THE CEREBRAL HEMISPHERES 



951 



THALAMO- 



FHONTAL 



TRACT 



GENICULATE 



PORTION OF 



MOTOR TRACT 



(FOR MUSCLES 



OF FACE AND 



TONGUE) 



last-mentioned system traverses the internal capsule to a greater or less extent, 

 hut does not continue into the crusta. These various systems are summarized 

 on page 957. It may here he mentioned that the internal capsule, topographically, 

 exhibits a functional dissociation in that its frontal or lenticulocaudate limb is 

 composed of preponderatingly corticipetal fibres, while corticifugal fibres form the 

 major portion of the thalamolenticular limb (Fig. 705). In the frontal limb 

 are the thalamofrontal and thala- 

 mostriate fibres, the former ending 

 in the cortex of the frontal lobe, 

 the latter in the caudate and 

 lenticular nuclei. The chief cor- 

 ticifugal components are the fron- 

 topontile tract, and fewer fronto- 

 thalamic and striatothalamic fibres. 

 The frontopontile tract arises in 

 the cortex of the prefrontal region, 

 traverses the frontal limb of the 

 internal capsule, forms the ental 

 sector (one-fifth) of the crusta, 

 and ends in the nuclei pontis. 



In the genu and the thalamo- 

 lenticular limb of the internal 

 capsule course several important 

 fibre tracts which are chiefly cor- 

 ticifugal. 



The pyramidal (motor) tract, in 

 its course from the precentral 

 cortex to the lower motor centres, 

 occupies the frontal half of this 

 limb. The portion in the genu, 

 often designated the geniculate 

 tract, comprises the pyramidal 

 fibres which are destined to go to 

 the facial and hypoglossal nerve 

 nuclei; farther caudad lie, in suc- 

 cession, the fibres going to the 

 motor centres for the upper and 

 the lower extremity. The most 

 caudal segment (also called the 

 retrolenticular part) of the internal 

 capsule contains (a) the optic 

 radiation, composed of fibres 

 coursing in both directions be- 

 tween the occipital cortex and the 

 pulvinar, external geniculate and 

 superior quadrigeminal bodies; (6) 

 the auditory radiation, composed of 

 fibres passing in both directions between the cortex of the temporal lobe (auditory 

 centre) and the posterior quadrigeminal and internal geniculate bodies; (c) the 

 occipitopontile and temporopontile tracts from the occipital and part of the temporal 

 cortex, coursing through the caudal segment of the internal capsule, constituting 

 the ectal (one-fifth) sector of the crusta and ending in the nuclei pontis. In ad- 

 dition there are scattered fibre bundles which arise from the ventral portion of 

 the thalamus, enter the internal capsule to pass toward the cortex, in part through 



OPTIC 

 RADIATION 



FIG. 705. Diagram of the tracts in the internal capsule. 

 Motor tract red. The sensor tract (blue) is not direct, but 

 formed of neurones receiving impulses from below in the 

 thalamus and transmitting them to the cortex. The optic 

 radiation (occipitothalamic) is shown in violet. 



