THE ANATOMY OF BIRDS. — PNEUMATOLOGY. 
201 
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 lungs, notwithstanding their 
fixed condition, are subject to due compression through the medium of the contiguous air- 
receptacles, and are aflFected 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 afford to the muscles of 
the wings. This was suggested by observing that an inflation of the au'-cells in the gigantic 
crane (Ciconia 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 provided 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; i. e., 
their respiratory apparatus. 
The Trachea (Gr. rpaxeTa, 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, i, o to 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 i, 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, & 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 
a h 
Fig. 96. —a, an inch of tra- 
chea, contracted to the utmost, 
the rings looking like alter- 
nating half-rings; 6, the same, 
stretched to two inches, the rings 
evidently complete, with inter- 
vening membrane. (After Mac- 
gillivray.) 
