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. 14.5). 



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 



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

 c6, cereljellum; /, flocculus; i/i/. infundibulum ; vi.o, medulla oblongata; o. /, 

 optic lobes; 0. (, optic tracts; p?i, pineal body; II-XII, cerebral or cranial nerves; 

 sp. 1, first spinal nerve. (After Parker.) 



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

 tion and expiration. The trachea and (he larger bronchi are kept 

 open by means of rings of carlilage; 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 mamnrals; the 

 mechanics of voice production in the biril depends upon forcing air 

 through a flexible valve, which is set in vibration. 



