Rhinencephalon 93 



nucleus and the endorhinal groove (Pis. XVIII., XIX.). This belongs with 

 the first group, but has been modified by the processes involved in the 

 formation of the newer centres. 



Medially, the tuberculum olfactorium (except a small part 

 anteriorly) joins the ventral edge of the septum, which forms 

 the medial wall of the anterior part of the lateral ventricle. 

 This structure appears as a fairly thick plate of gray matter 

 penetrated by a large number of medullated fibres, most of 

 which are directed dorso-ventrally. Anteriorly, it rapidly 

 tapers off between the tuberculum olfactorium and the hippo- 

 campus (praecallosal portion — ^fig. 8) and it is most easily 

 recognized under the corpus callosum. The upper part of 

 the septum extends postero-dorsally between the corpus 

 callosum and the fornix and hippocampal commissure (Fig. 8) 

 and in the human subject this part becomes secondarily 

 separated from the remainder. To it the term septum 

 pellucidum is applied, while the remainder is known as the 

 subcallosal gyrus or paraterminal body (PL XXII.). 



Thus the septum as a whole is composed of two parts, the area prae- 

 commissuralis septi or paraterminal body, and the pars supraforaminalis or 

 pars fimhrialis septi. According to Johnston, the latter is of pallial origin 

 and is alone entitled to the name of septum (Fig. 8), while the term par- 

 olfactory area may be applied to the remainder. This must not be con- 

 fused with Edinger's lobus parolfactorius, which is the tuberculum ol- 

 factorium. 



The myelinated fibres in the septum fall into medial and 

 lateral groups, the former consisting of numerous relatively 

 thick fibres which form a distinct layer on the medial side 

 of the septum, the bundle of Zuckerkandl or fasciculus prae- 

 commissuralis {tractus lobo-hippocampalis, tractus cortico- 

 olfactorius septi), while the lattei" is made up of finer fibres 

 more diffusely arranged. This system comprises several 

 different kinds of fibres, including axlons both to and from the 

 septum as well as others which pass right through it. At 

 least some of the fibres arising in this region go to the hypo- 



