THE CEREBRAL HEMISPHERES 207 



Nervns terminalis. — These unmyelinated fibers enter the brain in 

 small compact fascicles mingled with those of the olfactory nerve. 

 Their peripheral and central courses can be accurately followed only 

 in elective Golgi impregnations, which, fortunately, are frequently 

 obtained. All fibers of the olfactory nerve end in the olfactory bulb, 

 but none of the terminalis fibers do so. The latter enter the brain at 

 the ventral border of the olfactory bulb and course backward in sev- 

 eral small fascicles, which terminate in the septum, preoptic nucleus, 

 and hypothalamus. In Necturus some of them reach the interpedun- 

 cular nucleus, and this may be true in Amblystoma also. This nerve 

 is present in vertebrates generally, from fishes to man, but our knowl- 

 edge is incomplete about its terminal connections and functions 

 (McKibben, '11; for Necturus see my '336, p. 120, and '346, '34c; for 

 the frog, '09). It is regarded here as a sensory nerve, but even this is a 

 debatable question. 



Olfactory bulb. — In the olfactory bulb, as in the retina, the periph- 

 eral receptors discharge into a field which receives few afferent fibers 

 of other functional systems. In the other primary sensory centers 

 there is a common pool of neuropil within which terminals of periph- 

 eral fibers of diverse sensory modality are mingled, and to these there 

 are added terminals of other correlating fibers of central origin. The 

 bulbar formation of Amblystoma receives an enormous number of 

 fibers of the olfactory nerve and no others from the periphery. There 

 are also terminals of fibers ascending from other parts of the brain: 



(1) many of these are collaterals from the secondary olfactory tracts; 



(2) some are axons of cells of the anterior olfactory nucleus; (3) some 

 may be commissural fibers by way of the anterior commissure; (4) 

 some may come from more remote parts of the hemisphere. By far 

 the larger number of these ascending fibers belong to the first two 

 classes, in which olfactory influence is clearly dominant. From this 

 it follows that the impulses conducted by the secondary olfactory 

 tracts are influenced relatively little by other functional systems. In 

 this respect they differ from the lemniscus systems of the lower brain 

 stem; and the fact that these almost purely olfactory tracts reach all 

 parts of the cerebral hemisphere is probably the reason why this 

 hemisphere remains at a low level of structural differentiation and 

 physiological specificity. 



Necturus and Amblystoma exhibit two well-defined stages in the 

 histological differentiation of the olfactory bulb, but the mammalian 

 type of structure has not been attained ('246, '31). Throughout the 



