136 



THE INTERNAL CAPSULE. 



deepest layer of the grey cortex of the hemispheres. Most of the cells contain 

 yellow pigment. 



The internal capsule (Burdach) (figs. 97 to 100, c.i. and i.e.') is a lamellated 

 tract of white fibres which lies between the nucleus lenticularis laterally and the 

 nucleus caudatus, stria terminalis, and optic thalamus mesially. It is somewhat 

 interrupted, especially in front, by strands of grey matter which unite the caudate 

 and lenticular nuclei. In front, behind and above it is continuous with the white 

 substance of the hemispheres, its fibres diverging in a fan-like manner towards the 

 cortex (corona radiata of Reil). Below it passes directly into the crusta, of which it 

 is the immediate continuation. In horizontal sections (fig. 99) the internal capsule 

 shows a bend (genu) opposite the stria terminalis, the anterior third forming an angle 

 of about 120 with the posterior two- thirds ; these two parts are known as the ante- 

 rior and posterior segments respectively. In vertical sections (fig. 100) it appears 

 to take a straight course, upwards and outwards from the crus cerebri to the 



Fig. 100. SECTION ACROSS THE OPTIC 



THALAMUS AND CORPUS STRIATUM 

 IN THE REGION OF THE MIDDLE 



COMMISSURE. (E. A. S., after a 

 preparation by Mr. S. G. Shattock.) 

 Natural size. 



th., thalamus ; a, e, i, its anterior, 

 external and internal nuclei respectively ; 

 iv, its latticed layer ; m.c., middle com- 

 missure ; above and below it is the cavity 

 of the third ventricle ; c.c., corpus callo- 

 sum ; f, fornix separated from the third 

 ventricle and thalamus by the velum 

 interpositum. In the middle of this are 

 seen the two veins of Galen and the 

 choroid plexuses of the third ventricle ; 

 and at its edges the choroid plexuses of 

 the lateral ventricles ; t.s., tsenia semi- 



circularis ; cr. , forward prolongation of the crusta passing laterally into the internal capsule, i.e. ; 

 S.t.r., subthalamic prolongation of the tegmentum, consisting of (1) the dorsal layer, (2) the zona 

 incerta, and (3) the nucleus of Luys ; s.n., substantia nigra ; ii.c., nucleus caudatus of the corpus 

 striatum ; n.l., nucleus lenticularis ; e.c., external capsule ; cl., claustrum ; /, island of Reil. 



corona radiata. It has been determined as the result of experimental observa- 

 tions in animals, and from pathological and clinical observations in man, that the 

 fibres which course in the middle third of the internal capsule (i.e., which are 

 opposite the globus pallidus of the lenticular nucleus) are connected with the part 

 of the cerebral cortex excitation of which gives rise to movements in the various 

 parts of the body (Rolandic region of cortex), whilst those of the anterior third 

 are connected with the prefrontal region, andthose of the posterior third with the 

 occipito-temporal region. 



Besides these fibres which connect the cortex of the hemisphere with the crusta, 

 the internal capsule contains others derived from various sources, viz., from the 

 caudate and lenticular nuclei, the optic thalamus, and sub-thalamic region, and 

 also, through the corpus callosum, from the cortex of the opposite hemisphere 

 (Hamilton). The exact localization within the internal capsule of most of these 

 fibres has not as yet been determined. 



