6o 



Verlebrata. 



the impure or venous blood of the right auricle into 

 the lungs, and the other, or left ventricle, for driving 

 the purified blood after its return from the lungs, 

 through the body ; the opening into the right ventri- 

 cular cavity from the auricle is guarded by a muscular 

 flap. There is only one aortic arch developed in the 

 adult bird, and it arches to the right side, and in many 

 birds there is only one artery developed in the neck 

 for the supply of the head. The lungs are large, and 

 surrounded on their lower surfaces by large air-sacs, 

 into which the bronchial tubes distinctly open ; from 

 these cavities pass the membranous canals, which 

 convey the air to the principal bones. There is no 

 muscular layer underlying the ' lungs for the purpose 



FIG. 25. 



Organ of voice of the raven. 

 A, front view ; B, side view showing the muscles of vocalisation. 



of directly acting on them in respiration, except in the 

 ostrich and apteryx ; but as the sternal and vertebral 



