Inspiration, caused by the elevation of the ribs and depression of the diaphragm.

The intercostal muscles fill up the spaces between the adjacent ribs. They are arranged in two sets, external and internal, eleven pairs of each.

Expiration, caused by the slackening of the muscles, the weight of the thorax, and the elasticity of the lungs. The triangularis sterni is a special depressor of the ribs, and some anatomists consider the internal intercostal muscles to be so also.

Fig. 37. Intercostal Muscles.

Fig. 37. - Intercostal Muscles.

Action.

Muscle.

Origin.

Insertion.

Nerve-supply.

Inspiration

External intercostal

From the lower border of the rib between the tubercle and the costal cartilage. The fibres go downwards and forwards in series with the fibres of the external oblique

The upper border of the rib below from the tubercle to the costal cartilage

Intercostal

The space in front over the costal cartil ages is tilled by the ante rior intercostal aponeurosis

Inspiration (contd.)

Internal intercostal

From the lower border of the costal cartilage and inner edge of the subcostal groove ; from the sternum to the angle of the rib. The fibres go downwards and backwards in series with the fibres of the internal oblique

The upper border of the rib below from the sternum to the angle of the rib

Intercostal

The space behind, between the angles and the tubercles of the ribs, is filled by the posterior intercos tal aponeurosis

Scaleni

See Back Muscles

Serrati postici

See Back Muscles

Depressors of the ribs in expiration

Triangularis sterni

From the back of the ensiform cartilage and sternum as high as the third costal cartilage.

The cartilages of the second to sixth ribs inclusive

Intercostal

Internal intercostal

See above

Abdominal muscles

See Flexion of Spinal Column

Several other muscles are used in acts of extraordinary or forced inspiration, namely:

Quadratus lumborum. Pectorales major and minor. Serratus magnus. Latissimus dorsi. Sterno-mastoid. Extensors of the back.

The Deep Fascia of the trunk forms a complete investment for all the muscles. In the upper part of the body it is similar to the deep fascia of the limbs, forming an outside covering, and sending in septa between the muscles; but in the lumbar and abdominal regions it is rather more complicated.

The Lumbar Fascia is attached to the lumbar vertebrae in three layers-the outermost layer, to the spines; the middle layer, to the transverse processes; the innermost layer, to the bodies, near the roots of the transverse processes.

The outermost layer covers the dorsal surface of erector spinae.

The middle layer lies between erector spinae and quadratus lumborum.

The innermost layer covers the ventral surface of quadratus lumborum.

The middle layer then splits into two, so that four layers of fascia are formed for the three layers of abdominal muscles.

1. Covers the outer surface of external oblique.

2. Lies between external and internal oblique.

3. Lies between internal oblique and transversalis.

4. Covers the inner surface of transversalis.

At the outer border of rectus abdominis these four layers now join again to form two layers, which pass one in front of, the other behind, rectus abdominis, forming what is called the sheath of the rectus; this takes place in the upper three-fourths of the muscle. In the lower fourth both the layers of fascia pass in front of the rectus, so that on its ventral surface three-fourths of the way down there is a free edge of fascia, known as the fold of Douglas.