The figure may pass. It is a reasonable basis from which to calculate the packing capacity of a dormitory. So long as a man has 800 cubic feet of air to himself, he may safely feel that he has room to stretch his lungs. Dwelling on this figure may make him feel uncomfortable when he finds himself in a railway carriage, seated five on a side, with the windows closed. In the theatre or in church he may doubt whether he has all the fresh air to which his humanity entitles him. But, as a philosopher rather than as a physiologist, he reflects that, whether on the Antarctic icecap in a sleeping-bag or standing on a summit in the Alps, he takes all that he can get, for fresh air is one of the few good things of which one can never have enough. =Tissue Respiration.=—A frog will live for seventeen hours in an atmosphere of nitrogen. Under these circumstances it is clearly impossible for it to take up oxygen, yet for several hours it gives off as much carbonic acid as it would do if it were living in air. Such an observation as this proves that oxidation does not occur in the lungs, but deeper in the body. At one time the blood was regarded as the seat of oxidation; the products formed by the splitting up of proteins in the tissues were supposed to be passed into the blood, where they came in contact with the oxygen carried by hæmoglobin. A certain amount of oxidation does take place in the blood, as in all other tissues, for blood is a living tissue and needs to respire. But the oxidation which occurs in the blood is small in amount as compared with that in the organs which the vessels traverse. Muscle and other tissues detached from the body and free from blood give off carbonic acid. It is possible to wash the blood out of the vessels of a frog and to replace it with a solution of salt. In an atmosphere of oxygen such a “saline frog” lives for a day or two, taking in the same quantity of oxygen and giving off the same quantity of carbonic acid as a normal frog. The oxygen is chiefly absorbed through the skin, the carbonic acid discharged from the lung. This experiment shows that blood is not essential for oxidation. Oxidations do not occur in the salt solution with which blood is replaced. Taking all the evidence together, it seems to be safe to conclude that the tissues absorb the oxygen which the oxyhæmoglobin brings into their neighbourhood, and that they have some capacity of storing it. A piece of detached muscle which gives off carbonic acid in an atmosphere of nitrogen would appear to be holding a store of oxygen, much as hæmoglobin holds it. The proof is not quite so definite as might be desired; but we are probably justified in holding the belief that the main part of the respiratory exchange occurs in the tissues. Lymph dissolves oxygen which it obtains from the blood. The tissues take it from lymph. Tissues set free carbonic acid which lymph dissolves. Its tension being higher than in blood, carbonic acid