NERVOUS SYSTEM. 



dullary bands which radiate outwards to the cineritious exte- 

 rior lamellae, from the white fibrous parietes of the fourth 

 ventricle (98. B../J) produce an arborescent appearance when 

 the cerebellum is divided vertically. The ordinary great 

 component fasciculi of the medulla oblongata are distinctly 

 marked externally, and the same nerves arise from this part 

 as in reptiles and mammalia. Where the lateral tubercles or 

 rudimentary hemispheres of the cerebellum are most distinct, 

 a small transverse commissure is already perceptible between 

 them, asin the human embryo, but there is no corpus dentatum. 

 The optic lobes (98. A. c,) reduced in size and separated 

 from each other to the sides of the medulla oblongata by 

 the encreasing cerebrum and cerebellum, are covered ex- 

 ternally, like the other lobes of the brain, with cineritious 

 matter, consist internally of white fibres, and present a small 

 ventricle (98. B. g,) in each, like the cerebral hemispheres. 

 They are proportionally small and round in the strutheous birds 

 (98. B. g, g,) more lengthened and oval in the inferior birds 

 (98. A. c, c,) and present between them transverse medullary 

 bands, (98. B. k } ) forming a valvular commissure, on which 

 rests the pineal gland (98. B. i,) with its two peduncles ex- 

 tending forwards over the thalami optici. The optic thalami 

 covered by the cerebral hemispheres are connected by a cine- 

 ritious commissura mollis, the optic lobes are without trans- 

 verse exterior sulci, and the hemispheres are destitute of 

 convolutions though partially divided into two lobes, (98. A. 

 d, e,) by the fissura sylvii. The cerebral hemispheres, covered 

 externally with cineritious matter, smooth and undulated on 

 the surface, broad posteriorly, and tapering suddenly forwards 

 to the ethmoid bone, cover the thalami optici and the cor- 

 pora striata, and contain each a small ventricle which extends 

 forwards to the olfactory tubercles (98. A./.) The cerebral 

 hemispheres (98. B. i,) are composed chiefly of the ascending 

 and diverging fibres of the corpora pyramidalia and are con- 

 nected by an anterior commissure, and the crossing fibres of 

 a rudimentary corpus callosum. The olfactory tubercles 

 (98. A./,) commencing from two medullary tracts (98. A. h, 

 h,) on the inferior surface of the hemispheres, and containing 

 prolongations of the lateral ventricles, are still covered with 

 cineritious substance, and taper forwards to the olfactory 

 nerves, greatly reduced from their proportions in the inferior 



