BASAL GANGLIA 471 



the anterior commissure pass through it on their way to the 

 temporal lobe. Define the fibres of the anterior commissure 

 (Fig. 189) and trace them forward and medially, to the medial 

 face of the section, by removing the anterior perforated substance, 

 which lies below them. 



The fibres which lie anterior and posterior to the lentiform 

 nucleus are fibres of the internal capsule. The anterior fibres 

 can be traced downwards to the basis pedunculi, but those of 

 the posterior part, which are fibres of the acoustic and optic 

 radiations, turn medially towards the posterior part of the 

 thalamus and the medial geniculate body. 



Make a similar dissection on the upper segment of the right 

 hemisphere to expose the upper part of the lateral surface and 

 the upper border of the lentiform nucleus. Note that the white 

 matter which appears at the upper border of the lentiform 

 nucieus consists of fibres of the internal capsule, which are 

 passing vertically into the corona radiata, and of some longi- 

 tudinally directed fibres which form the superior longitudinal 

 fasciculus (Fig. 188). Now remove the lentiform nucleus and 

 expose the remainder of the lateral surface of the internal 

 capsule. Finally, trace the main mass of the capsule downwards 

 into the basis pedunculi of which they form the middle three- 

 fifths ; the lateral and medial fifths being formed by fibres 

 passing from the temporal and frontal lobes to the pons. Pre- 

 serve the pieces of the right hemisphere so that the continuity 

 of the motor fibres of the anterior two-thirds of the posterior 

 division of the capsule with the cerebro-spinal fibres of the pons 

 and medulla can be demonstrated at a later stage. 



When the dissection of the right hemisphere is completed 

 turn to the posterior vertical section of the left hemisphere and 

 expose the internal capsule from the lateral side by removing, 

 in turn, the remains of the insula, the claustrum, the external 

 capsule, and the lentiform nucleus ; then trace the fibres of 

 the internal capsule of the left side downwards into the basis 

 pedunculi. 



Complete the dissection of the right hemisphere by tracing 

 the fasciculus mamillo-thalamicus upwards from the mamillary 

 body into the anterior nucleus of the thalamus. 



Capsula Interna. -The internal capsule is a relatively 

 thick lamina of white substance by means of which associa- 

 tions are established between the cortex of the hemisphere, 

 its basal nuclei, the lower parts of the brain, and the medulla 

 spinalis, It lies between the caudate nucleus and the thalamus, 

 on the medial side, and the lentiform nucleus on the lateral 

 side, but it extends both anterior and posterior to the lentiform 

 nucleus, and therefore consists of lentiform, pre-lentiform, and 

 retro-lentiform portions. It is continuous, below, with the 

 basis pedunculi and above with the corona radiata, and the 

 lentiform part is bent upon itself, round the medial angle 

 of the lentiform nucleus. The bend, which is known as the 

 genu, lies between, and unites together, the anterior and 

 in 30 b 



