NERVOUS SYSTEM OF THE VERTEBRATA. 



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The most characteristic parts of the Mammalian cerebrum are 

 the commissures connecting the two hemispheres. These commissures 

 are (1) the anterior commissure, (2) the fornix, and (3) the corpus 

 callosum, the two latter being peculiar to Mammalia. 



By the fusion of the inner walls of the hemispheres in front of 

 the lamina terminalis a solid septum is formed, known as the septum 

 lucidum, continuous behind with the lamina terminalis, and below 

 with the corpora striata (figs. 255 and 259 s'pt). It is by a series of 

 differentiations within this septum that the above commissures 

 originate. In Man there is a closed cavity left in the septum 

 known as the fifth ventricle, which has however no communication 

 with the true ventricles of the brain. 



In the septum lucidum there become first formed, below, the 

 transverse fibres of the anterior 

 commissure (fig. 255 and fig. 

 259 cma), and in the upper 

 part the vertical fibres of the 

 fornix (fig. 255 and fig. 259 

 frx 2). The vertical fibres 

 meet above the foramen of 

 Munro, and thence diverge 

 backwards, as the posterior 

 pillars, to lose themselves in 

 the cornu ammonis (fig. 259 

 anmi). Ventrally they are con- 

 tinued, as the descending or 

 anterior pillars of the fornix, 

 into the corpus albicans, and 

 thence into the optic thalami. 



The corpus callosum is not 

 formed till after the anterior 

 commissure and fornix. It 

 arises in the upper part of the 

 region (septum lucidum) formed 

 by the fusion of the lateral 

 walls of the hemispheres (figs. 

 255 and 259 caV), and at first only its curved anterior portion — 

 the genu or rostrum — is developed. This portion is alone found in 

 Monotremes and Marsupials. The posterior portion, which is present 

 in all the Monodelphia, is gradually formed as the hemispheres are 

 prolonged further backwards. 



Primitively the Mammalian cerebrum, like that of the lower 

 Vertebrata, is quite smooth. In many of the Mammalia, Monotre- 

 mata, Insectivora, etc., this condition is nearly retained through life, 

 while in the majority of Mammalia a more or less complicated system 

 of fissures is developed on the surface. The most important, and first 

 formed, of these is the Sylvian fissure. It arises at the time when 

 the hemispheres, owing to their growth in front of and behind the 



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Fio. 262. Lateral view of the brain 

 OF A c.u,F EMBRYO OF 5 CM. (After Mihal- 

 kovics.) 



The outer wall of the hemisphere is re- 

 moved, so as to give a view of the interior of 

 the left lateral ventricle. 



hs. cut wall of hemisphere; st. corpus 

 striatum ; am. hippocamjius major (cornu am- 

 monis); d. choroid plexus of lateral ventricle ; 

 fm. foramen of Munro; op. optic tract; in. 

 infundibulum ; hi&. mid-hrain ; cfc. cerebellum ; 

 IV. V. roof of fourth ventricle; j)s. pons Va- 

 rolii, close to which is the fifth nerve with 

 Gasserian ganglion. 



