1446 A MANUAL OF ANATOMY 



anterior limb of the internal capsule, and most of them pass to the 

 cortex of the frontal lobe. The fibres of the parietal stalk pass partly 

 through the internal capsule and partly through the external cap- 

 sule to the cortex of the parietal lobe and of the Rolandic region of 

 the frontal lobe. The fibres of the occipital stalk pertain to the 

 optic radiation, to be presently described. The fibres of the in- 

 ferior or ventral stalk form the ansa lenticularis and ansa peduncu- 

 laris. The ansa lenticularis enters the nucleus lenticularis, within 

 which its fibres terminate. The ansa peduncularis passes beneath 

 the nucleus lenticularis and traverses the external capsule, the 

 destination of its fibres being the cortex of the temporal lobe and 

 island of Reil. 



The auditory radiation consists of fibres which arise as the axons 

 of the cells of the internal geniculate body. Having issued from 

 that body they traverse the post-lenticular part of the posterior 

 limb of the internal capsule, and pass to the cortex of the middle 

 part of the first temporal gyrus of the temporal lobe. 



The corticipetal fibres of the optic radiation are associated with 

 the corticifugal fibres, already described. The corticipetal fibres 

 arise as the axons of the cells of the pulvinar, corpus geniculatum 

 externum, and upper quadrigeminal body. They traverse the 

 post-lenticular part of the posterior limb of the internal capsule, 

 and thereafter pass to the cortex of the occipital lobe. 



Commissural Fibres. — ^These fibres are disposed transversely, 

 and serve to connect the grey cortex of one hemisphere with 

 that of the other. They constitute the following commissures : 

 (i) the corpus callosum ; (2) the anterior commissure ; and (3) the 

 lyra or psalterium, which is known as the hippocampal commissure. 

 The fibres of the corpus callosum, as they enter each hemisphere, 

 are disposed so as to form an extensive callosal radiation, and 

 serve to connect the cortex of one hemisphere with that of the 

 other. The individual portions of cortex so connected may be 

 symmetrical, but to a large extent they are asymmetrical. The 

 fibres arise on one side as (i) the axons of pyramidal or of poly- 

 morphous cells, or (2) collaterals of projection or of association 

 fibres ; and on the opposite side they tenninate in delicate arboriza- 

 tions. 



The anterior commissure, which crosses from side to side in front 

 of the anterior pillars of the fornix, divides on either side into two 

 parts, olfactory and temporal. The olfactory portion enters the 

 olfactory tract. Some of its fibres serve to connect the olfactory 

 bulb of one side with that of the other side ; and other fibres con- 

 nect the olfactory bulb of one side with the temporal lobe of the 

 opposite. The temporal portion enters the white matter of the 

 temporal lobe on either side. 



The lyra or psalterium consists of transverse fibres which con- 

 nect the hippocampus major of one side with its fellow of the 

 opposite side ; hence the name hippocampal commissure. 



Association Fibres. — These fibres serve to connect different parts 



