The above statements will be illustrated by some account of the manner in which the body is supplied with healthful nutriment. There are two modes of nourishing the body, one is by food and the other by air. In the stomach the food is dissolved, and the nutritious portion is absorbed by the blood, and then is carried by blood-vessels to the lungs, where it receives oxygen from the air we breathe. This oxygen is as necessary to the nourishment of the body as the food of the stomach. In a full-grown man weighing one hundred and fifty-four pounds, one hundred and eleven pounds consists of oxygen, obtained chiefly from the air we breathe. Thus the lungs feed the body with oxygen, as really as the stomach supplies the other food required. The lungs occupy the upper portion of the body from the collarbone to the lower ribs, and between their two lobes is placed the heart. [Illustration: Fig. 28.] [Illustration: Fig. 29.] [Illustration: Fig. 30.] [Illustration: Fig. 31.] Fig. 28 shows the position of the lungs, though not the exact shape. On the right hand is the exterior of one of the lobes, and on the left hand are seen the branching tubes of the interior, through which the air we breathe passes to the exceedingly minute air-cells of which the lungs chiefly consist. Fig. 29 shows the outside of a cluster of these air-cells, and Fig. 30 is the inside view. The lining membrane of each air-cell is covered by a net-work of minute blood-vessels called _capillaries_, which, magnified several hundred times, appear in the microscope as at Fig. 31. Every air-cell has a blood-vessel that brings blood from the heart, which meanders through its capillaries till it reaches another blood-vessel that carries it back to the heart, as seen in Fig. 32. In this passage of the blood through these capillaries, the air in the air-cell imparts its oxygen to the blood, and receives in exchange carbonic acid and watery vapor which are expired at every breath into the atmosphere. By calculating the number of air-cells in a small portion of the lungs, under a microscope, it is ascertained that there are no less than eighteen millions of these wonderful little purifiers and feeders of the body. By their ceaseless ministries, every grown person receives, each day, thirty-three hogsheads of air into the lungs to nourish and vitalize every part of the body, and also to carry off its impurities. [Illustration: Fig. 32.]