THE ANATOMY OF BIRDS. — PNEUMATOLOGY. 



207 



and the thoracic cavity proportionally enlarged ; the air then rushes into the lungs and tho- 

 racic receptacles, while those of the abdomen become flaccid ; when the sternum is 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 luugs, notwithstanding their 

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

 receptacles, and are afl"ected equally and regularly by every motion of the sternum and ribs. 

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

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

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

 cells, and by the rarification of the contained air from the heat of the body. ... A fourth use 

 of the air-receptacles relates to the mechanical assistance which they afi'ord to the muscles of 

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

 crane {Oiconia argald) was followed by an extension of the wings, as the air found its way 

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

 our wood ibis, Tantalus 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 other than a secondary and probably partial service of the air-cells. In the 

 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 pro\-ided with the mechanism necessary to pro- 

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

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



Though nothing like them exists in mammals, it must not be inferred that these air- 

 pouches are unique in birds. The general pulmonary mechanism 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 

 pulmonary, though fishes' gills are functionally, and therefore analogically, their lungs; «. e., 

 their respiratory apparatus. 



The Trachea (Gr. rpaxeia, tracJieia, 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, i, o to q). It 

 commences at the root of the tongue by a chink 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 forking 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 ^, 2) borrowed from 

 Macgillivray's admirable account. When contracted, the rings 

 look like an alternating series of lateral half- hoops, as in fig. 

 96, a; when stretched to the utmost, as in fig. 96, b 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, ^, 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 tlie utmost, 

 the rings looking like alter- 

 nating lialfriiigs; h, the same, 

 stret(lio<l to two inches, the rings 

 eviilenfly complete, with inter- 

 vening membrane. (After Mac- 

 gillivray.) 



