THE WHITE MATTER OF THE CEKEBEAL HEMISPHERES. 651 



The fasciculus occipito-frontalis superior is a bundle of fibres which runs in a 

 sagittal direction in intimate relation to the lateral ventricle (Fig. 576, p. 649). It may 

 be regarded as the medial edge of the superior longitudinal bundle. It has been pointed 

 out (Forel, Onufrowicz, and others) that, in cases where the corpus callosum fails to 

 develop, the tapetum remains apparently unaffected, and Dejerine has endeavoured to 

 prove that the fibres of this layer really belong to the fasciculus occipito-frontalis. The 

 fasciculus occipito-frontalis lies on the medial aspect of the corona radiata in intimate 

 relation to the caudate nucleus, and posteriorly it spreads out over the superior and 

 lateral aspect of the lateral ventricle, immediately outside the ependyma, where, 

 according to Dejerine, it constitutes the tapetum (see p. 632). 



Projection Fibres. We have already seen that every part of the cerebral 

 cortex is linked to other cortical areas, not only in its own neighbourhood (short 

 association fibres) (Fig. 578), but also 

 in the most distant parts of the hemi- 

 sphere (long association fibres), as well 

 as to the cortex of the other hemi- 

 sphere (commissural fibres). In 

 addition there are two large series of 

 fibres : (i.) an ascending group which 

 conveys to the cerebral cortex im- 

 pulses coming from the thalamus and 

 metathalamus, the corpora quadri- 

 gemina and the red nucleus, and the 

 various other sensory nuclei scattered 

 throughout the brain stem and spinal 

 medulla ; and (ii.) a descending group 

 connecting the cerebral hemisphere 

 with the corpus striatum, various parts 

 of the diencephalon, mesencephalon 

 and cerebellum, as well as with all the 

 motor nuclei scattered throughout the 

 central nervous system. These two 

 groups of tracts, respectively passing 



r , , J . & 



to and from the cerebral cortex, are 

 known collectively as its projection 

 fibres. 



While examining the general ar- 

 rangement of these projection fibres 

 of the cerebral hemisphere it is con- 

 venient to refer incidentally to certain 

 other fibre -tracts which do not fall 

 strictly within this group. 



The Sensory Tracts. A certain 

 proportion of the fibres that enter the 



spinal medulla by its posterior root, which are supposed to be the sensory nerves^ of 

 muscles, tendons, and joints, pass upwards without interruption in the posterior 

 funiculi throughout the whole length of the spinal medulla until they reach the 

 medulla oblongata, where they end in the nucleus gracilis and nucleus cuneatus. 

 From these nuclei, arcuate fibres (fasciculus bulbothalamicus) arise and, after crossing 

 the median plane, proceed upwards in the medial lemniscus of the other side to end 

 in the ventro-lateral nucleus of the thalamus, from which a third group of neurones 

 arises and proceeds upwards through the internal capsule to the cerebral cortex, 

 where the impulses conveyed by it excite a consciousness of position and movement. 

 But other sensory fibres end in the spinal medulla near their place of entry into 

 it, and from the cells related to the endings of these fibres a new tract (fasciculus 

 spinothalamicus) arises, crosses the median plane to reach the antero-lateral funiculus 

 of the opposite side, in which it proceeds upwards throughout the whole length of 

 the spinal medulla (that lies above its origin), the rhombencephalon and mesen- 

 cephalon to the thalamus, where it ends alongside the bulbo-thalamic tract in 



FIG. 579. DIAGRAM OF THE SENSORY TRACTS FROM THE 

 SPINAL MEDULLA TO THE CEREBRAL CORTEX. 



