CLASS AVES 



395 



among which, however, they are not developed to the degree seen in 

 birds. 



The kidneys have a reduced renal-portal system. There is no 

 urinary bladder and the urinary secretion passes directly out with 

 the feces. The female bird has but a single ovary, the right one dis- 

 appearing during development. 



The brain of the bird is relatively short and broad (Fig. 270). The 

 cerebral hemispheres are large, but the olfactory lobes are very small, 

 indicating a poor development of the sense of smell. The large optic 

 lobes correspond to the great development of the power of sight, and 

 the very large and much convoluted cerebellum indicates the delicate 



Cere bell urn 



Cerebrum 



Optic lobe 



Optic 



tract Hypophysis 



Fig. 270. — Brain of pigeon, viewed from the side. {From a Ziegler model, after Wieders- 

 heim.) The roots of the cranial nerves are marked by roman numerals. The hypophysis 

 is also called the pituitary body. 



sense of equilibrium and the great power of muscular coordination 

 belonging to birds. 



Although the sense of taste is present it is not well-developed. The. 

 ear is more complex than that of reptiles and the sense of hearing rela- 

 tively acute. The eyeball is large, is much elongated, and the cornea 

 is very convex. Bony plates are developed in the sclerotic coat. There 

 is also a peculiar structure known as the pecten, suspended in the vitreous 

 body. The pecten is highly vascular, pigmented, and fan-shaped. 

 Its function is not known, although it may have something to do with 

 the nutrition of the eyeball — or, possibly, it assists in accommodation, 

 which in birds is remarkably well-developed. Birds of prey, which 

 plunge at high speed from great heights upon their quarry on the ground 

 or which follow it through the branches of a woodland, need the maxi- 



