420 



PHYSIOLOGY 



by smaller 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 Monakow (Fig. 211, p. 419). 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 b"y 

 the various blocks and facilitations 

 imprinted on the nerve-paths by the 

 experience of the individual himself. 



The original cavities of the hemi- 

 spheres 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 ven- 

 tricle we see on its floor the body 

 of the fornix, a flattened tract of white 

 matter with longitudinal fibres, which 



the 



the 



FIG. 212. Horizontal section through 

 optic thalamus and corpus striatum, 

 ' basal ganglia.' (Natural size.) 

 vl, lateral ventricle, its anterior cornu ; 

 cc, corpus callosum; si, septum lucidum ; 

 af, anterior pillars of the fornix ; v3, third 

 ventricle ; th, thalamus opticus ; st, stria 

 medullaris ; nc, nucleus caudatus, and 

 nl, nucleus lenticularis of the corpus stria- 

 tum ; ic, internal capsule ; g, its angle or 

 genu ; nc, tail of the nucleus caudatus 

 appearing in the descending cornu of the 

 lateral ventricle ; cl, claustrum ; /, island 

 of Reil. 



in front bifurcates into two cylin- 

 drical bundles which pass vertically 

 downwards in front of the foramen of 

 Monro into the mesial part of the sub- 

 thalamic tegmentum. Internal to 

 the fornix is a layer of pia mater, 

 including the choroid plexus. On 



removing this the third ventricle 



is opened, so that in this region the wall of the cerebral hemispheres, like the 

 roof of the third ventricle, is limited to a simple layer of ependyma. At the 

 margin of the choroid plexus can be seen a part of the superior surface of the 

 optic thalamus, separated, however, from the cavity of the ventricle by 

 a layer of ependyma. Outside and in front of the optic thalamus are the 



