THE CENTRAL NERVOUS SYSTEM. 83 



the two hemispheres. The projection fibres are those which connect 

 the cerebral cortex with the lower parts of the central nervous system, 

 and they are either afferent or efferent. The associational fibres link 

 up different parts of the cortex. The fact that nerve fibres acquire 

 their myelin sheaths at the time at which they become functionally 

 active has been made use of by Flechsig in studying the nerve tracts 

 of the cerebral hemispheres, and, from his observations, he has come to 

 conclusions as to the functions of the different areas of the cortex 

 which are as a whole identical with those based upon the study of the 

 structure of the grey matter in different regions of the adult cortex. The 

 first fibres to become myelinated in the cerebral hemispheres are those 

 afferent projection fibres which are distributed to the sensory areas of 

 the brain. The last to acquire a myelin sheath are those which are 

 connected with the higher centres of association. 



The Projection Fibres. The efferent projection fibres are the 

 pyramidal (cerebro-spinal) and the cerebro-cerebellar. The pyramidal 

 fibres are the axons of the Betz cells in the pre-central convolution or 

 motor area. They converge towards the base of the hemisphere, and 

 occupy the genu and the anterior two-thirds of the posterior limb of 

 the internal capsule (fig. 27). From this .situation they descend in 

 the pes of the cerebral peduncle and through the pons and medulla 

 oblongata to the spinal cord in the manner already described. 



The cerebro-cerebellar fibres are the fronto-pontine from the frontal 

 lobe and the temporo-pontine from the temporal lobe. These descend, 

 the former in the anterior limb of the internal capsule, the latter in 

 the posterior third of its posterior limb, to the base of the cerebral 

 peduncle, after passing through which they reach the pons to arborise 

 round the cells of the nuclei pontis. 



The afferent projection fibres are the thalamo-cortical, the optic 

 radiation, the auditory radiation, and some fibres of the superior 

 cerebellar peduncle. 



The thalamo-cortical fibres form the final relay on the path of 

 afferent impulses from the lower centres to the cortex of the cerebral 

 hemisphere, and they are distributed to all parts of the cortex. The 

 thalamo-frontal fibres run towards the frontal lobe in the anterior limb 

 of the internal capsule. The thalamo-parietal fibres pass through the 

 posterior limb of the internal capsule to reach their destination in the 

 parietal lobe. Those to the island of Reil run under the lenticular 

 nucleus. Those to the temporal lobe run in the posterior end of the 

 posterior limb of the internal capsule, and are joined by other fibres 

 from the inferior colliculus of the corpora quadrigemina and the medial 

 geniculate body to form the auditory radiation. Those to the occipital 



