INTRODUCTION. XV 



rings being thin, and the membranes hirge and contrac- 

 tile. The great volume of air, contained in the large 

 air-cavities, contributes much to the strength of the 

 voice. 



The brain is large, often indeed proportionally larger 

 than in Mammals ; and has a larger mass than the spinal 

 marrow. The hemispheres of the cerebrum are without 

 convolutions ; but the lateral ventricles and corpora striata 

 are large. There is only a rudimentary corpus cullosum 

 connecting the two hemispheres, but no pons varolii. 

 The olefactory tubercles are situate beneath the front of 

 the hemisphei-es. The cerebellam is almost without lateral 

 lobes ; being chiefly formed by the central lobe or ver- 

 miform process. The spinal cord has two eidarge- 

 ments, corresponding to the origin of the extremities. 

 The cerebral nerves are the same as in Mammals. 



The sense of sight in Birds is very perfect and highly 

 developed. The bony orbits are of great magnitude, and 

 the eye is proportionally large, and placed laterally on the 

 head. It is composed of two unequal segments ; the smallest 

 being the anterior, and at the junction of the two there is 

 a bony ring of usually fourteen to fifteen bony plates 

 lodged in the front of the sclerotic membrane. The crys- 

 talline lens is usually lather flat, but more convex in 

 those birds which do not require long vision. In certain 

 birds, such as Eiigles, Vultures, Falcons, &c., some muscles 

 are attached from the bony ring to the cornea, and by 

 their contraction, the convexity of the cornea is increased 

 so that the eye can readily adapt itself to the great dif- 

 ferences in the reach of their vision, and they can thus 

 equally distinguish a small object when soaring high in 

 mid-air, and the same object when they have descended 

 upon it ; or, as in the case of Penguins and other water- 



