HIGH SCHOOL ZOOLOGY. 129 



and destined for shaking the plumage, attain a g-eater develop 

 ment in the birds than they do in the lower forms. 



8. A great advance is to be seen in the brain of a bird as 

 compared with that of any reptile, for not only is the cere- 

 brum much larger, but the cerebellum is so also, with the result 

 that the optic lobes are thrust aside right and left towards the 

 base of the brain. The surface of the cerebrum is smooth, but 

 that of the cerebellum is much folded so that the white matter 

 i« arranged in a tree-like fashion in its interior. Of the senses, 

 sigth is decidedly the most acute ; the birds of pi'ey especially are 

 gifted with extraordinary powers of vision, and in association 

 therewith the bulb of the eye has a very different shape from 

 tlie globular one present in other Vertebi'ates. Its principal 

 axis is much elongated, the posterior part of the bulb being a seg- 

 ment of a sphere, while the anterior is drawn out in a tubular 

 fashion. As in some of the reptiles, the sclerotic coat has a 

 circlet of bony plates foi*med in it. Hearing is also more acute 

 than in the reptiles ; the tympanic membrane is situated at the 

 bottom of a short extei-nal auditory passage (surrounded by 

 special " auricular " feathers) and the Eustachian tubes con- 

 verge to a common aperture in the palate. 



9. Although organs both of touch and taste are present in 

 the mouth-cavity of the bird, yet the tongue is generally clad 

 to a great extent with horn, which varies in shape in different 

 species. The most constant peculiarity of the oesophagus is the 

 presence of a crop, which may be a projection from one side, as 

 in the fowl (Fig. O-t), or from both, as in the pigeon. The 

 stomach is divided into a smaller glandular cardiac end, the 

 proventriculus, and a larger muscular pyloric end, the gizzard. 

 In the latter the muscular coat is very thick in two places, and 

 the epithelial lining is converted into horny pads, which serve 

 for the grinding of the food in the granivorous birds. As a 

 compensation for small salivary glands, the pancreas is large, 

 and the length of the intestinal surfiice is increased by two 



