1 8o C. Judson Herrick 



parts of the thalamus are under the influence of the olfactory system in- 

 directly by way of correlation tracts connected with the hypothalamus 

 and epithalamus. 



The ventral zone. — The portions of the telencephalon which are 

 directly contiguous with the three major divisions of the diencephalon 

 are profoundly influenced thereby. Under the influence of ascending 

 fibers froni the hypothalamus the ventral part of the telencephalon mé- 

 dium is differentiated on each side of the median plañe as nucleus preop- 

 ticus and arca olfactoria medialis. AU parts of this ventro-medial zone 

 are connected by ñbers running fore-and-aft in the medial forebrain bundle. 

 Its efferent fibers pass to both the hypothalamus and the epithalamus. 



The intermedíate zone. — At the caudal end of the telencephalon mé- 

 dium above the nucleus preopticus there is an intermedíate zone which is 

 directly continuous with the somatic sensory (optic and somesthetic) 

 región of the thalamus. These parts are connected by ascending and 

 descending somatic projection fibers which in the Amphibia are increased 

 in number and condensed, thus forming important components of the 

 lateral forenbrain bundle. The telencephalic part of this cell column is 

 the área somática. In fishes it does not receive any fibers of the tractus 

 olfactorius; but in elasmobranchs, Avhere its connections are best known, 

 it is connected with the región immediately in front (área olfactoria late- 

 ralis) by an olfacto-somatic correlation tract. 



The área olfactoria lateralis, área olfacto-somatica, área somática, and 

 somatic thalamic centers thus form a longitudinal cell-column whose dis- 

 tinctive physiological character is determined largely by the diencephalic 

 connections. This intermedíate zone, like the ventral zone, is dominated 

 by the olfactory connections in front and by the diencephalic connections 

 behind, with varying proportions of admixture of these two systems 

 between. The discharge path is partly into the ventral thalamus and 

 pedunculus cerebri (for somatic motor responses), and partly into the 

 epithalamus and hypothalamus for responses in which the olfactory com- 

 ponent of the complex predominates. 



In Acipenser, in teleosts and in urodele Amphibia there is within the 

 field of the área olfactoria lateralis a poorly differentiated área olfacto- 

 somatica which is reached by somatic projection fibers from the thalamus 

 and whose efferent neurons discharge chiefly into the ventral thalamus 

 and pedunculus cerebri. This seems to imply an invasión of territory 



