THE TELENCEPHALON. 



number, the lenticular nucleus and the external capsule and proceed to the cortex of the hind 

 part of the frontal and of the parietal lobe; and (2) probably some thalamo-lcnticnlar fibres 

 which pass from the thalamus to the lenticular and, perhaps, the caudate nucleus. 

 The corticifugal fibres include : ( i ) the important motor cortico-bulbar and cortico- spinal 

 tracts, collectively often called the pyramidal tracts, which descend from the precentral 

 (Rolandic) cortical region through the corona radiata and the fore-part of the posterior limb of 

 the internal capsule into the crusta of the cerebral peduncle and thence to the appro- 

 priate levels of the brain-stem or of the spinal cord. A tract supplementary to the pyramidal 

 motor paths, the cortico-rubral fibres, must be mentioned. These arise from the cortex 

 (perhaps of the parietal lobe) and descend through the lenticular portion of the posterior 

 limb to the mid-brain where they end in relation with the red nucleus. (2) The cortico- 

 thalainic fibres, which converge from the cerebral cortex to the thalamus. The retro- 

 lenticular portion of the posterior limb is traversed by important corticipetal fibres con- 

 cerned in conveying impressions of special sense, as ( i ) those of the optic radiation, 

 which, issuing as the occipital stalk, connect the thalamus and the lateral geniculate and 

 the superior quadrigeminal body with the occipital cortex ; and (2) those of the auditory 

 radiation, which link together the mesial geniculate and the inferior quadrigeminal body 

 with the auditory cortical area in the temporal lobe. The corticifugal fibres are represented 

 by (i) the temporo-occipito-pontile tracts, which pass from the cerebral cortex through the 

 retrolenticular portion of the capsule into the crusta of the cerebral peduncle and thence to the 

 pontile nucleus within the ventral part of the pons ; and (2) cortico-thalamic fibres, which 

 course in reverse order through the optic radiation to end within the thalamus and lateral 

 geniculate body. 



The relative positions of the longer tracts composing the internal capsule, as seen in hori- 

 zontal sections, are, in a general way, indicated schematically in Fig. 1012. The anterior limb 

 is shared, from before backward, by the fronto-thalamic and the fronto-pontile tracts in the 

 order named. The genu is appropriated by the cortico-bulbar tracts, the facial fibres lying 

 immediately in advance of the hypoglossal. The succeeding part of the posterior limb, 

 approximately one-third, affords passage to the cortico-spinal or pyramidal tracts. Next follows 

 a narrow segment devoted to the tegmental sensory tracts, behind which the occipito-temporo- 

 pontile tract occupies a small area, the last part of the retrolenticular field being taken up by 

 the optic radiation. 



STRUCTURE OF THE CEREBRAL CORTEX. 



FIG. 1013. 



The surface of the hemispheres is everywhere clothed with a thin continuous 

 stratum of cortical gray matter, which encloses the white medullary substance com- 

 posed of the interlacing tracts of nerve-fibres. This cortical sheet varies in thick- 

 ness not only in the same area, being 

 thicker over the summit than at the sides 

 of the convolutions or at the bottom of 

 the bounding fissures, but in different 

 regions of the hemisphere. Its average 

 thickness is about 3 mm., but where it 

 borders the upper end of the Rolandic 

 fissure, particularly in the paracentral 

 lobule, this increases to over 5 mm., 

 whilst over the frontal and occipital poles 

 the thickness of the cortex is reduced to 

 almost 2 mm. The entire superficial 

 extent of the cortex of the two hemi- 

 spheres has been estimated to be about 

 2000 sq. cm. , of which scarcely one- 

 third is exposed surface, the remainder calcarine fissure 



Stratum zonale- 



Externalgray stratum- 

 Outer stripe of 

 Baillarger, 

 (Stripe of Gcnnari) 

 Internal gray stratum 



Medullary fibres.'^ 



Frontal section of hemisphere including cortex sur- 

 frp^h rounding calcarine fissure; stripe of Gennari (outer stripe 

 of Baillarger) is here unusually distinct. X 3- 



being sunken. 



On examining sections of 

 brain, the cortex does not appear 

 uniformly tinted, but exhibits, even to the unaided eye, an indistinct division 

 into alternate light and dark layers. From without in these are: (i) a thin 

 peripheral layer of whitish color, the stratum zonale; (2) a thicker layer of 

 grayish hue, the external gray stratum ; (3) a thin lighter band, the outer stripe 

 of Baillarger ; and (4) a somewhat broader, yellowish-red zone, the internal gray 



