RESPIEATOrvY SYSTEIM OF BIRDS. 217 



Agreeably to tliis view of the function of the air-cells, it is 

 found that the quantity of air admitted into the system is in pro- 

 portion to the rapidity and continuance of the bird's flight; and, 

 where it is limited, the air is distributed to those members which 

 are most employed in locomotion ; thus the air is admitted into 

 the wino;-bones of the Owl, but not into the femur : while in the 

 Ostrich the air penetrates the femur, but not the humerus or other 

 bones of the wing. 



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 air-cells in a 

 Gigantic Crane ( Ciconia Ai^gala) 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, 

 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 dis- 

 tended 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 lio-ht mav be reo-arded 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 

 aro'ument aa'ainst this function that the air-cells exist in birds 

 which are not provided with the mechanism necessary to produce 

 tuneful notes ; since it was not pretended that this was the ex- 

 clusive and only oflfice of the air-cells. 



§ 158. Air-passages in Birds. — The air-passages in Birds com- 

 mence by a simple superior larynx, from which a long trachea 

 extends to the anterior aperture of the thorax, where it divides 

 into the two bronchi, one to each lung. At the place of its divi- 

 sion there exists, in most birds, a complicated mechanism of bones 

 and cartilages moved by appropriate muscles, and constituting the 

 true organ of voice : this part is termed the inferior lan/n.r. 



The tendency to ossification, which is exemplified in the bony 

 condition of the sternal ribs and tendons of the muscles, is again 

 manifested in the framework of the larynx and the rings of the 

 trachea, whicli, instead of being cartilaginous, as in Beptiles and 

 Mammals, are in most birds of a bony texture. 



The superior lari/nx, figs. 73, e — li, 99, and 100, is situated be- 

 hind the root of the tono:ue, and rests upon the urohyal, fig. 73, 43, 

 to which it is attached by dense cellular texture. 



It is composed of several bony and cartilaginous pieces, varying 

 in number from four to ten. The largest of these pieces constitutes 



