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

traction in all parts of the abdominal cavity; and through this contraction, as before stated, expiration is brought about and strengthened.

SECTION 2.

THE DIAPHRAGM.

The diaphragm is a flat and sinewy muscle. It is attached to the interior surface of the lower ribs, and also to the vertebral column. It forms a wall between the thoracic and the abdominal cavity. The part of the muscle extending toward the chest-cavity is arched. In the act of inspiration it contracts, i. e., it flattens itself, and by this means increases the chest-cavity; but that an empty space should not result in consequence, we must, by means of the larynx and trachea, inhale fresh air into the lungs. Through relaxation, i. e., by the re-arching of the diaphragm, the lower part of the chest-cavity is made smaller; and, in this way, the air from the lungs is expelled (Fig. XIV).

The diaphragm, although considered an involuntary organ, can, owing to the diverse nature of its nervous fibres, be made voluntary to a certain extent; and it is this which enables us to sing and speak, as far as inspiration and expiration are concerned.

A, the diaphragm in inspiration; B, the diaphragm in expiration. Tr, trachea; St, sternum (breastbone); D, diaphragm (midriff); Ab, Abdomen. The shading roughly indicates the stationary air.

We shall see further on how much depends upon our ability to cause the diaphragm to perform certain movements, upon the power of controlling its action.

CHAPTER IV.

ORGANS OF RESPIRATION.

SECTION 1.

THE CHEST (THORAX).

THE bony frame of the chest is composed of the following parts: The upper twelve (cervical or

AN ANTERIOR VIEW OF THE THORAX.

1-12, the ribs; a, the breastbone; b, the clavicle.

dorsal) vertebræ, the ribs (twelve on either side, Fig. XV, 1-12), the sternum (a bone in the anterior and