270 



VERTEBRATE ZOOLOGY 



right aortic arch carries all of the arterial blood to the system. Fur- 

 ther details of the circulatory system are best understood from the il- 

 lustration (Fig. 145). 



Respiratory System. Large lungs each with nine thin-walled 

 air-sacs. Air enters bronchi, passes to air-sacs and thence in a warmed 

 condition into the alveoli of the lungs and makes its exit through the 



elf 



o.t 



FIG. 146. Brain of Bird (Pigeon). A, dorsal; V, ventral; C, left lateral view; 

 cb, cerebellum; /, flocculus; inf. inf undibulum ; m. o, medulla oblongata; o. I, 

 optic lobes; o. t, optic tracts; pn, pineal body; II-XII, cerebral or cranial nerves; 

 sp. I, first spinal nerve. (After Parker.) 



excurrent bronchi. A complete change of air occurs at each inspira- 

 tion and expiration. The trachea and the larger bronchi are kept 

 open by means of rings of cartilage; the trachea is enlarged, just be- 

 fore it divides, into a syrinx or voice box, a structure limited to birds 

 and that is in no way homologous with the larynx of mammals; the 

 mechanics of voice production in the bird depends upon forcing air 

 through a flexible valve, which is set in vibration. 



