KESPIKATORY SYSTEM OF BIRDS. 



'211 



97 



an air-space of its own ; they interlace in every direction, formino- 

 a cubic mass of capillaries permeated everywhere by air. In 

 fig. 97, a is the cavity of a bronchial tube, h its lining mem- 

 brane supporting blood-vessels with large areolae ; c, c, perforations 

 in the membrane at the orifices of the lobular passages, d,d\ e, e, 

 are interlobular spaces containing the terminal branches of the 

 pulmonary vessels supplying the 

 capillary plexus, /, f, to the 

 meshes of which the air o-ets ac- 

 cess by the lobular passages. 



§ 157. Air-cells of Birds. — 

 The thoracic-abdominal cavity is 

 subdivided and intersected by a 



the 



thus 



The 



p OS- 



number of 



'^ 



«fe 



Section through a broucbial tube, limg of Bird, 

 magn. XLV. 



membranes ; 

 greater part of the cells 

 formed are filled with air. 

 texture of their parietes 

 sesses considerable firmness in 

 the larger birds, as the Ostrich 

 and Cassowary. 



The innermost layer of the air- 

 receptacles can be separated from the outer layer, and is a con- 

 tinuation of the lining membrane of the bronchial tube ; the outer 

 layer is a serous membrane, and appears to form the cells by a 

 series of reflections of what may be regarded as the pleura or 

 peritoneum. 



These large membranous receptacles, into which the extremi- 

 ties of the bronchial bifurcation and also some of the preceding 

 branches open, are disposed with suflficient general regularity to 

 admit of a definite description and nomenclature. 



The first or interclavicular air-cell, fig. 98, a, extends from the 

 anterior part of each lung, forward to the interspace of the fur- 

 culum, anterior to which it dilates in the Gannet and many other 

 birds into a large globular receptacle. In the Vultures it is 

 divided into two lateral receptacles, between which the large crop 

 is situated. A thin fan-shaped muscle is extended from the 

 anterior edge of the furculum, over the interclavicular air-cell in 

 these and some other birds. 



The anterior thoracic cell, ib. b, contains the lower larynx and 

 bronchi, and the great vessels with their primary branches to the 

 head and wings. It is traversed by numerous membranous septa, 

 which connect the different vessels together, and maintain them 

 in their situations. The air passes into the posterior part of this 



r 2 



