THE BEAIN OF MAMMALS AND THE OLFACTORY APPARATUS. 315 



intended to simplify the comparison. The entire formation of the lulhus is 

 there designated as the cortex of the lohus. 



The gray mass of the bulbus sends backward the central secondary and 

 tertiary pathways of the olfactory apparatus. First of all, such a pathway 

 extends constantly to the surface of the lobus, where, now splitting up into 

 several, now into fewer, strands, it passes backward. In so doing small 

 fibers continually pass down from this lateral olfactory radiation deep into 

 the cortex of the lobus. Yet the tract is not exhausted by this; on the con- 

 trary, its fibers, passing over the olfactory area, extend farther backward 

 until in the region of the nucleus amygdalae. All of these fibers have a 

 large diameter, and have been known for a long time as the roots of the 

 olfactory nerve. 



In the description of the lower vertebrate brain they have been more closely 

 followed, and we have been able to confirm the fact that a part certainly passes 

 into the cortex of the olfactory lobe, just as in the mammals above, and that, in 

 addition to these tractus bulbo-corticales, a tract of fibers is present, which, running 

 in a similar course, soon separates and ends in the epistriatum. This epistriatum has 

 not yet been found in mammals, because only now are we in a position to seek for 

 it, seeing that its existence has been recognized so distinctly in lower brains. Never- 

 theless, it is very probable that good guides to that still-undiscovered portion of the 

 brain are possessed in those tracts of fibers that extend into the more posterior re- 

 gion: into the lobus pyriformis and into the neighborhood of the nucleus amygdalae. 



Prom the olfactory radiations from the biilbus there must be dis- 

 tinguished a tract which hitherto has been regarded as part of it, and called 

 the mesial root of the olfactory nerve. There develop, namely, from the 

 medulla of the hulbus, numerous finer nerve-fibers, which pass away beneath 

 the cortex into the medulla of the lobus. In it they mix with the medullated 

 fibers of the lobus in such a manner that they are not separable at present. 

 At the posterior end of the lobus, directly in front of the olfactory field, 

 however, a tract, which lies in its continuation, leaves the lobus and passes 

 beneath the thin cortex of the olfactory lobe upon the inner surface of the 

 brain. The cortex is raised up somewhat by this tract. This mesial olfactory 

 radiation, passing on the inner side of the brain to the septum pellucidum, 

 extends over this into the fornix, and from there into the cornu Ammonis. 

 It is always less white than the lateral radiation, on account of the thin cor- 

 tical covering. 



A fissure separates the lobus olfactorius posterior from the lolus hippo- 

 campi. This structure, always uncommonly large in the osmatic animals, 

 contains the inroUing of the cornu Ammonis at its mesial edge. It is hardly 

 to be compared with the small, relatively atrophied convolution of the cornu 

 Ammonis, the gyrvis hippocampi, in man. At the base of the brain the 

 gyrus hippocampi follows the entire edge of the hemisphere, passes up pos- 



