From their external forms, the muscles may be divided, like the bones, into long, broad and short. Their arrangement varies according to these three general forms.
I. Forms of the Long Muscles.
The long muscles occupy in general the limbs, to the conformation of which theirs is accommodated. Separated from the skin by the aponeuroses, from the bone by the periosteum, they are situated in a sort of fibrous gutter, which retains them powerfully, and in which they are arranged in layers more or less numerous, the deep ones are confined in their place by the superficial ones, which, in their turn, have the aponeuroses to support them. These last are very long; they commonly belong to the motions of three or four bones and even more, examples of which we have in the sartorius, the semi-tendinosus and membranosus, the biceps, the flexors and the extensors. As they become deeper, they are also shorter and generally destined to the motions of two bones only, as the adductors, the pectineus, &c. are a proof.
Cellular layers separate them; they are loose where great motions take place, more compact where these motions are less, very thick where vessels and nerves go between the muscular fasciculi. Often spaces more or less considerable, filled with cellular texture, separate these fasciculi from each other. We divide the long muscles into simple and compound. They are simple when a single fasciculus forms them, compound when they arise from the union of many. These fasciculi are found then in two different manners; sometimes in fact its division is at the top of the muscle, as we see in the brachial and femoral biceps; sometimes it is at the inferior part, at the most moveable side that this division is met with, as in the flexor and extensor muscles of the leg and the fore-arm.
The long muscles often separate from each other, are sometimes held together by means of aponeuroses, which confound a more or less considerable portion of two, three and even four of these neighbouring organs. The origin of the muscles of the internal and external tuberosities of the humerus exhibits this arrangement, whence results an essential advantage in the general motions of the limb. Then in fact the contraction of each muscle serves, both to cause a motion below in the moveable part to which it is attached, and to strengthen above the fixed point of the neighbouring muscles which contract at the same time with it.
Every long muscle is in general thicker in its middle than at its extremities, a form which arises from the manner of the insertion of the fleshy fibres, which arising above and terminating below, some successively below others, are so much the less numerous as we approach nearer each extremity, whilst in the middle they are all found in juxta-position. The anterior rectus, the long supinator, the external radial muscles, &c. have manifestly this arrangement.
There is a particular kind of long muscles, which has no analogy but in external appearance with that of the muscles of the limbs. They are those that are embedded in front and especially behind the spine. Though they appear simple at first view, these muscles have as many distinct fasciculi as there are vertebræ. The transversalis colli, the sacro-lumbalis, the longus colli, &c. represent very well an elongated fasciculus like the sartorius, the anterior rectus of the thigh, &c.; but the structure of this fasciculus has nothing in common with that of these muscles; it is a series of small fasciculi, which have each their distinct origin and termination, and which appear to be confounded into one muscle only because they are in juxta-position.
II. Forms of the Broad Muscles.
The broad muscles occupy in general the parietes of the cavities of the animal economy, those of the thorax and abdomen especially. They form in part, these parietes, defend the internal organs, and at the same time by their motions assist their functions.
Their thickness is not great; most of them appear like muscular membranes, sometimes arranged in layers, as in the abdomen, sometimes covering the long muscles, as on the back; they are in the first case, much more extensive superficially than deeper seated.