DiENCEPHALON 83 



this nucleus is considerably larger in the rat than in man. 

 It consists of two distinct cell masses — a smaller, dorsal, 

 large-celled group and a larger, ventral and lateral, small- 

 celled group. The anterior thalamic nucleus sends fibres to 

 the caudate nucleus (p. 105) and probably to the cerebral 

 cortex and also has connections with the medial thalamic 

 nucleus. It is probably of special importance in olfactory 

 exploration. 



The anterior thalamic nucleus in the human brain is 

 separated from the adjacent gray matter by a layer of white 

 fibres formed by the bifurcation of the dorsal edge of a vertical 

 plate of such fibres which separates the medial and lateral 

 nuclei. This layer of w^hite matter is the internal medullary 

 lamina {lamina medullars s interna thalami), a very distinct 

 object in sections of the human thalamus, but very much 

 less so in that of the rat, through most of which there is a 

 more or less diffuse scattering of white fibres. 



The medial nucleus {nucleus medialis thalami) (Pis. XVHI- 

 XX.) comprises several distinct cell groups and probably has 

 various functions in its different parts. It has both ascending 

 and descending connections with the corpus striatum and 

 probably with the cerebral cortex. This group and the 

 anterior nucleus are the older part of the thalamus phylo- 

 genetically and contain the intrinsic reflex mechanism of 

 that region of the brain. The medial nucleus probably 

 includes among its functions the more primitive and ele- 

 mentary forms of consciousness. 



This mass is separated from the third ventricle in man 

 by a thin layer of central gray matter which contains scattered 

 cells constituting the nucleus paramedianus. The latter 

 is probably a visceral — particularly vasomotor — reflex centre. 

 The enlargement of the thalamus has reduced the ventricle 

 to a narrow slit and has brought its two walls into contact 

 over part of their area in a great many cases, forming a con- 

 tinuous mass of gray matter across the median plane — the 



