CHAPTER III.

DIVISIONS OF THE MUSCLES.

THE muscles are divided into voluntary and involuntary.

To the latter belong the diaphragm, the heart and the intestines.

To the voluntary belong all the remaining muscles.

We must distinguish (a) muscles of the head, (b) muscles of the trunk, (c) muscles of the upper limbs and (d) muscles of the lower limbs.

After having treated of muscles in general, we have only to observe particularly the muscles of the trunk, since our method of tone-formation, as far as breathing is concerned, is based chiefly upon the correct activity of the abdominal muscles and the diaphragm.

Of the muscles of the trunk we only require to observe more closely the chest and abdominal muscles, as well as the diaphragm.

The muscles of the chest serve in breathing to move the chest, as also at times the arms, and to

lower the shoulders. They lie upon the front surface of the thorax (without covering the middle), toward and at the sides, and fill out the spaces between the ribs.

The abdominal muscles serve particularly (for our purpose) for expiration. Besides this they shield the intestines, promote the bending of the body, and help to contract the abdominal cavity.

"The ribs are attached to the spine so as to be freely movable upon it; but when left to themselves they take a position which is inclined obliquely downward and forward. Two sets of muscles, called intercostals, pass between the successive pairs of ribs on each side. The outer set, called external intercostals, run from the rib above obliquely downward and forward to the rib below. The other set, internal intercostals, cross these in direction, passing from the rib above, downward and backward, to the rib below. The action of these muscles is somewhat puzzling at first, but is readily understood if the fact that when a muscle contracts, it tends to make the distance between its two ends as short as possible, be borne in mind. Consequently the external intercostals must raise, and the internal intercostals must depress, the bony ribs."*

The other muscles of the chest, which connect the ribs with parts of the spine above them, and with the shoulders, require no special description. The function of all these muscles is merely either to raise single groups of ribs, or to raise them and at the same time force them outward so as to considerably expand the chest.

* Huxley's "Lessons in Elementary Physiology."

SECTION 1.

THE ABDOMINAL MUSCLES.

The abdominal muscles are, like all voluntarily movable muscles, thicker in the middle than at either end, where they terminate in shorter or longer tendons, by which they are fastened to the bones or to other tendons. They lie in layers one upon another, form the fore and side covering of the abdominal cavity, and are always found in pairs. This is because the single muscle does not extend the whole width of the abdomen, but only to the middle where it ends with its tendon on the musculus rectus, in the middle of the latter and in or on the linea alba (by which the musculus rectus is equally divided). A similar formation is found on the other side, so that the two muscles are connected by their tendons and practically form one.

2. Musculus transversus abdominis.

3. Musculus obliquus descendens.

oblique.)

These

(The external

4. Musculus obliquus ascendens. (The internal oblique.)

Imagine the entire wall of the abdomen freed from

all its insertions, and stretched out flat, and it will show about the following form and position of its muscular fibres (Fig. XIII).

In a perpendicular line from a to b, musculus rectus abdominis; transversely from c to d, musculus transversus abdominis; obliquely from e to f, musculus obliquus descendens; obliquely from g to h, musculus obliquus ascendens.

The action of the muscle oblique descendens, and that of the ascendens, if acting separately, are dissimilar. The descendens, marked e, f, e, f, when alone active, or specially so, presses the contents of the abdomen upward, the ascendens, g, h, g, h, more downward. Owing to this the first-mentioned

oblique descendens is active particularly in singing and speaking.

If, however, the descendens and ascendens of both sides co-operate, i. e., all four act uniformly and simultaneously, then the combined action in all parts of the abdomen, especially at the sides, will be a contraction of the abdominal cavity from the front backward.

The musculi transversi abdominis, marked c, d, contract the abdominal cavity in a horizontal direction. The shortening of the fibres causes the side walls of the abdomen to become flattened, and the middle wall to be drawn nearer to the vertebral column. Their fibres can all contract simultaneously, or one division can be especially active; in every case, however, both sides operate simultaneously. The upper fibres, which are attached to the ribs, can only then contract powerfully when the ribs are fixed.

The musculi recti abdominis, those marked a, b, draw the sternum down, contracting the abdominal cavity in a vertical direction. These muscles are comparatively the least active, being very narrow, whilst the musculi transversi abdominis, which are spread over the entire abdomen, are the most active.

The co-operation of all the muscles causes con