200 GENERAL OBNITHOLOGY. 



in that system of enormous air-spaces f(]r which the respiratory system of "birds is so remarkably 

 distinguislied, — like a heap of soap-bubbles, blown up en masse frfim a IkjwI of fluid ; the extra- 

 pulmouary air-spaces being the larger superficial bubbles, the minute vesicles of lung-tissue 

 proper being little bubbles just formed. In tliis way air penetrates even the hollow skeleton of 

 most birds (p. 135). 



The Lungs of Birds (fig. 10], t, t), notwithstanding their heated energy of respiration, 

 are anatomically more like those of r(!iitiles than of mammals. They are not shut by a dia- 

 phragm in a special division of the great thciracic-abdomiual cavity of the body, but extend from 

 the apex of the ch(^st as far as the kidneys, in the i:)elvic regi(jn. They are not divided into lobes, 

 as in niamuials, nor do tbey as in that class float freely in the chest by their mooring at their 

 roots; nor, again, are tliey cdinjiletely invested by a serous membrane formiug a closed pleural 

 cavity. Tliey are hx(>d in the dorsal region of the general cavity, covered in front with pleura, 

 with which slips of tlii^ rudimentary diaphragm (c, v, v) arc connected ; but on the dorsal surface 

 are accurately moulded to the iiitercd.stal spaces, showing the impressions of the ribs and verte- 

 bras, — just as the lolmlated kiiliieys are stamped with the sacral inequalities of surface. They 

 are, as usual, two, riglit and left : their " roots" are the bronchi (r, r), the jjulmonary arteries 

 and veins, nerves, and connrcti\'i' tissue. 



The Pneumatoej'sts. -A liird is literally inflated with these great membranous recepta- 

 cles of air, and draws a remarkably "long breath," — all through the truuk (jf the body, in 

 several pretty definite compartmonts ; in nniny, or most, or all, of the bones; in many inter- 

 muscular spaces; in some birds also tliroughout tin- cellular tissue immediately beneath the 

 skin. They vary .so much in extent ;nid disposition as to be not easily described except eitber 

 in the most general terms already used, orwitli particularity of detail for difi'erent species. Ac- 

 cording to Owen, however, tlie iisiud disposition is: An hitcr-clavicidar air-space, quite con- 

 stant: this, with its cervical prcjiongations, furnishes the great "air-drums" of our pinnated 

 gro\ise and cock-of-the-plains. Anterior thoracic, about the roots of the lungs. Lateral tho- 

 racic, prolonged to axillary, and to spaces and passages in tlie wings, incduding the hollow 

 humerus. Large hepatic nr posterior thoracic, about the lower part of the lung and the liver. 

 Abdominal, riglit and h'ft, of great size, from the lower ]iart of tlie lung where the longest bron- 

 chial tulies ojien very freely; extending to pelvic and iiujiiinal compartments, whence femoral 

 sacs, the hollow of the femur, etc. The suhcutaneoas cells are en(a-mously developed in the 

 pelican and ganuet ; tlie extcusive areolar tissue being thoroughly pneumatic, and furnished 

 with an arraugemeut of tlie cutaueous muscle {panniculvs carvosus) whereby, apparently, the 

 air may be rapidly and forcibly expelh^d by compression. A similar muscle develops in some 

 birds in connection with the interclavicular air-space. (For pneumaticity of the skeleton, see 

 p. 185.) 



The ]iur])ose of this extensive i('s|iiRitory apparatus is thus dwelt upon by the great "New- 

 ton of Anatomy " just cited: "The extension from the lungs of continuous air-receptacles 

 throughout the body is subservient to tlie function of respiration, not only by a change in the 

 blood of the puluKmary circnhitiou etfected by the air of the receptacles on its repassage through 

 the bronchial tubes; but alscj, and more especially, by the change which tin' blood undergoes 

 in the capillaries of the systemic circulation which are in contact with the air-receptacles. 

 The free outlet to tlie air by the bronchial tubes does not, therefore, aftbi'd an argument against 

 the use of the air-cells as subsidiary respiratory organs, but I'nther supports that <ipinion, since 

 the inlet of atmo.spheric oxygenated air to be diffused over the body must be equally free. A 

 second use may be ascribed to the air-cells as aiding mechanically the action of respiration in 

 birds. During the act of inspiration the sternum is depressed [lowered from the back-bone in 

 horizontal position of a bird], the angle between the venebral and sternal ribs made less acute, 



