THE THALAMUS 139 



tiles (Huber and Crosby, 1926) these fibers connect 

 for the most part the medioventral nuclei of the 

 thalamus with the hypopallium; whether any of them 

 reach the cerebral cortex directly is uncertain. 



3. The mammalian stage. — In lower mammals the 

 elaboration of thalamic nuclei is further advanced 

 and extensive connections are made directly with the 

 cerebral cortex, chiefly from the lateral nucleus and 

 lateral and medial geniculate bodies (for somesthetic 

 sensibility, vision and hearing, respectively). Cor- 

 tical localization of "projection centers" associated 

 with these three great systems of projection fibers is 

 more or less clearly defined. 



4. The human stage. — In man the thalamus is tre- 

 mendously enlarged and complicated and the cortical 

 connections are more extensive than in other mam- 

 mals. The lateral nuclei and pulvinar are elaborated 

 parallel with enlargement of association fields of the 

 cortex and the greater differentiation of the supra- 

 granular layers of the cortex of Bolton (p. 91). Thus 

 the thalamus, as well as the cortex, is seen to reflect in 

 its structure the change in behavior patterns as we 

 pass from brute to man. 



In reviewing these stages, it is to be remembered 

 that the thalamus is older than^the cerebral cortex, 

 though not older than the primordia out of which 

 cortex has been differentiated. The simplest known 

 type of thalamus is a very slightly modified sector of 

 the primitive neural tube in which the bodies of the 



