200 GENERAL ORNITHOLOGY. 
in that system of enormous air-spaces for which the respiratory system of birds is so remarkably 
distinguished, —like a heap of soap-bubbles, blown up en masse from a bow] 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 this way air penetrates even the hollow skeleton of 
most birds (p. 135). 
The Lungs of Birds (fig. 101, ¢, t), notwithstanding their heated energy of respiration, 
are anatomically more like those of reptiles than of mammals. They are not shut by a dia- 
phragm in a special division of the great thoracic-abdominal cavity of the body, but extend from 
the apex of the chest as far as the kidneys, in the pelvic region. They are not divided into lobes, 
as in mammals, nor do they as in that class float freely in the chest by their mooring at their 
roots; nor, again, are-they completely invested by a serous membrane forming a closed pleural 
cavity. They are fixed in the dorsal region of the general cavity, covered in front with pleura, 
with which slips of the rudimentary diaphragm (v, v, v) are connected; but on the dorsal surface 
are accurately moulded to the intercostal spaces, showing the impressions of the ribs and verte- 
bree, — just as the lobulated kidneys are stamped with the sacral inequalities of surface. They 
are, as usual, two, right and left; their ‘‘ roots” are the bronchi (r, ”), the pulmonary arteries. 
and veins, nerves, and connective tissue. 
The Pneumatocysts. — A bird is literally inflated with these great membranous recepta- 
cles of air, and draws a remarkably ‘long breath,” —all through the trunk of the body, in 
several pretty definite compartments; in many, or most, or all, of the bones; in many inter- 
muscular spaces; in some birds also throughout -the cellular tissue immediately beneath the 
skin. They vary so much in extent and disposition as tu be not easily described except either 
in the most general terms already used, or with particularity of detail for different species. Ac- ° 
cording to Owen, however, the usual disposition is: An iter-clavicular air-space, quite con- 
stant: this, with its cervical prolongations, furnishes the great ‘‘air-drums” of our pinnated 
grouse 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 the wings, including the hollow 
humerus. Large hepatic or posterior thoracic, about the lower part of the lung and the liver. 
Abdominal, right and left, of great size, from the lower part of the lung where the longest bron- 
chial tubes open very freely; extending to pelvic and inguinal compartments, whence femoral 
sacs, the hollow of the femur, ete. The subcutaneous cells are enormously developed in the 
pelican and gannet; the extensive areolar tissue being thoroughly pneumatic, and furnished 
with an arrangement of the cutaneous muscle (panniculus carnosus) whereby, apparently, the 
air may be rapidly and forcibly expelled by compression. A similar muscle develops in some 
birds’ in connection with the interclavicular air-space. (For pneumaticity of the skeleton, see 
p. 135.) 
The purpose of this extensive respiratory 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 the function of respiration, not only by a change in the 
blood of the pulmonary circulation effected by the air of the receptacles on its repassage through 
the bronchial tubes; but also, and more especially, by the change which the blood undergoes 
in the capillaries of the systemic circulation which are in contact with the air-receptacles. 
The free outlet to the air by the bronchial tubes does not, therefore, afford an argument against 
the use of the air-cells as subsidiary respiratory organs, but rather supports that opinion, since 
the inlet of atmospheric 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 
norizontal position of a bird], the angle between the vertebral and sternal ribs made less acute, 
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