Rhixen'cephalox 97 



From the secondary olfactory areas, tertiary fibres run 

 to the habenula (stria meduUaris thalami) and to the hypo- 

 thalamus {iractus oljacto-hypothalamictis, basal olfactory tract, 

 basal olfactory projection tract (Pis. X\1I.-XX.), while others 

 descend to the lower parts of the brain. All these tracts 

 together form the large, more or less diffuse medial forebrain 

 bundle (Pis. XX., XXL). The farther pathways from the 

 lower centres have been considered in the previous chapter. 

 A great many of the tertiary olfactory fibres, however, belong 

 to the olfacto-cortical group, which ends in the hippocampus^ 

 (vide infra). 



The olfactory centres of the two sides are, moreover, 

 connected by the anterior commissure {commissura anterior 

 cerebri), a large structure crossing between the hemispheres 

 in the anterior wall of the telencephalon medium (lamina 

 terminalis). The bundle consists of two parts which are 

 quite distinct, the more anterior being composed of large 

 fibres deeply stained by the Weigert method, while the 

 remainder is made up of smaller, less heavily myelinated 

 fibres which take a paler stain. The anterior part is generally 

 held to be made up largely of secondary olfactory fibres con- 

 necting the two olfactory bulbs, while the posterior or temporal 

 portion is chiefly a commissure between the two pyriform 

 lobes.2 As might be expected, the anterior portion, which 

 is large in the rat and other macrosmatic animals, is very 

 much smaller in the human brain, where also the commissure 



^Kappers differentiates the cortex of the olfactory lobe (cortex 

 piriformis and praepiriformis), which receives secondary olfactory fibres 

 as palaeocortex, and terms the hippocampus, which receives olfactory 

 fibres only of the third and higher orders, archicortex. The latter term is 

 frequently applied by other writers to the whole of the olfactory cortex. 



The olfacto-cortical fibres from the tuberculum olfactorium (p. 94) 

 reach the hippocampus (and indusium) through the fimbria, the fornix 

 superior, and the striae Lancisii. 



2A narrow zone of fibres on the lateral aspect of the bulbar portion 

 of the commissure connects the anterior parts of the lateral olfactory areas 



