I40 BRAINS OF RATS AND MEN 



nerve cells are crowded in a central gray layer next to 

 the ventricle with dendrites and axons extending out- 

 ward into a superficial white layer where the synaptic 

 junctions are made. Into this nucleus diffusus thai- 

 ami there enter a few optic fibers directly from the 

 retina and a larger number of fibers from the mid- 

 brain roof carrying somesthetic, optic, and perhaps 

 auditory influences. The mesencephalic fibers come 

 from the most elaborately developed correlation cen- 

 ter of these primitive brains, and the nervous im- 

 pulses which they transmit may be the resultants of 

 the interaction of all forms of external stimulation 

 to which the body is subjected (except the olfactory, 

 and these may enter this field by another pathway). 



In the Amphibia a few fibers enter the thalamus 

 directly from the primary optic, tactile, and acoustic 

 centers, but most of them come from the midbrain 

 roof. In the lower species of this group all of these 

 fibers converge upon the same neurons and there is no 

 precise localization of thalamic centers for the various 

 sensory functions represented in the incoming fibers. 

 This can only mean that the thalamus here acts as a 

 whole, equipotentially, in exerting some sort of an 

 influence upon processes already going on in the lower 

 centers from which it is activated and into which its 

 own efferent fibers discharge. 



Even in the salamanders, however, we can recog- 

 nize the beginnings of the process of sorting out of the 

 incoming fibers, with a tendency for those of certain 



