ORIGIN OF THE CEREBRAL HEMISPHERES 449 



in the olfactory area and terminating either in various higher 

 correlation centers or in reflex centers of the betweenbrain and 

 midbrain. The tracts of the third order are usually named by 

 hyphenated compound words, of which the second member 

 designates the place of termination, such as tractus olfacto- 

 habenularis, tractus olfacto-corticahs, etc. 



It is probable that primitively the olfactory area was an undif- 

 ferentiated secondary olfactory nucleus extending backward 

 from the base of the olfactory bulb and recei\dng various ascend- 

 ing non-olfactory systems from the betweenbrain. A remnant of 

 this primitive undifTerentiated nucleus persists in all vertebrates 

 at the base of the olfactory bulb, in most cases extending outward 

 more or less into the bulb, and is called the anterior olfactory 

 nucleus. The remainder of the olfactory area has been differ- 

 entiated in conformity with the specificity of the various ascend- 

 ing systems of fibers into the several olfactory nuclei, each of 

 which is concerned with a particular complex of correlations 

 between olfactory nervous impulses, on one hand, and certain 

 non-olfactory systems, on the other hand, thus giving rise to the 

 lateral, medial, and intermediate olfactory nuclei and their 

 subdivisions. 



The regional differentiation of the anatomically distinct centers 

 of the entire endbrain behind the olfactory bulbs, therefore, 

 primitively arose as a result of the invasion of the original second- 

 ary olfactory area by diverse non-olfactory systems, and the 

 entire history of the subsequent evolutionary differentiation of 

 this part of the brain can be written in terms of the interaction 

 of these two systems of conduction fibers — those descending 

 from the olfactory bulb and those ascending from the between- 

 brain. Increasingly complex correlations between the olfactory 

 centers in front and the non-olfactory centers of the midbrain 

 led to the forward growth of tracts from the lower reflex centers 

 of touch, taste, vision, hearing, etc., into the olfactory territory 

 of the endbrain (and probably in still earlier stages the ascending 

 tracts into the betweenbrain were led forward by the same 

 motive). 



