The vertebral fascia is strong and thin ; it confines the muscles of the spine and head in a hollow between the spinous processes of the dorsal vertebrae and the angles of the ribs ; it joins the fascia of the neck and the lumbar fascia. The latter is a dense structure which gives support to the lumbar muscles, and origin to the three flat muscles of the abdominal walls. It is in three layers, and encloses the erector spinae and. quadratus lumborum.

The superficial fascia of the pectoralis major contains the mammary gland. The deep fascia is thin ; it invests the pectoralis major, and is attached to the clavicle and to the front of the sternum. At the lower border of the pectoralis it is continuous with the axillary fascia, which is very dense and stretches across the armpit, and is continuous with the fascia of: the serratus magnus and latissimus dorsi.

The superficial fascia of the abdomen is continuous with that of the chest. Above it is thin, but becomes dense at the lower part, where it consists of a fatty layer which is continuous with the superficial fascia of the thigh and a membranous layer which blends with the fascia lata of the thigh. The superficial fascia of the abdomen is more elastic than that of the rest: of the body, owing to the elastic fibres in its membranaceous, part.

The deep fascia of the abdomen lines the visceral portion of the deep muscles and the back of the abdominal cavity ; it is called the transversalis fascia and the iliac fascia.

The trunk has two chambers separated by a muscle called the diaphragm, which is the chief muscle of respiration, and, after the heart, the most important muscle in the body.

Its origin is : in front, by fleshy attachment from the back of the ensiform cartilage ; behind, by two pillars, or crura, from the bodies of the upper three lumbar vertebrae—laterally, from the lower six costal cartilages, and interdigitating with the transversalis abdominis. It has three openings

1. The aortic.

2.   The vena-caval.

3.   The oesophageal.

Through the aortic pass the aorta, the thoracic duct, and the vena azygos major. Through the vena-caval, the inferior vena cava, and through the oesophageal the oesophagus and the two pneumogastric nerves. There are small openings in the pillars for the passage of the small azygos vein and splanchnic nerves.

Fig. 73.—Diaphragm. 1, Aorta ; 2, œsophagus ; 3, opening for inferior vena cava ; 4, crus.

The upper chamber of the trunk is the thorax.

The lower chamber is the abdomen.

The thorax is a conical flexible case formed by the dorsal vertebrae, the ribs with their cartilages, and the sternum. It contains :

Lungs.
Heart.
Part of the trachea .
Part of the oesophagus.
Part of the thoracic duct.
Part of the great bloodvessels.

Fig. 74.—Thorax.

The spaces between the ribs are occupied by the internal and external intercostal muscles.

The trachea, which is the principal air-tube, 4.5 inches long and 0.75| inch wide, composed of membrane and imperfect rings of cartilage, passes from the throat into the thorax, and divides into the right and left bronchi, which go to the right and left lung respectively ; they divide and subdivide, after the manner of the branches of a tree, till the tubes reach every part of the lungs. The smaller tubes have no cartilages, but have coats of unstriped muscle which contract ; the ends of the tubes terminate in sacs, the walls of which are covered with saccular dilations, the air-cells or alveoli.

These air-cells are the special respiratory parts of the lungs. The capillaries from the pulmonary artery form a close network around them ; the blood in the capillaries comes almost in contact with the inspired air, having only the thin walls of the air-cells and capillaries between. The air-passages are lined with epithelium, and from the surfaces of the epithelial cells delicate filaments called cilia project, which, with a wavelike movement, send forward mucus with dust and other particles breathed in with the air, thus keeping the lungs comparatively free from foreign matter.

Relations.—In the thorax the trachea lies in front of the œsophagus, between the pleura ; it is behind the upper part of the sternum, the arch of the aorta, the innominate and left • carotid arteries, and the deep cardiac plexus. The pneumo-gastric nerve is on either side.

The right bronchus is about an inch long, and enters the root of the right lung opposite the fourth dorsal vertebra.

The left bronchus is nearly 2 inches long, and enters the root of the left lung opposite the fifth dorsal vertebra.