NERVOUS SYSTEM OF BIRDS. 377 



ground; and it would seem that the eye can adapt itself to these 

 great differences in the reach of its vision, by the contractions of 

 its motor muscles ; which, acting on the bony circle of the 

 sclerotic coat, compress the humours with which the eye is filled, 

 and thus produce distension of the cornea, so as to increase its 

 curvature, when the animal requires to become, as it were, short- 

 sighted for a time, to distinguish clearly very near objects. 



341. The Nervous System, which influences the functions of 

 the organs we have just described, presents remarkable pecu- 

 liarities of structure. The Encephalon is less developed than in 

 the Mammalia ; the cerebral hemispheres are still the most 

 voluminous parts of it : but they have no convolutions, and they 

 are not united in a manner as complete ; 

 for the great commissure, of which we have 

 before spoken under the name of corpus cal- 

 losum, is wanting in this class. The optic 

 lobes, which in the Mammalia are small, and 

 concealed between the cerebrum and the cere- 

 bellum, here, on the contrary, assume a great 

 development, and appear uncovered behind 

 " and externally to the cerebral lobes ; and in- 



stead of being solid, they are hollow like the 

 cerebral lobes. The cerebellum is transversely 

 a, cerebrum ; b, optic furrowed by parallel and converging lines ; it is 

 formed almost entirely by the central lobe, 

 which, in the Mammalia, is small in comparison 

 with the lateral lobes or hemispheres of the cerebellum ; these 

 remain in an almost rudimentary state, especially in Birdsthat fly 

 badly. The spinal cord of Birds is generally very long, and 

 has two enlargements corresponding with the origins of the 

 nerves of the wings and feet ; in those that fly best, the upper 

 enlargement is more developed than the lower ; whilst in those 

 that employ their feet more than their wings, there is an inverse 

 arrangement. 



342 The food of Birds is of various kinds; some feed 

 exclusively on Seeds, others on Insects, others on Fish, others 

 again on the flesh of Mammalia or Birds still living, and soire 



