THE CLASS BIEDS. 293 



furnished with osseous plates anteriorly, whose form varies 

 with the genus and species. Lachrymal glands always exist, 

 and besides the horizontal eyelids, they have a third, vertical, 

 moveable, and elastic, which can be drawn completely over the 

 surface of the eye. 



Birds are well known to have a piercing and most distinct 

 vision at all distances, that is to say, the most complete 

 adaptation of sight. On what this depends is not clearly 

 known, some ascribing it to the mobility of the osseous plates 

 and to the varying form of the lens in this class of animals.* 



The nervous system presents the following peculiarities. 

 The encephalon is less developed than in mammals, but the 

 cerebral hemispheres (Fig. 235) still 

 maintain their superiority over the other 

 parts. The great commissure, called 

 corpus callosum, is wanting, and there 

 are no cerebral convolutions. The 

 optic lobes, or thalami (0), which in 

 mammals are small, and concealed by 

 the overlapping of the hemispheres, are 

 here exposed and visible without dis- 

 section. They are proportionally much 

 larger, and instead of being solid, are 

 hollow, like the cerebral lobes. The cere- f. , _ B . 

 bellum (y) is grooved transversely by the Ostrich. 1 



parallel and converging lines; it is 

 almost wholly formed by the median portion; this in 

 mammals is small compared with the hemispheres or lateral 

 portions, which remain rudimentary, as it were, in birds, espe- 

 cially in bad flyers. The annular protuberance, or pons of 

 Varolius, is wanting in birds, as well as in reptiles and fishes. 

 Finally, the medulla spinalis (e) is generally very long, and 

 has two swellings in its course, corresponding to the going off 

 of the nerves of the wings and limbs or feet. The former is 

 the stronger in birds of powerful flight, and vice versd. 



435. The food of birds is very varied : grains, insects, 

 fishes, flesh, fresh or putrid. They use the feet sometimes as 

 instruments of prehension, but the bill is always the prin- 

 cipal organ employed for this purpose; thus its nature 

 varies with the food, and it becomes an important character 

 in classification. A horny covering, solid, and more or 



* The highly-developed annulus albus seems connected with this adaptive 

 power. B. K. 



