too far from the subject at hand; but anyone who would pursue this further can consult Professor Huxley’s ‘Elementary Physiology,’ or any other elementary text-book upon physiology. When a mammal--a human being, for example--breathes certain muscles are called into play. If a person is watched, it will be seen that the chest expands during inspiration, and that its calibre diminishes during expiration. What happens is this. The lungs are contained in a cavity which contains no air. This cavity can be increased in size in two directions. When the ribs are moved out--which they can be by the movements of the muscles called intercostal, which lie between them--the cavity of the chest from before backwards is evidently enlarged. On the other hand there is the diaphragm, which we have already spoken of as bounding the chest cavity below. Now this diaphragm is muscular, with a tendinous centre. When the muscles contract, like all muscles do, the surface of the diaphragm, which was before rather convex towards the chest cavity, becomes more flat; hence the cavity lying above it, _i.e._ the chest cavity, becomes larger in a downward direction also. When it is increased in this way by the action of the two separate sets of muscles, some space--more space than before--is left between its walls and the lungs which lie within it; it follows, therefore, that, as there is no air in the cavity, the pressure of air outside the body forces more air into the lungs, because there is no counterbalancing pressure to prevent this. The principle is the same in the bird, but the details are different. If you will turn again to the bird’s skeleton, you will see that the backbone and ribs and sternum form a bony box, which is jointed in the middle; this acts precisely like a pair of bellows: the bones at top and bottom represent the wood, and the soft intervening leather of the bellows is represented by the muscles which lie between, and which connect the sternum with the abdomen and with the ribs. When these muscles contract, the sternum is obviously brought nearer to the backbone, and air is expelled from the inside; when they are relaxed, a vacuum is created and air rushes in. The air-spaces, then, are really ramified tags of lung which have no blood-vessels in their walls, and are therefore not meant for respiration, but serve as reservoirs of air, lightening the body of the creature. It is curious that birds are not the only animals which possess expansions of lung that are apparently useless for breathing purposes. The lungs of the Chameleon have quite similar sacs appended to them. There is, it is true, no such complicated a ramification as that which we find in the bird, but still there is no doubt that the structure is of the same nature. It looks almost like a first step in the path towards a bird. Very possibly the extinct Pterodactyles, which flew through the woods of the middle ages of the earth, had bodies lightened in the same or a similar way; for we know