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 pleuræ.* 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 pleuræ, 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 framework for the thorax is supplied by the ribs which connect with the spinal column behind and with the sternum, or breast-bone, in front. They form joints with the spinal column, but connect with the sternum by strips of cartilage. The ribs do not encircle the cavity in a horizontal direction, but slope downward from the spinal column both toward the front and toward the sides, this being necessary to the service which they render in breathing.