in an untrained person, the puffing and blowing is not only pronounced but even distressful. We shall see presently how muscular exercise brings these changes about, but before doing so it will be necessary for us to take up the movement of the gases between the lungs and the tissues, by way of the blood; oxygen from lungs to tissues, carbon dioxide from tissues to lungs. By lung ventilation the air in the tiny individual lung sacs is kept supplied with oxygen and also measurably free from carbon dioxide. From this air there is a continuous diffusion of oxygen into the blood. The first oxygen that diffuses in may dissolve in the blood liquid just as oxygen will dissolve in any water to which it is exposed, but as this goes on the hemoglobin of the red corpuscles begins to take up oxygen, forming a chemical compound to which is given the name of _oxyhemoglobin_. If there is enough of the gas present, every molecule of hemoglobin will take up oxygen to its full capacity. The amount that will dissolve directly in blood is so slight that to all intents and purposes the ability of the blood to carry oxygen depends on the extent to which hemoglobin can combine with it. It is important to emphasize this, because it means that there is a definite limit to the amount of oxygen that the blood can carry, a limit which is reached as soon as the hemoglobin is saturated. Hemoglobin has so great a power of combining with oxygen that the moderate lung ventilation which ordinary quiet breathing gives suffices usually to saturate it. Now and then we encounter statements which give the impression that there is a virtue in deep breathing in improving the amount of oxygen which becomes available for our tissues. As a matter of fact, this is not the case; ordinary quiet breathing when the body is at rest saturates the blood with oxygen, which means that it is carrying its full cargo, and evidently no amount of deep breathing can make it do more than that. We should not be understood as intimating that deep breathing is not a valuable exercise; the point which we wish to emphasize is that its value does not lie in affording an increased supply of oxygen.