DiEXCEPHALON 79 



frontal extremity of the subthalamic nucleus as a similar 

 looking region known as the regio innomifiata, in which run 

 a large number of thalamic fibres to and from the cerebral 

 hemispheres. 



Perhaps to be included with the subthalamus (Herrick '18), 

 is the nucleus reticularis thalami (formatio reticulata, lattice 

 nucleus, radiate nucleus), which occurs between the zona 

 incerta and the overlying white fibres (ventral medullary 

 lamina) in the more anterior part of the extent of the former. 

 It is continuous with a more lateral portion in the lateral part 

 of the thalamus and anteriorly extends between the nucleus 

 anterior and the fibres of the internal capsule (Pis. XIX., XX.). 

 Its names are derived from the many large bundles of cortico- 

 thalamic and thalamo-cortical fibres which run through it. 



The dorsal portion of the thalamus is very large as com- 

 pared with the parts of the diencephalon hitherto considered, 

 this disproportion being even much greater in man than in 

 the lower mammals. The great increase in size is correlated 

 with the development of the cerebral cortex, the increase 

 being chiefly in those parts which serve as relay stations on 

 the sensory paths to the hemispheres — and every such path 

 is interrupted by a synapse in the thalamus. 



One of the first parts of the thalamus to appear as one 

 passes forward in serial sections is the medial or internal 

 geniculate body (cor pics geniculatum mediate), which forms a 

 marked eminence on the lateral surface of the brain near the 

 frontal end of the anterior colliculi (Pis. III., XV. -XML). 

 The nucleus is surrounded and penetrated by many white 

 fibres, largely derived from the brachium of the inferior 

 coUiculus, which ends in this centre. Thus the body receives 

 auditory impulses, and these it transmits along its axons to 

 the auditory area of the cerebral cortex. Not all its axons 



these fibres are not very distinct in the writer's preparations of the rat 

 brain, they appear to be present between the levels of Plates XIV and 

 XV. 



