296 GENERAL ORNITHOLOGY part u 



heavy with, effete carbonaceous matters, is there relieved of its 

 burden and aerated by the action of oxygen ; the products of com- 

 bustion being exhaled in the form of carbonic dioxide and water. 

 Aside from the proper lung-tissue, the capillary substance of the 

 immense air-sacs tends to the same result. There is likewise, in 

 birds, a lesser system of ventilation, by which air is admitted to 

 cranial bones through the Eustachian tubes ; but this is unconnected 

 with the proper respiratory office. Pulmonary tissue consists chiefly 

 of a wonderful net (a rde mirabile) of capillaries, interlacing in every 

 direction, bound together and supported by fine connective tissue, 

 and invested with membrane so delicate that their walls seem naked, 

 their exposure to the air being thus very thorough. Air gains such 

 intimacy with the capillaries through the larynx, trachea (Fig. 101, 0), 

 and bronchial tubes (r, r), these being the primary air-passages. But 

 all the bronchial tubes do not subdivide into the ultimate air-cells ; 

 some large ones run through the lung, pierce its surface (as at u, 11, 

 Fig. 101), and end 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 bowl of fluid ; 

 the extra-pulmonary 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. 



The Lungs of Birds (Fig. lOl, t,t), notwithstanding their heated 

 energy of respiration, are anatomically more like those of reptiles 

 than of mammals. They are not shut by a diaphragm 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 vertebrse, — 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, r), the pulmonary arteries and veins, nerves, and 

 connective tissue. 



The Pneumatoeysts. — A bird is literally inflated with these 

 great membranous receptacles 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 intermuscular spaces ; in some birds also throughout the 

 cellular tissue immediately beneath the skin. These cysts vary so 



