IX 



MID- AND INTER-BEATN 



491 



At the ventral part of the posterior end of the optic thalamus 

 are two oval prominences, the corpora geniculata. The corpus 

 geiiiculatuin internum is the smaller ; it is connected with the 

 posterior quadrigeminal body hy a bundle of medullated fibres 

 known as the brachium posticum. The corpus geuiculatum 

 externum or laterale lies directly below the pulvinar, and is con- 

 siderably larger; it receives the external root of the optic tract, 

 and is united to the anterior quadrigeminal body by a bundle of 

 medullated fibres known as 

 the brachium antieum. 



A frontal section of 

 the optic thalamus shows 

 that the grey mass of 

 which it is composed is 

 divided into three distinct 

 nuclei by a medullary 

 layer : an internal nucleus, 

 lying between this layer 

 and the third ventricle ; an 

 external nucleus between 

 the internal nucleus and 

 the so-called internal cap- 

 sule ; and an anterior or 

 superior nucleus which 

 corresponds to the anterior 

 tubercle of the thalamus 

 (Fig. 247). 



A horizontal section 

 through the thalamus 

 shows the same three 

 nuclei (internal, external, 

 anterior) under another 

 aspect (Fig. 247). 



The optic thalamus at 

 its lower and external surface is in direct relation with the 

 bundles of fibres coming from the upper end of the peduncle. 

 These are the fibres of the superior cerebellar peduncle; fibres 

 which arise from the cells of the red nucleus ; the fibres of the 

 dorsal longitudinal bundles ; and part of the fibres of the median 

 lemniscus or fillet of Reil. 



Other fibres connect the optic thalamus with the nuclei of the 

 corpus striatum. These take origin partly in the caudate nucleus, 

 partly in the lenticular nucleus. They cross the genu and posterior 

 segment of the interior capsule, and penetrate the lateral border of 

 the thalamus. Larger bundles issue from the ventral surface of 

 the lenticular nucleus, and enter the ventral surface of the thalamus. 

 The fibres that unite the thalamus to the cerebral cortex spring 



FIG. 246. Mesencephalon and its relations. (Testut.) 

 1, third ventricle ; 2, epiphysis or pini'al gland ; 3, 

 trigonum habenulae ; 4, posterior end of optic thalamus ; 

 5, external ; 6, internal geniculate bodies ; 7, optic 

 tract with its two roots ; 8, anterior ; 9, posterior 

 corpora quadrigemina ; 10, anterior ; 11, posterior 

 brachium of corpora quadrigemina ; 12, pons ; 13, 

 valve of Vieussens ; 14, superior cerebellar peduncles ; 

 15, trochlear nerve ; 16, lateral bundle of isthmus 

 cerebri ; 17, fourth ventricle ; 18, middle cerebellar 

 peduncles ; 19, inferior cerebellar peduncles. 



