THE CEREBRAL HEMISPHERES 3G9 



Iv, to end with wide arborizations in the granular layer of the bulb. 

 Most of the smaller cells have short, much branched axons, which 

 participate in the formation of the dense axonic neuropil of this 

 region. 



Olfactory tracts. — Strictly defined, a tractus olfactorius includes 

 axons of olfactory neurons of the second order only, that is, axons of 

 mitral cells ; but practically all these fibers are mingled with those of 

 higher order from the anterior olfactory nucleus and other parts of 

 the hemisphere, so that the tracts so designated on the figures are all 

 mixed })undles. These axons of mitral cells stream backward from all 

 margins of the olfactory bulb. Only the shorter fibers to the anterior 

 nucleus are drawn in figures 111 and 112. As shown in figure 6, these 

 are accompanied by longer fibers, which join the tracts descending 

 from the anterior nucleus. Olfactory tracts from the lateral and ven- 

 tral borders of the bulb take direct courses backward in three series. 

 The more dorsal fibers enter tr. olfactorius dorsolateralis for distribu- 

 tion to the dorsolateral olfactory nucleus, which is primordium piri- 

 forme. This is the largest of the olfactory tracts and is comparable 

 with the lateral olfactory stria of mammals. Other lateral fibers pass 

 to the corpus striatum and amygdala. Some of these fibers join tr. 

 olfacto-peduncularis, most of the fibers of which arise in the anterior 

 nucleus and primordial caudate nucleus. Fibers from the ventral 

 border of the bulb enter tr. olfactorius ventralis and descend for an 

 undetermined distance in the medial forebrain bundle. 



As shown by figure 4, the lateral ventricle extends forward almost 

 to the anterior end of the olfactory bulb. Many of the longer fibers 

 from the bulb take tortuous courses to reach their terminal stations. 

 They accumulate in the medial sector of the anterior olfactory nu- 

 cleus and primordium hippocampi, where they form a very large 

 compact sheet of fibers termed "fasciculus postolfactorius" (fig. 100, 

 f.po.; '27, figs. 2, 3). These fibers run vertically around the tip of the 

 lateral ventricle, some directed ventrally to enter tr. olfactorius 

 ventralis and some dorsally to enter tr. olfactorius dorsolateralis 

 (fig. 5). 



Olfactory tracts of the third and higher orders, i.e., those sepa- 

 rated by two or more synapses from the periphery, are generally 

 designated by hyphenated compound words, as tr. olfacto-peduncu- 

 laris; but, as mentioned above, many of these tracts are mixtures con- 

 taining some axons of mitral cells. For further details of these con- 

 nections see the summaries ('33&, p. 124; '27, p. 282). 



