BASAL GANGLIA 519 



peduncle. The dissector can easily satisfy himself in the 

 specimens before him of this continuity ; it is apparent in the 

 more posterior of the frontal sections which he has made 

 through the left portion of the cerebrum. 



It has been mentioned already that the fibres which occupy the middle 

 third or more of the basis of the cerebral peduncle belong to the motor 

 cerebro- spinal fasciculus. In the internal capsule these fibres occupy the 

 anterior two- thirds of the posterior limb, being thus placed immediately 

 posterior to the genu. The fibres which constitute the medial third of the 

 basis pedunculi come from the anterior limb of the internal capsule ; whilst 

 the fibres which form the lateral third of the basis pedunculi are situated 

 in the posterior limb. 



When the fibres of the internal capsule are traced upwards, they are 

 found to spread out in a radiating manner so as to reach the various 

 gyri of the cerebral hemispheres. This arrangement is termed the 

 corona radiata. As the fibres . of the corona radiata are liberated from 

 the internal capsule, and spread out to reach their destinations, they are 

 intersected by fibres of the corpus callosum, which also radiate in 

 every direction to gain the cortex of the cerebrum, and they are inter- 

 sected also by bundles of association fibres. 



Nuclei of the Thalamus. When a horizontal section 

 is made through the thalamus in a fresh brain, or in one 

 which has been preserved by means of formalin or a chromic 

 salt, the grey matter composing it is seen to be broken up 

 into a lateral, a medial, and an anterior nucleus by thin 

 white medullary laminae. 



Capsula Externa. This term is applied to the thin 

 stratum of white matter which intervenes between the 

 lentiform nucleus and the claustrum (Figs. 213 and 216). 



Dissection. The fasciculus thalamo-mamillaris and the anterior com- 

 missure should now be followed, as far as this is possible, in what remains 

 of the right half of the cerebrum. The dissection is not difficult. 



By the removal of the remains of the lentiform nucleus the anterior com- 

 missure will be exposed in its course towards the temporal lobe. In the first 

 instance, it passes transversely, then postero-laterally below the putamen. 

 Next it bends suddenly in a posterior direction above the inferior horn of 

 the lateral ventricle to reach the medullary centre of the temporal lobe. 

 If the dissection be successfully accomplished, the anterior commissure 

 will be seen to present a twisted or rope-like appearance. 



The fasciculus thalamo-mamillaris may be traced from the corpus 

 mamillare upwards into the anterior nucleus of the optic thalamus by 

 scraping away the grey matter on the side of the third ventricle. The 

 continuity between the column of the fornix and the corpus mamillare 

 should at the same time be established. Within the corpus mamillare there 

 is a nucleus of grey matter. 



The dissectors have now examined all those portions of the brain which 



lie in the anterior and middle cranial fossce, and certain terms, not hitherto 



mentioned in connection with the various structures which have been 



studied, now require consideration. It has been noted already that the 



II 33 



