All medical writers agree that want of sufficient exercise--as by lying too much in bed, riding in a carriage, &c.--is favourable to the development of obesity. The explanation is simple. We are all cognizant of the fact, that the body is sustained chiefly by means of food; but we also know that the atmosphere by which we are surrounded, plays an important part in the nourishment of the body. The atmosphere we inspire contains oxygen gas, a portion of which is destined to revivify the blood in its passage through the lungs; another portion we expel, we expire, no longer pure, but in combination with carbon obtained from the body, in the form of carbonic acid gas. In proportion as the respiration is more active, a larger quantity of oxygen is taken into the system, and more carbon in combination with oxygen is expelled as carbonic acid gas. There is consequently a less amount of carbon left in the system to form fat. The greater the activity of the animal, the more frequent do the respirations become. Having said this, it is readily understood why want of exercise, riding in a carriage, lying too much in bed, tend to the development of fat; because, with this want of activity, respiration is less frequent, and the oxygen combines with a less amount of carbon, and a larger quantity is left to enter into combination with the existing hydrogen, forming fat. Consequently the mountaineer, breathing an atmosphere rich in oxygen, is generally less prone to the formation of fat than the dweller in the valley. The Bedouin Arab, owing to the activity of a nomadic life, is never fat. Our peasantry are rarely over fat, unless they have acquired wealth sufficient to relieve them from the necessity for labor. Animals which are in constant motion, such as the roebuck and the deer, although feeding upon substances rich in carbon and in hydrogen, have usually but little fat. Those birds which are continually on the wing are never very fat. On the other hand, birds or animals leading an inactive life readily take on fat. A means frequently resorted to, in order to fatten them, is to feed them in a small enclosure. Some domestic animals are even deprived of all power of motion in order to hasten their fattening. Among orientals, where the men remain seated the greater part of the day, and the women are obliged to stay in the house without ever going out, frequent examples of obesity are to be met with.