CORONA RAD I AT A 709 



situations where the different tracts cross one another tlie arrangement l^ecomes 

 more compHcatecl. This is notably the case in the region where the fibres of the 

 corpus callosuni intersect the fibres of the inner capsule. 



The corona radiata is the name given to the prolongation of the fibres of the 

 inner capsule into the hemisphere. It is continued upwards from the inner capsule 

 as a somewhat narrow band (peduncle of the corona radiata), but the fibres soon 

 spread out like a fan and proceed to the cerebral cortex. It will be well at this 

 stage to re-examine the constitution of the inner capsule; the latter contains three 

 sets of fibres, namely: (a) Fibres which pass frcnn the crusta of the crus cerebri 

 directly into the corona radiata without the intervention of nerve-cells; (h) fibres 

 ])assing from the mesencephalon int(j the optic thalamus and corpus striatum ; and 

 (e) fibres from the thalamus (jjart of the corona radiata of that body); and from 

 the corpus striatum to the cerebral cortex. We have already seen (page 701) that 

 the inner capsule shows in horizontal section an anterior limb, a posterior limb and 

 a genu. The fibres which occupy the most anterior part of the anterior limb belong 

 to the corona radiata of the optic thalamus. Behind these fibres is a tract which 

 extends nearly to the genu; this tract is formed by fibres which are derived from 

 the frontal lobe, and although their function is not accurately known they are sup- 

 posed to be associated with the higher intellectual functions. Traced downwards, 

 this bundle passes into the crusta, forming rather more than the inner fifth of that 

 body, and from thence enters the pons, where a part of it seems to terminate 

 (frontal pontine tract). Its further course is unknown. The portion of the inner 

 cai)sule immediately in front of the genu is intimately associated "with the faculty 

 of speech; it passes in the crusta immediately external to the frontal pontine tracts. 

 The genu conveys fibres from the cortex cerebri to some of the motor cranial nerves. 

 These fibres pass immediately external to the preceding (aphasia) bundle, and 

 probaiily decussate in the pons to become connected to the nuclei of the facial and 

 hypoglossal nerves, and to the motor nucleus of the trigeminal. The anterior two- 

 thirds of the posterior limb of the inner capsule is formed by the pyramidal tract 

 Avhich is described elsewhere (page 741). The posterior third of the posterior limb 

 of the inner capsule is formed by sensory fibres comprising the oi:»tic radiation; the 

 latter proceeds exclusively from the occipital lobe. The remaining sensory fibres 

 are derived from the occipital and temporal, and to a certain extent from the 

 parietal, lobes. They descend in the crusta, occupying the outer third of that body. 

 As this sensory tract traverses the crusta, its outermost part is formed liy fibres 

 derived from the fillet. 



The commissural system, consisting of the corpus callosum and the anterior 

 commissure, lias l)een already described (]iages 692, 701). 



The intrahemispheric or association fibres unite different portions of the 

 cortex of the same hemisphere to one another. These fibres are in two sets — along 

 and a short. The short fibres (fibrse arcuata? seu proprise) unite two adjacent con- 

 volutions, the fibres passing round lieneath the fissure which separates the two 

 convolutions. The long fibres are disposed in several more or less well-defined 

 tracts, namely: the cingulum, the fasciculus arcuatus, the fasciculus uncinatus, the 

 perpendicular occipital fasciculus, and the fasciculus longitudinalis .inferior. The 

 cingulum is a well-defined band, which occupies the core of the gyrus fornicatus. 

 Near the splenium of the corpus callosum it receives an accession of fil ires from the 

 occipital lobe. The fasciculus arcuatus passes from the cortex of the frontal to 

 the occipital lobe; it also sends some filires into the temporal lobe. The fasciculus 

 uncinatus springs from the orbital gyri and from the inferior frontal convolution, 

 and, after traversing the lower part of the claustrum, ends in the cortex of the 

 temporal lobe near the amygdaloid nucleus. The perpendicular occipital 

 fasciculus passes from the inferior parietal ]ol)ule to the inferior occipito-temporal 

 convolution. The fasciculus longitudinalis inferior is an extremely well-defined 

 bundle which ]>asses from the occipital to the temporal lobe. 



The external capsule also belongs to the association system, as it is chiefly 

 concerned with the convolutions of the island of Reil. 



Dissection. — Tlie stiirlent should remove the pia mater from the anterior part of the upper 

 surface of tlie cerebellum, and also any of that membrane which may be still adherent to the 

 quadrigeminal bodies. The anterior part of the cerebellum should now be gently i)ushed back- 



