The lungs are also supplied with nerves.
The position of the lungs is shown in Fig. 133 (p. 2040 One lung occupies the right side, the other the left. They meet in the middle line, but the lung of the right side reaches farther down in the middle line than that of the left, because a deep notch exists in the front border of the left, into which the heart projects. When both lungs are well filled with air the heart is covered except the portion in the position of this notch. As seen in the figure (133) a line drawn downwards and to the side to the level of the tenth rib, from about the position where the sixth rib joins the breast-bone, will indicate the extent to which the lungs reach downwards. On the left side, however, the long extends down wards farther than on the right side by the breadth of a rib. On each side the lungs reach upwards higher than the first rib, the top of each lung passing up into the neck on each side for an inch and a half above the level of the first rib. Refer also to Plate XIX.
The Diaphragm. —The base of the lungs rests on the muscular partition — the dia phragm—which separates the cavity of the chest from that of the belly. This muscular partition is one of the main agents in the act of breathing, and its position must be under stood. It forms the floor of the chest cavity and the roof of the abdominal cavity, just as the roof of one room is the floor of the room above it, and separates the two from one another. It is not flat, however, but arched, the arch being directed upwards. This will be understood by a reference to Fig. 152, which is a section of the body carried from back to front, and shows the cavities of chest and belly, with their contents removed. The diaphragm is also shown in section, arching from a in front It is seen that it is convex towards the chest and concave towards the belly. It is attached all round to the breast
bone and ribs in front, to the ribs at the side, and to the ribs and backbone behind, being attached lower down at the sides and back than in front. On its upper surface the base of the lungs rests, the pleura being connected with it and the investing membrane of the heart also. This is well shown in Fig. 104 (p. 198), where c points to the diaphragm. It also shows that immediately below the dia phragm, and mainly on the right side, the liver is situated, while mainly to the left side is the stomach. If the liver be gorged with blood—con gested, or the stomach distended with wind, pres sure will be ex erted upwards, will force up the diaphragm, will prevent the proper filling of the lungs with air, by di minishing the space for their d tentioo, and so will occasion breath lessness. Now the diaphragm, being partly composed of muscle, is capable of contraction. When it contracts, the sides being fixed by their attachments to the ribs, &c., its centre will be pulled on and therefore lowered. That is to say, the floor of the chest will be lowered, the cavity of the chest will thereby be enlarged, and room will be afforded for the expansion of the lungs. On the other hand, when the diaphragm becomes relaxed, its contraction being over, it will rise by its own elasticity, the cavity of the chest will be diminished, and air will be forced out of the lungs to afford room for the rise. As we shall see, it is chiefly by the regular succes sion of contraction and relaxation of this mus cular partition that the movements of breath ing are carried on.