Through the movements of the blood and the lymph, materials entering the body are transported to the cells, and wastes formed at the cells are carried to the organs which remove them from the body. We are now to consider the passage of materials from outside the body to the cells and _vice versa_. One substance which the body constantly needs is oxygen, and one which it is constantly throwing off is carbon dioxide. Both of these are constituents of *The Atmosphere.*—The atmosphere, or air, completely surrounds the earth as a kind of envelope, and comes in contact with everything upon its surface. It is composed chiefly of oxygen and nitrogen,(29) but it also contains a small per cent of other substances, such as water-vapor, carbon dioxide, and argon. All of the regular constituents of the atmosphere are gases, and these, as compared with liquids and solids, are very light. Nevertheless the atmosphere has weight and, on this account, exerts pressure upon everything on the earth. At the sea level, its pressure is nearly fifteen pounds to the square inch. The atmosphere forms an essential part of one’s physical environment and serves various purposes. The process by which gaseous materials are made to pass between the body and the atmosphere is known as *Respiration.*—As usually defined, respiration, or breathing, consists of two simple processes—that of taking air into special contrivances in the body, called the lungs, and that of expelling air from the lungs. The first process is known as _inspiration_; the second as _expiration_. We must, however, distinguish between respiration by the lungs, called _external respiration_, and respiration by the cells, called _internal respiration_. _The purpose of respiration_ is indicated by the changes that take place in the air while it is in the lungs. Air entering the lungs in ordinary breathing parts with about five per cent of itself in the form of oxygen and receives about four and one half per cent of carbon dioxide, considerable water-vapor, and a small amount of other impurities. These changes suggest a twofold purpose for respiration: 1. To obtain from the atmosphere the supply of oxygen needed by the body. 2. To transfer to the atmosphere certain materials (wastes) which must be removed from the body. The chief organs concerned in the work of respiration are *The Lungs.*—The lungs consist of two sac-like bodies suspended in the thoracic cavity, and occupying all the space not taken up by the heart. They are not simple sacs, however, but are separated into numerous divisions, as follows: 1. The lung on the right side of the thorax, called the right lung, is made up of three divisions, or _lobes_, and the left lung is made up of two lobes. 2. The lobes on either side are separated into smaller divisions, called _lobules_ (Fig. 33). Each lobule receives a distinct division of an air tube and has in itself the structure of a miniature lung. [Fig. 33]