapulo-humeral the supra-spinalis and the teres minor, in the hand the muscles of the palmar eminences, in the foot various fleshy fasciculi, in the vertebral column the interspinal, in the head the small and great anterior, posterior and lateral recti, exhibit more or less regularly the form of which we are treating, and answer the double object that I have indicated, on the one hand by the very considerable number and on the other by the shortness of their fibres.
The short muscles are, more often than the broad, united to each other, either in their origin or termination, as we see in the foot and the hand. Sometimes they are of a triangular form, as in these two parts; sometimes they approach to the form of a cube, of which there is an example in the masseter and pterygoid muscles. In general they are rarely covered by aponeuroses, undoubtedly because the shortness of their fibres prevents them from being liable in a great degree to considerable displacements.
Besides, the division of the muscles into long, broad and short, is, like that of the bones, subject to an infinite number of modifications. In fact many of these organs have mixed characters; thus the sub-scapularis and the infra-spinalis have a form intermediate to the broad and short one; thus the cruræus, the gemelli of the leg, &c. cannot be considered precisely long or broad muscles. Nature varies, according to the functions of the organs, the conformation of the agents of their motions, and we can only establish approximations in our anatomical divisions.
ARTICLE SECOND.
ORGANIZATION OF THE MUSCULAR SYSTEM OF ANIMAL LIFE.
The part peculiar to a muscle is what is commonly called the muscular fibre; the vessels, the nerves, the exhalants, the absorbents and the cellular texture, which is very abundant around this fibre, form its common parts.