Lungs and Air-sacs. It is not only by virtue of their powerful muscles and stiffened fore limbs that birds can fly. The body is rendered lighter in proportion to its bulk by air-cavities, which permeate everywhere, even into the substance of the bones. So thorough is this aëration in the Screamer of South America, that when the skin of the recently dead bird is roughly pressed it crackles. Curiously enough, there seems to be no very definite relation between the degree of thoroughness to which the aëration of the body is carried out and the capacity for flight. The Screamer, that has just been mentioned, is fuller of air-cavities than the Frigate-bird, in which the art of flying is carried to the highest extreme--the ‘triumph of the wing,’ as Michelet says in ‘L’Oiseau.’ Anyone who has the opportunity of dissecting a Hornbill will be struck by the large and abundant air-spaces between the muscles. This applies even to the Ground Hornbill of Abyssinia; and yet the latter, as its name denotes, lives upon the ground, while the flight of other hornbills is heavy and most unsuggestive of lightness of body. These air-spaces are in direct communication with the windpipe. It is much easier to understand their arrangement by the actual dissection of a bird. We must first get a notion of the position and form of the lungs, which differ very much from the lungs of other animals. In a rabbit, for example, or any other mammal, the lungs lie freely on each side of the heart, and are capable of being pushed here and there after the body is opened, and of much expansion and diminution of volume during the movements of respiration. But the lungs of all birds are tightly fixed to the wall of the chest cavity, being, as it were, moulded on to the ribs and vertebræ; when they are carefully picked away from their place, they retain the impressions of the bones which they touch. There is no great possibility here of independent movements on the part of the lungs. Respiration is effected in a totally different manner; it is, in fact, bound up with the mechanical filling of the air-spaces. Each of the two lungs is contained within a large compartment, which is bounded externally by an obliquely disposed septum, often spoken of, on account of its direction, as the ‘oblique septum.’ Others call it the diaphragm, imagining that it is the equivalent of the diaphragm in the mammal, that partly fleshy, partly tendinous plate which shuts off the cavity of the chest, in which lie the heart and lungs, from the cavity of the abdomen, in which lie the intestines, stomach, and liver. Now, this oblique septum does not by any means closely invest the lungs; on the contrary, a deep space is thereby shut off, at the bottom of which are the lungs. This cavity is subdivided by two partitions into three separate compartments. It requires a very skilful manipulation to show the fact, but it can, with care, be demonstrated that each of these