864 THE NERVOUS SYSTEM 



tions, the fibres of the internal capsule are dispersed to the cortex in all directions. 

 The appearance known in coronal sections of the hemispheres as the corona radiata 

 is produced both by the upper continuations of the internal capsule and by the 

 radiations of the corpus callosum. The radiations derived from the internal capsule 

 may be divided into a frontal, parietal, and occipital part, corresponding to the 

 frontal, parietal, and occipital peduncles of the thalamus. 



The radiation derived from the occipital portion of the internal capsule is sub- 

 divisible into (1) an anterior part, passing between the lateral portion of the thala- 

 mus and the cortex directly above and the cortex of the temporal lobe, and (2) a 

 posterior part, passing from the region of the lenticular nucleus backwards into the 

 occipital lobe. The latter part accumulates into a well-defined band of fibres spread 

 in the lateral wall of the posterior cornu of the lateral ventricle immediately outside 

 the tapetum. This band consists, for the most part, of fibres arising in the pulvinar, 

 the lateral geniculate body, and the superior quadrigeminate body, and is known as 

 the occipito-thalamic radiation, or, more commonly, the optic radiation, since the 

 fibres of which it is composed are chiefly concerned in the optic apparatus. 



The external capsule is, as already noted, a thin sheet of white substance spread 

 between the claustrum and the lenticular nucleus. It owes its appearance as such 

 to the presence of the claustrum. It joins the internal capsule at the upper, pos- 

 terior, and anterior borders of the putamen, and below the claustrum it is continuous 

 with the general white substance of the temporal lobe. Thus it contributes to an 

 encapsulation of the lenticular nucleus by white substance. Most of the fibres con- 

 tained in it belong to the association system. Its projection fibres consist of those of 

 the inferior peduncle of the thalamus, which pass from the under surface of the thala- 

 mus and, instead of continuing below to the cortex of the temporal lobe and insula, 

 turn upwards, around the lenticular nucleus to the cortex above the insula. 



The ascending projection fibres arise mostly from the cells of the nuclei of the 

 thalamus and hypothalamic nucleus; some arise from nuclei in the mesencephalon 

 and cerebellum. They may be summarised as follows: 



(1) The terminal part of the general sensory pathway of the body. The portion of the 

 medial lemniscus which arises in the nuclei of the fasciculus gracilis and cuneatus 

 terminates in the hypothalamic nucleus and the ventral portion of the lateral nucleus 

 of the thalamus. The projection fibres given off by the latter nuclei pass chiefly 

 through the anterior part of the occipital portion of the internal capsule and radiate 

 to and terminate in the somscsthetic area of the cortex. Some few pass outside 

 around the lenticular nucleus, and ascend by way of the external capsule. 



(2) The terminal part of the general sensory pathway of the head and neck. The 

 nuclei of termination of the sensory portions of the cranial nerves of the rhomben- 

 cephalon (except the nuclei of the auditory nerve) give fibres which course upwards 

 in the medial lemniscus (fillet) and reticular substance of the same and (chiefly) the 

 opposite sides and terminate in the ventro-lateral portions of the thalamus and in the 

 hypothalamic nucleus. Thence arise projection fibres which ascend to the somEesthetic 

 area by practically the same route as those of the general sensory system for the body. 



(3) The terminal part of the auditory pathway. The ventral and dorsal nuclei of 

 termination of the cochlear nerve establish connections with the inferior quadri- 

 geminate body, the medial geniculate body, and the nucleus of the lateral lemniscus. 

 These nuclei send projection fibres through the anterior part of the occipital por- 

 tion of the internal capsule, and thence by the temporal portion of the corona radmtu 

 to the cortex of the superior temporal gyrus (auditory area) . Probably some of these 

 fibres pass by way of the inferior peduncle of the thalamus. Some of the fibres arising 

 in the nuclei of termination of the vestibular nerve convey impulses which reach 

 the somaesthetic area, but the origin of the terminal portion of this system is uncertain. 



(4) The terminal part of the visual pathway. The cells of the pulvinar, the lateral 

 geniculate body, and the nucleus of the anterior quadrigeminate body, all serving as 

 nuclei of termination of the optic tract, give off projection fibres which pass by way 

 of the occipital portion of the internal capsule and the occipito-thalamic radiation to 

 the cortex of the occipital lobe, chiefly the region about the posterior end of the 

 calcarine fissure the visual area. 



(5) The terminal ascending cerebellar pathway. The fibres of the brachium con- 

 junctivum, after decussating, terminate both in the red nucleus and in the lateral 

 nucleus of the thalamus. Some fibres from the red nucleus become projection fibres 

 direct, others terminate in the medial and anterior portion of the lateral nucleus of 



