ibula, the little arteries
branch into the capillaries that penetrate the infundibular walls (Figs.
38 and 39). From these capillaries the blood is conveyed by the pulmonary
veins to the left auricle.

The lungs also receive blood from two (in some individuals three) small
arteries branching from the aorta, known as the _bronchial arteries_.
These convey to the lungs blood that has already been supplied with
oxygen, passing it into the capillaries in the walls of the bronchi,
bronchial tubes, and large blood vessels, as well as the connective tissue
between the lobes of the lungs. This blood leaves the lungs partly by the
bronchial veins and partly by the pulmonary veins. No part of the body is
so well supplied with blood as the lungs.

[Fig. 40]

Fig. 40--*The pleurae.* Diagram showing the general form of the pleural sacs
as they surround the lungs and line the inner surfaces of the chest (other
parts removed). _A, A'._ Places occupied by the lungs. _B, B'._ Slight
space within the pleural sacs containing the pleural secretion, _a, a'._
Outer layer of pleura and lining of chest walls and upper surface of
diaphragm. _b, b'._ Inner layer of pleura and outer lining of lungs. _C._
Space occupied by the heart. _D._ Diaphragm.

*The Pleura.*--The pleura is a thin, smooth, elastic, and tough membrane
which covers the outside of the lungs and lines the inside of the chest
walls. The covering of each lung is continuous with the lining of the
chest wall on its respective side and forms with it a closed sac by which
the lung is surrounded, the arrangement being similar to that of the
pericardium. Properly speaking, there are two pleurae, one for each lung,
and these, besides inclosing the lungs, partition off a middle space which
is occupied by the heart (Fig. 40). They also cover the upper surface of
the diaphragm, from which they deflect upward, blending with the
pericardium. A small amount of liquid is secreted by the pleura, which
prevents friction as the surfaces glide over each other in breathing.

*The Thorax.*--The force required for breathing is supplied by the box-like
portion of the body in which the lungs are placed. This is known as the
thorax, or chest, and includes that part of the trunk between the neck and
the abdomen. The space which it incloses, known as the thoracic cavity, is
a _variable_ space and the walls surrounding this space are _air-tight._ A