446 



Vertehrata. 



into a tracliea of considerable length, furnished with numerous 

 cartilaginous or bony rings, and dividing below into two branches, one 

 for each lung. Unlike most other air-breathing Yertebrata there are 

 no Tocal cords in the larynx ; but most Birds possess a peculiar lower 

 larynx {syrinx) at the junction of the trachea, with the upper ends 

 of the two large bronchi; and here the walls of the bronchi form 

 membranous folds (me and mi, Pig. 374), which are caused to vibrate 

 by the exhalent current of air (for details see below) . The lungs 

 are spongy organs, closely apposed to the dorsal wall of the body- 

 cavity ; in structure, they resemble very closely those of most Eeptiles. 

 A tubular continuation of the bronchus runs through the lung. 



Fig. 372. 



Pig. 372. Lungs of an eleven days Chick embryo, t trachea, I rudiments of 

 hranohial sacs. — After Selenka. 



Pig. 373. Lungs of a Pigeon, ir trachea, o apertures from the lungs into the sacs 

 which have been removed. — After J. Parker. 



dividing and sub-dividing on its way ; the delicate terminal branches 

 are the actual respiratory organs. All these branches are closely 

 bound together by connective tissue; some of them are continued 

 into large thin-walled air sinuses, which extend back between the 

 viscera, between certain of the subcutaneous muscles, and even send 

 long processes into many of the bones, e.g., the limb bones, in 

 which they occupy the position of the marrow cavities; the bones 

 are thus to a large extent pneumatic* This development of air 

 «acs is of significance, from the small specific gravity that the body 

 thus acquires, in other words, the lungs of Birds, like the tracheae of 



* It may be mentioned here, that Birds are not the only animals with pneumatic 

 bones ; the Pterodactyles, and many Diuosaurians had them, which leads to the 

 •conclusion that they, also, were provided with air sacs. 



