THE THALAMUS 137 



the overlying cortex when this latter is present. Par- 

 allel with the four types of cortical differentiation (p. 

 70) we may outline four stages of thalamic evolution, 

 as follows: 



1. The ichthyopsid stage. — ^The thalami of fishes 

 are simply organized, and in many cases they have 

 differentiated in aberrant directions. In the lower 

 Amphibia (as in Necturus and Amblystoma, Herrick, 

 1 91 7, igi^a) the thalamus is seen in diagrammatic 

 simplicity (Fig. 30). The bodies of the neurons are in 

 the primitive position as central gray matter border- 

 ing the ventricle (shown as coarse stipple in the draw- 

 ing). Within the dorsal part of the thalamus there are 

 no differentiated cell clusters or "nuclei," and the 

 whole of this region may be termed "nucleus diffusus 

 thalami" (the nucleus sensitivus of Cajal, or general 

 sensory correlation center). The ventral thalamus 

 (subthalamus) is chiefly efferent in function, dis- 

 charging downward toward the motor centers. Most 

 of the functions of this thalamus are local reflexes in- 

 trinsic to the thalamus itself, but there are small wisps 

 of fibers passing from the dorsal part of the thalamus 

 to the lateral wall of the cerebral hemisphere — the 

 beginnings of the sensory radiations or projection 

 fibers. The preoptic nucleus of Figure 30 is continu- 

 ous behind with the hypothalamus and may be re- 

 garded physiologically as part of it. 



2. The sauropsid stage. — In the frog the nucleus 

 diffusus thalami has begun to break up into separate 



