CHAP.TEK XI. 

 The Cerebrum : The Prosencephalon. 



From those brain-segments already considered in different classes of 

 vertebrates very varied direct tracts pass into the anterior segment: the 

 cerebrum. In frogs the interbrain only is connected with it; in other 

 vertebrates the midbrain also, and, finally, in mammals it receives a con- 

 nection with the spinal cord, whose centers are, in other animals, much more 

 independent of it. A direct tract from cerebrum to cerebellum is not 

 known, but even here there is, in mammals, a possible connection through 

 the tegmental nucleus and through the frontal ganglia. The influence which 

 the cereirum must exert over the lower centers direct is in varying strength 

 according to the vertebrate class. This fact, so evident to the comparative 

 anatomist, remained, curiously enough, up to the present quite ignored in 

 the interpretation of physiological and psychological phenomena. 



It is important, naturally, to designate ' which part of the cerebrum is 

 connected with other parts of the brain. You will see at once that especially 

 important tracts, those from the cortex, appear relatively late in the series, 

 ahd much later still do they reach comparative perfection; indeed, it is 

 only in mammals that such tracts pass to most of the other parts of the 

 brain. 



"We can imagine a schematic cerebrum. Picture to yourself the ovoid 

 vesicle which evaginates from the common ventricle near the terminal 

 lamina on either side. It increases in thickness at the base, forming there 

 a large body: the Corpus striatum. In the floor of the vesicle end the olfac- 

 tory nerve-fibers, and we may at once differentiate the olfactory apparatus 

 from the corpus striatum. Thus we have a second — indeed, characteristic 

 — division of the cerebrum, — sometimes, in fact, given the dignity of a po- 

 sition co-ordinate with the brain-segments, and called the Ehinencephalon. 

 The portion which remains of the vesicle — namely, the roof and sides — is 

 ■called the Pallium, or Mantle. 



The mantle may consist of (1) a simple epithelial plate, as in teleosts; I 

 (3) the lateral portions may thicken into nerve-areas, as in cyclostomes;/ 

 (3) lateral walls and anterior walls may be thickened, as in selachians, or, 

 finally (4) nearly the complete mantle may be transformed into brain-sub- 

 stance, only the most posterior part retaining its epithelial character and 

 persisting as the Tela chorioidea. Thus is constructed the brain-mantle in 



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