MORPHOLOGY OF THE FOKEBRAIN 487 



to the secondary olfactory nucleus, are partially isolated physio- 

 logically by the interruption of the ancient path of longitudinal 

 correlation. Ascending fibers have grown forward in the mean 

 time from the hypothalamus into the ventro-median part of the 

 hemisphere and from the thalamus into the ventro-lateral part, 

 and these parts became important correlation stations between 

 the olfactory bulb and the hypothalamus and thalamus respec- 

 tively. 



The overlying dorsal parts are profoundly modified by the change 

 in the character of these ventral parts. They receive secondary 

 olfactory fibers from the olfactory bulb in front, but are not 

 directly connected with the great pathways of efferent discharge. 

 They are therefore unfavorably located to serve as organs of the 

 direct and stereotyped olfactory reflexes, but by that very fact 

 are in a favorable position to serve those reactions which involve 

 more extensive correlation and more deliberate response. 



The function of the amphibian dorso-lateral part, as of the 

 pyriform lobe of mammals, is evidently the correlation of olfactory 

 with other exteroceptive impressions belonging to the somatic 

 sensory systems. In this con plex the olfactory element clearly 

 predominates in the frog and the type of organization is really 

 no higher than that of the ventro-median part in which the olf acto- 

 hypothalanic (and presun ably visceral) reflex systems predomi- 

 nate. 



The dorso-median part receives a smaller number of direct 

 olfactory fibers. Its other afferent elements come from two sources, 

 partly from the nucleus n.edianus septi and partly as association 

 fibers from the dorso-lateral part. The rostral end develops under 

 the influence of the olfactory and septal factors chiefly, the caudal 

 end under the lateral influence (olfacto-somatic), and the com- 

 missura pallii anterior effects a thorough coordination of these 

 elements, its fibers arising throughout this part and spreading 

 through the whole of the opposite dorso-median and dorso-lateral 

 parts. Efferent fibers leave the medial edge of the dorso-median 

 part for the hypothalamus, and its posterior end for the epithal- 

 amus. They leave its lateral edge for the thalamus by way of 

 the lateral forebrain tract. 



