270 



ANATOMY OF VERTEBRATES. 



as in fig. 142, tlie cerebellum, d, olfactory lobes 



146 



a ' corpus callosum ' or great commissure, fig. 145, /. The rliinen- 

 ceplialon, fig. 142, a, is in contact with the prosencephalon, 

 Z», and sends off numerous olfactory nerves which perforate a 

 ^ cribriform ' plate of the prefrontal. 



§ 173. Mammalian Subclasses. — The primary subdivisions of 

 the present class are characterised by conditiolis of the brain.' 



When the hemisj^heres are connected by the ^ round commis- 

 sure ' and ' hippocampal commissure' only, fig. 144, m, this cerebral 

 condition is associated with the absence of a vascular chorion or 

 placenta, and wdth prematurely born young, compared Avith the 

 rest of the class (fig. 138 show^s the natural size of the new-born 

 Kangaroo of the largest species). 



The cerebral hemispheres are usually without folds, and leave, 



a, and part of 

 the optic lobes, c, exposed. The sub- 

 class so characterised is called Lyen- 

 CEPHALA.2 Mammals of this low^ type 

 existed as far back, in time, as the 

 oolitic and triassic periods, and are 

 the oldest known.^ 



The next stage of complexity in the 

 Mammalian brain is w here the * corpus 

 callosum,' fig. 145, Z, is present; but 

 connects hemispheres as little ad- 

 vanced in relative bulk or outward 

 character as in the preceding sub- 

 class ; the 



146, e, or with folds 

 members of the group, not more nu- 

 merous or complex than in the larger 

 Lyencephala. The hemispheres, ib. 

 e, leave the cerebellum, e, d, and part 

 of the olfactory lobes, /, exposed. 

 The subclass so characterised is called Lissencephala.'^ In the 

 species with this condition of brain the testes remain in the 

 abdomen, or are protruded into a temporary scrotum only at the 

 breeding period, to be again retracted : in most there is a common 

 external urogenital aperture : there are two precaval veins. The 

 squamosal in most, and the tympanic in many, retain their primi- 

 tive condition as distinct bones. The orbits have not an entire 



surface being smooth, fig. 

 in the largest 



Tppcr surf;ice of brain, Agouti. 



T.XVIII-. and lxiv. 

 xvjr. p. 338. 



- Xvoo, I loose ; iyKefaAos, brain. Lxx* 

 ■• AicraSs, siDOOtli ; eyiii(pa\os, brain. 



