134 BRAINS OF RATS AND MEN 



similar to those of the midbrain roof; but there are 

 significant differences. It receives some optic fibers 

 directly from the retina, but these are relatively few in 

 number. Most of its afferent fibers come from the 

 midbrain roof and the sensory centers of the medulla 

 oblongata and the spinal cord, regions in which the 

 sensory impressions discharged directly from the pe- 

 riphery are locally subjected to a certain measure of 

 correlative modification or reflex control through in- 

 terconnecting fibers coming from other primary sen- 

 sory centers. In other words, the afferent fibers reach- 

 ing the thalamus (except the small optic tracts) do not 

 transmit crude sensory data directly from peripheral 

 sense organs but more or less fully elaborated reflex 

 patterns in which several simultaneous or successive 

 excitations of diverse sorts are represented. Like the 

 midbrain roof, the thalamus is concerned chiefly with 

 excitations coming from outside the body (exterocep- 

 tive in Sherrington's sense) and with the adjustments 

 of the body to these external conditions. It also re- 

 ceives a generous contribution of fibers from the pro- 

 prioceptive centers, especially those concerned with 

 muscle sense and allied sensory systems of motor con- 

 trol. 



The thalamus is a true correlation center in the 

 sense that it is not dominated by any single system of 

 sensory excitations. In none of these lower verte- 

 brates is it very large or very complex in mternal 

 structure. In terms of the physiological conceptions 



