THE NERVOUS SYSTEM 14^3 



The inner two zones, which axe somewhat yellowish, are together 

 known as the globus paUidus. The putamen and globus palhdus, 

 which consist of grey matter, are traversed by white fibres. 



The grey matter of the nucleus caudatus and nucleus lenticularis 

 comes to the surface at the base of the brain in the region of the 

 locus perforatus anticus, where it is continuous with the grey 

 matter of the cerebral cortex. 



The internal capsule is the thick tract of white matter which lies 

 between the nucleus lenticularis externally, and the nucleus cau- 

 datus, taenia semicircularis, and optic thalamus internally. As 

 seen in horizontal section it describes a bend opposite the front 

 part of the optic thalamus. This bend is called the genu, and its 

 convexity is directed inwards. The part of the internal capsule 

 in front of the genu is called the anterior limb. It forms about 

 one-third of the entire capsule, and its direction is forwards and out- 

 wards. The part behind the genu is called the posterior limb. It 

 forms about two- thirds of the entire capsvde, and its direction is 

 backwards and outwards. 



The anterior limb of the internal capsule is situated between the 

 front part of the nucleus lenticularis and the nucleus caudatus. 

 Anteriorly it is intersected by the striae of grey matter which pass 

 between the two nuclei. 



The fibres which compose the anterior limb are partly corticipetal 

 and partly corticifugal. The corticipetal fibres are as follows: 

 (i) thalamo-frontal fibres, which pass from the optic thalamus to 

 the cortex of the frontal lobe ; (2) thalamo-striate fibres, which pass 

 from the optic thalamus to the corpus striatum (thalamo-caudate 

 and thalamo-lenticular fibres) ; and (3) strio-frontal fibres, which 

 pass from the corpus striatum to the cortex of the frontal lobe. 



The chief corticifugal fibres constitute the fronto-pontine tract. 

 The fibres of this tract arise in the cortex of the prefrontal region. 

 They traverse the anterior limb of the internal capsule, and then 

 descend in the inner part of the crusta of the cms cerebri to the 

 pons Varolii, within which they terminate in connection with the 

 ceUs of the nucleus pontis. 



Other centrifugal fibres constitute fronto-thalamic, fronto-siriaie, 

 and strio-thalamic tracts. 



The posterior limb of the internal capsule is situated between the 

 back part of the nucleus lenticularis and the optic thalcimus, and 

 is prolonged backwards for a little beyond the posterior limit of 

 the nucleus lenticularis. It is therefore conveniently divided into 

 two parts, lenticular, representing the anterior two-thirds, and 

 postlenticular, representing the posterior third. 



The lenticular part of the posterior limb, like the anterior limb, is 

 composed of centripetal and centrifugal fibres. The centripetal 

 fibres arise in the optic thalamus, and their destination is the cerebral 

 cortex. The centrifugal fibres represent the pyramidal or motor fibres, 

 and they occupy the anterior portion of the lenticular part of the 

 posterior limb of the internal capsule. These fibres descend from 

 the Rolandic region of the cerebral cortex. Some of them pass to 



