THE ANATOMY OF BIRDS. ~ PNEUMATOLOGY. 



201 



.•iiul the thoracic, cavity proportionally enlarged ; the air tlicn rushes into the lungs and tho- 

 racic receptacles, while those of the abdomen become Haccid ; when the sternum Ls raised or 

 approximated towards the spine, part of the air is expelled from the lungs and thoracic cells 

 through the trachea, and part driven into the abdominal receptacles, which are thus alternately 

 enlarged and diminished with those of the thorax. Hence the lungs, notwithstanding their 

 fixed condition, are subject to due compression through tlie medium of the contiguous air- 

 r(K'eptaclcs, and are affected equally and regularly by every motiou of the sternum and ribs. 

 A third use, and pei'haps the one which is most closely related to the peculiar exigencies of the 

 bird, is that of rendering the whole body specifically lighter; tliis must necessarily follow from 

 the desiccation of the uuirrow and other fluids in those spaces which are occupied by the air- 

 cells, and by the rariticaticm of the contained air from tlie lieat of the body. ... A fourth use; 

 of the air- receptacles relates to the mechanical assistance which they aflbrd to the muscdcs of 

 the wings. This was suggested by observing that an inflation of the air-cells in the gigantic 

 crane (Cicoiiia nrgala) was followed by an extension of the wings, as the air found its way 

 along the lirachial and anti-brachial cells. In large liiids, therefore, which, like the argala [or 

 our wood ibis, Tantalits loculator], hover with a sailing motion for a long-continued period in 

 the upper regions of the air, the muscular exertion of keeping the wings outstretched will be 

 lessened by the tendency of the distended air-cells to maintain that condition. It is not meant 

 to advance this as otlier than a secondary and prol)ably partial service of the air-cells. In tlie 

 same light may be regarded the use assigned to them by Hunter, of contributing to sustain the 

 song of birds and to impart to it tone and strength. It is no argument against this function 

 that the air-cells exist in birds which are not provided with the mechanism necessary to pro- 

 duce tuneful notes; since it was not pretended that tliis was the exclusive and only office of the 

 air-cells." (Owen, Aiiat. Vert, ii, 1866, p. 216.) 



Tliough notliing like them exists in mammals, it must not be inferred that these air- 

 pouches are unique in birds. The general pulmonary mecha\iism is reptile-like, and the or- 

 nithic development is simply a logical extreme of arrangements found in reptiles and lower 

 vertebrates, — even to the swim-bladder of a fish, which is morphologically and homologically 

 puhnouary, though fishes' jfiH-s are functiiaially, and therefore auah>gically, their lungs: i.e., 

 their respiratory apparatus. 



The Trachea (Gr. rpaxela, tracheia, rough) or " asper-artery " 

 answers perfectly to its English name, wind-pipe. It is the tube 

 which conveys air to and from the lungs (fig. 101, l, o to q). It 

 commences at the root of the tongue by a cliinlc in the floor of the 

 mouth (fig, 101, 3, c), runs down the neck in front between the 

 gullet and the skin, and ends below by finlcing into right and left 

 bronchus (fig. 101, i, r, r). It is composed of a series of very 

 numerous gristly or bony rings connected together by elastic 

 membrane. Lengthening and shortening, effected by muscles 

 to be presently noted, is permitted by a very ingenious and in- 

 teresting construction of these rings, which will be clearly under- 

 stood with the help of the figures (96, a, h,97^,'^) borroM-ed from 

 Macgillivray's admirable account. When contracted, the rings 

 look like an alternating series of lateral lialf-lioops, as in fig. 

 96, a; when stretched to the utmost, as in fig. 96, 6 they are 

 clearly seen to be annular, or completely circular. The curious 

 bevelling of the right and left sides of each ring alternately is 

 shown in fig. 97, ', 2; and fig. 97, 1, 2, represents the same two 

 rings put together. The principle by which any two rings slip 



Fig. 96. —a, an inch of tra- 

 chea, contracted to the utmost, 

 the rings lool<ing like alter- 

 nating half-rings; I, the same, 

 strctchefl to two inches, the rings 

 evidenHy complete, with inter- 

 vening membrane. (After Jlac- 

 gillivray.) 



