CLASS AVES 



587 



fine down, but soon acquire a covering of contour feathers. 

 During their early life as nestlings they are fed upon " pigeon's 

 milk," a secretion from the crop of the adult. 



The Nervous System. - - The brain of the pigeon (Fig. 476) 

 is very short and broad. The cerebellum (cb) is comparatively 

 large, as are also the optic lobes (o.l), showing that birds have 



FIG. 476. The brain of the pigeon, side view, cb, cerebellum; c.h, cerebral 

 hemispheres; /, flocculus; m.o, medulla oblongata; o.l, optic lobes; o.t, optic 

 tracts; pn, pineal body; II-XII, cerebral nerves. (From Parker and Haswell.) 



well-developed powers of coordination and of sight. The ol- 

 factory lobes (olf), on the other hand, are very small, indicating 

 poorly developed olfactory organs. 



The Sense-organs. - - The bill and tongue serve as tactile 

 organs. Tactile nerves are also present at the base of the feathers, 

 especially those of the wings and tail. Birds are unable to dis- 

 tinguish delicate odors, and on the whole their sense of smell is 

 very poor. The sense of taste is also very poorly developed, 

 but is nevertheless present, as can easily be proved if a bad- 

 tasting morsel of food is presented to a bird. 



The cochlea of the ear is more complex than that of reptiles. 

 The Eustachian tubes open by a single aperture on the roof of the 

 pharynx. Birds have acute and discriminating powers of hear- 

 ing a power correlated with their singing ability. 



The eyes of birds are very large, and have a biconvex shape. 

 They are surrounded by bony sclerotic plates, and contain a fan- 



