THE NERVOUS SYSTEM 



capsule may be divided into a frontal, a parietal and an occipital part, corres- 

 ponding to the frontal, parietal and occipital peduncles of the thalamus, or to the 

 parts of the internal capsule. 



The radiation derived from the posterior segment of the occipital part of the 

 internal capsule, the visual path, accumulates into a well-defined band of fibres 

 which passes posteriorly into the occipital lobe, spreading in the lateral wall of the 

 posterior cornu of the lateral ventricle immediately lateral to the tapetum. This 

 band consists for the most part of fibres arising in the pulvinar of the thalamus and 



Fig. 700.- 



-DiAGRAM TO Indicate the Relative positions of the Projection Fibres in 

 THE Internal Capsule. (In part after Villiger.) 



stria terminalis of 

 thalamus 



■^ Fronto -thala- 

 mic path 



Fronto-pon- 

 tile path 



— — Cortico-medul- 

 lar path (Genu) 



- — Cortico-spinal 

 path (arm) 



■~ — Cortico-spinal 

 path (leg) 



^- General sen- 

 sory path 



^ ,. Temporal 

 and occipital 

 pontile path 



'^ Visual and 

 auditory path 



Frontal 

 part 



Fronto- 

 parietal 

 part 



Occipital 

 part 



in the lateral geniculate body and going to the visual area of the occipital cortex, 

 and of fibres arising in this cortex to terminate in the thalamus and mesenceph- 

 alon. Being thus concerned with the optic apparatus, it is known as the occipito- 

 thalamic radiation or optic radiation (fig. 699). 



The external capsule is, as ah-eady noted, a thin sheet of white substance 

 spread between the clau.strum and the lenticular nucleus. 



It owes its appoaranoo as sucli to tlu; i)rosonco of the claustrum. It joins the internal cap- 

 sule at the upper, po.stc^rior, and ant(!rior Ijorders of tl>e putanien, and below the claustrum it 

 is continuous with the general white .sub.stance of tli(! temporal lobe. Thus it contributes to 

 an encapsulation of the lenticular nucleus by white substance. Most of the fibres contained 

 in it belong to the as.sociation system. Its projection fibres consist of those of the inferior 

 peduncle of the thalamus, which pass from the basal surface of the thalamus and, instead of 

 continuing below to the cortex of the temporal lobe and insula, turn upward, around the lenticu- 

 lar nucleus to the cortex above the insula. .Some of these thalamus fibres are known to pass 

 upward through the lamina; of the lenticular nucleus instead of through the external capsule. 



