CORTEX AND CORPUS STRIATUM 109 



dominate, though the primitive olfactory component is not en- 

 tirely lost. 



The history of the development of the corpus striatum com- 

 plex may be summarized as follows: 



1. In the lowest vertebrates there is a primordial striatum 

 dominated by thalamic connections and directly or indirectly 

 connected with adjacent olfactory nuclei. In many fishes and 

 lower Amphibia this primordial striatum is incompletely sepa- 

 rated from the olfactory nuclei. 



2. In the dogfish and frog there may be recognized within 

 the striatal field a somatic striatum (precursor of the lentiform 

 nucleus) with well-defined thalamic connections, and an olfacto- 

 strio-amygdaloid complex which is incompletely separated from 

 the lateral olfactory nucleus and primordial pyriform lobe. 



3. In reptiles the somatic striatum^ olfacto-striatum, primitive 

 amygdala^ and pyriform cortex are separately differentiated, and 

 there is added a new structure, the "dorsal ventricular ridge" of 

 Johnston, whose rostral end, the hypopallium^ is apparently re- 

 lated anatomically and physiologically with the overlying neo- 

 pallial cortical field and whose caudal end, the amygdaloid ridge^ 

 is similarly related with the pyriform cortex. 



4. In mammals the hypopallium is added to the olfacto- 

 striatum to form the caudate nucleus; the amygdaloid ridge is 

 incorporated into the amygdala. The lentiform nucleus is the 

 persistent somatic striatum somewhat further differentiated. 

 From reptiles onward the somatic striatum is clearly separated 

 into the two parts which together comprise the lentiform nucleus: 

 (i) an afferent part, the putamen^ and (2) an efferent part, the 

 globus pallidus. 



5. The globus pallidus is the common efferent center for a 

 large part of the striatum complex and its large motor cells can 

 be recognized as far down as the fishes, mingled with other cells 

 of the somatic striatum. The globus pallidus is primarily the 

 efferent nucleus of the somatic striatum, but by reason of the 



