NERVOUS SYSTEM OF AVES. 121 



the fornix. The round anterior commissure, k, is prolono-ed on 

 either side into the substance of the hemispheres. 



The optic thalami, ib. I, are of small size, and not united by a 

 soft commissure : between them is the ca\aty called the third ven- 

 tricle, ib. m ; and above and behind they give off the peduncles 

 of the pineal gland. This body does not hang freely suspended 

 by the pedicles, but seems to form a rounded and thickened 

 anterior border of the valvula Yieussenii or lamelliform commis- 

 sure of the optic lobes. It adheres firmly to the confluence of 

 the great veins situated at the anterior orifice of the aqueduct of 

 Sylvius : it is usually of a conical or pyriform shape. The valve 

 which closes the upper part of the passage from the third to the 

 fourth ventricle, is a thin lamella of great width, in consequence 

 of the distance at which the optic lobes are separated from one 

 another. Anteriorly the third ventricle communicates with the 

 infundibulum, which terminates in a large hypophysis. 



Besides the cavities or ventricles above mentioned, there are 

 also two others situated in the optic lobes, fig. 45, o, or bigeminal 

 bodies, each of which, when laid open, is seen to be occupied by a 

 convex body, ib. jj, projecting from the posterior and internal 

 side of the lobe ; these ventricles communicate with the others in 

 the aqueduct of Sylvius. 



The brain of the Bird differs from that of the Reptile in tho 

 superior size of the cerebrum and cerebellum, together with the 

 folding of the latter, which relates probably to the higher locomo- 

 tive powers of the Bird : it differs from the brain of the Mammal 

 in the absence or small beginning of the fornix, and of the lateral 

 lobes of the cerebellum : it differs from the brain of every other 

 class in the lateral and inferior position of the optic lobes. In a 

 pigeon weighing eight ounces with and seven ounces mthout the 

 feathers, or 3360 grains, the myelenceplialon weighs 48 grains, 

 the weight of the myelon being 1 1 grains, and that of the brain 

 37 grains. The proportion of the weight of the brain to the 

 body is much greater in the Humming- Bird : whilst in the huge 

 Dinornis, the brain does not exceed two inches and a half in 

 length, and two inches in width. It thus presents a limited range 

 of size, and much sameness of form and structure in the different 

 orders of the class of Birds. 



§ 138. Nerves of Birds. — The olfactory or first pair, usually 

 of a simple rounded form, proceed from the small pyriform 

 rhinencephalon, fig. 44, r, continued from the apex of the hemi- 

 sphere, and usually somewhat deflected. The nerve runs along 

 an osseous canal, accompanied by a venous trunk above the 



