DiENCEPHALON 85 



medial, trigeminal, and spinal lemnisci; the former^ chiefly 

 medially, the latter laterally, and between them the trigeminal 

 lemniscus, which goes mainly to the nuclei centralis and arcu- 

 atus. While the ventral nucleus as a whole gives rise to an 

 extensive cortical radiation, some of its constituent centres 

 seem to have few or no such fibres, and the mass is apparently 

 rather intimately related with the medial nucleus. 



The lateral nucleus, on the other hand, is overwhelmingly 

 cortical in its connections, and enlarges enormously as the 

 cerebral cortex becomes more extensive. It is thus much 

 smaller in the rat than in man, though the exact functional 

 significance of this difference is not clear. A considerable 

 part of the increased bulk in the higher form is represented 

 in the pulvinar,'- which is not distinguishable in the rodents. 



The lateral and ventral surfaces of the ventro-lateral 

 nucleus are covered by the lattice nucleus (nucleus reticularis 

 thalami) (Pis. XIX., XX.), which is much more extensive in 

 the rat than in man. In the latter it is marked off from the 

 lateral nucleus by white fibres, the external medullary lamina 

 {lamina medullaris externa), but this lamina is not definite 

 in the rat — chiefly owing to its being more largely developed 

 and scattered through the nucleus. 



A conspicuous object in the diencephalon is the crusta of 

 the cerebral peduncle (basis or pes pedunculi), which runs 



'In the rat, "the principal terminals course along and in the ventral 

 surface of the ventral and lateral nucleus of the thalamus, forming a 

 narrow, crescentic, lemniscal stratum. The degeneration of these terminals 

 is limited almost exclusively to this nucleus", (Papez.) 



2This is apparently a centre concerned largely with the correlation 

 of optic and cerebellar impulses, though it, no doubt, receives many others. 

 It has a largely developed connection with the cerebral cortex, particularly 

 with the area of the latter which seems to be largely concerned in spacial 

 recognition. 



The suggestion has been made that the great enlargement of the 

 lateral nucleus is related to the emancipation of the fore limb, which has 

 reached such importance in the primates, and especially in man. 



