STRUCTURAL ARRANGEMENTS OF CEREBRUM 419 



fissures. The extent of these secondary fissures varies from brain to brain, 

 the higher types of brain being richer in convolutions than those of the more 

 primitive races. 



The gradual evolution of the cerebral cortex, and the concomitant shifting 

 of the chief afferent impulses, arising in the projicient sense organs, from the 

 lower ganglia to the higher educatable cortex, is well shown in the diagrams 

 from Moankow (Fig. 211, p. 418). In the lower fishes practically all the 

 reactions to visual impressions are carried out by the optic lobes. In the 

 higher types the reflexes through 

 these lobes become subordinated, first 

 to the more complex organ of the 

 optic thalamus (where representatives 

 from all the afferent tracts of the body 

 assemble), and later to the still more 

 complex occipital cortex, when the 

 reactions are determined not only by 

 inherited nerve paths but also by 

 the various blocks and facilitations 

 imprinted on the" nerve paths by 

 the experience of the individual 

 himself. 



The original cavities of the hemispheres 

 form the lateral ventricles, each of which, 

 in the adult brain, is prolonged into the main 

 divisions of the hemispheres as the anterior 

 horn, the posterior horn, and the inferior 

 horn. Each lateral ventricle is roofed over by 

 the corpus callosum and the adjoining white 

 matter of the hemispheres. On opening the 

 ventricle we see on its floor the body of the 

 fornix, a flattened tract of white matter with 

 longitudinal fibres, which in front bifurcates FlG 2 12. Horizontal section through the 

 into two cylindrical bundles which pass verti- optic thalamus and corpus striatum, the 

 cally downwards in front of the foramen of 'basal ganglia.' (Natural size.) (QTJAIN.) 

 Monro into the mesial part of the subthala- vl, lateral ventricle, its anterior cornu ; 

 mictegmentum. Internal to the fornix is a cc ; cor P us callosum; si, septum lucidum ; 



., a/, anterior pillars of the fornix; v3, third 



layer of pia mater, including the choroid ventricle . thf thalamus opticus ; *t, stria 

 "exus. On removing this the third ventricle medullaris ; nc, nucleus caudatus, and 

 opened, so that in this region the wall of nl, nucleus lenticularis of the corpus stria- 



e cerebral hemispheres, like the roof of tum ' ic > ^*f^ f P? ule ' ?' its ang > c 



genu ; nc, tail of the nucleus caudatub 



third ventricle, is limited to a simple ppearing ^ the descending comu of the 

 yer of ependyma. At the margin of the lateral ventricle ; r 7. claustrum ; /, island 

 horoid plexus can be seen a part of the supe- of Reil. 

 ior surface of the optic thalamus, separated 



lowever from the cavity of the ventricle by a layer of ependyma. Outside 

 and in front of the optic thalamus are the masses of nervous material con- 

 stituting the corpus striatum. These present two nuclei of grey matter, known 

 as the nucleus caudatus and the nucleus lenticularis (Fig. 212). The crusta 

 of the crura cerebri as it ascends to the cerebral hemispheres passes behind between 

 the optic thalamus and the corpus striatum, and in front between the nucleus lenticu- 



