posterior aponeurosis. 8, Its anterior
aponeurosis. 11, The right rectus muscle. 13, The crest of the ilium, or
haunch-bone.

_Practical Explanation._ The rectus muscle, 11, bends the thorax upon the
abdomen when the lower extremity of the muscle is the fixed point; but
when the upper extremity is the fixed point, the effect is to bring
forward and raise the pelvis and lower extremities. They likewise depress
the ribs in respiration. The transverse muscle, 6, 7, 8, lessens the
cavity of the abdomen, and presses the intestines; stomach, and liver
upward, against the diaphragm, in expiration.]

3d. The region of the back, in consequence of its extent, is common to
the neck, the upper extremities, and the abdomen. The muscles of which
it is composed are numerous, and are arranged in six layers.

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What is represented by fig. 40? Give the function of some of the
muscles represented by this figure.

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[Illustration: Fig. 41 The first, second, and part of the third layer of
muscles of the back. The first layer is shown on the right, and the
second on the left side. 1, The trapezius muscle. 2, The spinous
processes of the vertebrae. 3, The acromion process and spine of the
scapula. 4, The latissimus dorsi muscle. 5, The deltoid muscle. 7, The
external oblique muscle. 8, The gluteus medius muscle. 9, The gluteus
maximus muscle, 11, 12, The rhomboideus major and minor muscles. 15, The
vertebral aponeurosis. 16, The serratus posticus inferior muscle. 22, The
serratus magnus muscle. 23, The internal oblique muscle.

_Practical Explanation._ The muscles 1, 11, 12, draw the scapula back
toward the spine. The muscles 11, 12, draw the scapula upward toward the
head, and slightly backward. The muscle 4 draws the arm by the side, and
backward, The muscle 5 elevates the arm. The muscles 8, 9, extend the
thigh on the body. The muscle 1 draws the head back and elevates the
chin. The muscle 16 depresses the ribs in expiration. The muscle 22
elevates the ribs in inspiration.]

159. The diaphragm, or midriff, is the muscular division between the
thorax and the abdomen. It is penetrated by the oesophagus on its way
to the stomach, by the aorta conveying blood toward the lower
extremity, and by the ascending vena cava, or vein, on its way to the
heart.

[Illustration: Fig. 42. A representation of the under, or abdominal side
of the diaphragm. 1, 2, 3, 4, The portion which is attach