II74 



HUMAN ANATOMY. 



IOI2. 



Seen in frontal sections passing some distance behind the preceding section, 

 whilst the capsule is limited laterally by the lenticular nucleus, its mesial boundary 

 now includes the caudate nucleus, the taenia semicircularis and the thalamus. Still 

 farther back (Fig. 968), the internal capsule is bounded internally in addition by 

 the subthalamic structures and becomes continuous below with the crusta of the cere- 

 bral peduncle. An upper and a lower part of the capsule are therefore recognized, 

 the former — between the lenticular nucleus on the one side, and the caudate nucleus 

 on the other — is known as the thalamic regioyi (regio thalamica capsiilae internae), 

 whilst that between the lenticular nucleus and the subthalamic structures is termed 

 the subthalamic region (regio subthalamica). , 



Viewed in horizontal sections (Fig. loii. A), the capsule appears not only 

 much more extensive, but is seen to consist of two mesially converging parts, a 

 shorter anterior limb (pars frontalis) and a longer posterior limb (pars occipitalis). 

 The two limbs form an angle which opens outward and encloses on two sides the 

 gray triangle of the lenticular nucleus. The junction of the two mesially converging 

 limbs forms the knee, or genu, of the internal capsule which points inward and lies 

 opposite the taenia semicircularis, between the caudate nucleus and the thalamus. 

 At deeper planes (Fig. loii, t?), passing through the level of the continuity 

 between the two parts of the corpus striatum, the anterior limb is greatly reduced 

 in length or entirely disappears, the posterior one being prolonged into the cerebral 

 peduncle. 



The importance of the internal capsule will be appreciated when its function as 

 the great pathway connecting the cerebral cortex with the lower lying centres is 

 recalled. Its fibres, both corticipetal and corticifugal, after passing beyond, or before 



coming under the restraint of the boundaries of the 

 capsule, as the case may be, radiate to and from all 

 parts of the hemisphere, and in this manner form the 

 striking fan-shaped fibre-mass known as the corona 

 radiata, which continues the internal capsule upward 

 to the cerebral cortex. The radiating strands of this 

 great tract interlace with the radiation of the corpus 

 callosum and thereby contribute a large part of the 

 fibres composing the oval centre of white matter within 

 the hemisphere. 



The anterior limb of the internal capsule (pars lenticulocau- 

 data) includes the front third of the tract and extends from the 

 genu forward and outward. It contains fibres passing both 

 toward and away from the cortex. Its corticipetal fibres are : 



( 1 ) the thalamo-frontal, which pass from the thalamus by way 

 of its frontal stalk through the anterior limb of the internal cap- 

 sule and the corona radiata to the cortex of the frontal lobe ; 



( 2 ) the ihalanio-striate, which also pass from the thalamus into 

 the internal capsule and proceed to the caudate and lenticular 

 nuclei. The corticifugal fibres include : {i) th^.fronio-pontile, 

 which arise in the cortex of the frontal lobe and descend by 

 way of the corona radiata, the anterior limb of the internal 

 capsule, the crusta of the cerebral peduncle and the ventral 

 tracts of the pons to end around the cells of the pontile nucleus 

 as links in the connection between the cerebral and the cere- 

 bellar cortex (page 1094) ; (2) the fronto-thala^nic, which 

 extend from the cortex of the frontal lobe to the thalamus ; 

 and (3) the strio-thalavtic, which proceed from the caudate and 

 lenticular nuclei to the thalamus. 



The posterior limb of the internal capsule (pars lenticulo- 

 thalamica) extends backward, outward and downward from 

 the genu, and includes the remaining two-thirds of the tract. Its hind part extends beyond 

 the posterior limit of the lenticular nucleus, hence the posterior limb is subdivided into a 

 lenticular and a retrolenticular portioti. As does the anterior limb, so also does the posterior 

 limb of the capsule contain both corticipetal and corticifugal fibres. 



The lenticular portion includes corticipetal fibres: ( i ) the thalaino-cortical, which issue from 

 the lateral and lower aspect of the thalamus, traverse the int(5rnal capsule and to a considerable 



Diagram showing relative posi- 

 tions of chief tracts in internal cap- 

 sule (A) and in crusta of cerebral 

 peduncle {B) ; F-T, fronto-thala- 

 mic; F-P, fror.to-pontile; T-O-P, 

 tetnporo-occipito-pontile ; C-B, cor- 

 tico-bulbar ; C-Sy cortico-spinal ; 5, 

 tegmental sensory ; OR, optic ra- 

 diation. 



