THE CLASS BIRDS. 323 



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. 301) 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 (o), which in mammals 

 are small, and concealed by the over- 

 lapping of the hemispheres, are here 

 exposed and visible without dissection. 

 They are proportionally much larger, and 

 instead of being solid, are hollow, like 

 the cerebral lobes. The cerebellum (v) Fi 301 _ Bra j n O f 

 is grooved transversely by parallel and the Ostrich, 



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, especially in bad 

 flyers. The annular protuberance, or pons of Yarolius, 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 versa. 



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 clas- 

 sification. A horny covering, solid, and more or less hard, 



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

 power. See Trans. R. S. ofEdin., 1827. K. K. 

 Y2 



