'////; i'i<:iti':ni{.\L u KM isi'H /;/>/.>>' 



the greater mass of them pass obliquely outwards over it and gradually disappear in 

 the aniero-lateral port ion of t lie anterior perforated substance, in which some of them 

 terminate, but through which most of them pass to curve into the anterior end of the 

 hippocampal gyrus and terminate there, chiefly in the minis. In most of the mam- 

 mals the lateral stria is so strong that it appears as a superlicial white band passing 

 directly into the uncus. 



(4) The parolfactory area (Hroca's area) involves the mesial extension of the 

 olfactory trigone, and is concerned with the nmliul nljn/lnri/ stria. < >n tin- basal sur- 

 face < if I he hemisphere this area in \olves the posterior extremity of the gyms reel us 



u portion of which is sometimes separated from the remainder of the gyrus by a 

 ventral prolongation of the unit riar i><ir<>l/iictt>ri/ mildix of the medial surface (>ee ligs. 

 626 and lii'l). This prolongation when present has ix>en called the fissura serotina. 

 On the medial surface the parolfactory area apj)ears as a definite gyrns bearini: its 

 name. In front this is separated from the superior frontal gyms by the anterior 



parolfactory sulcus, and from the subcallosal gyms behind by the deeper posterior 



parii/factori/ xiilciiN (lig. CrJIi). It is continuous above into the gyms cinguli of the 

 limbic lobe, the posterior part of the rhiiiencephalon. A large portion of the fibres 

 of the medial stria are lost in the parolfactory area, and are known to terminate 

 about the cells there. This stria or root of the olfactory tract forms a slight ridge 

 on the ventral surface of the area, which is frequently prominent enough to retain 

 the name medial olfactory gyrus applied to it in the foetal brain (fig. 627). 



(5) The subcallosal gyrus (peduncle of the corpus callosum) is the narrow fold 

 of the pallium which lies between the posterior parolfactory sulcus and the rostral 

 lamina and the ventral continuation of the latter into the lamina terminalis. It 

 begins above, in part fused to the rostrum of the corpus callosum, and in part continu- 

 ous with the gyrus cinguli, and below it goes over into the anterior perforated sub- 

 stance. Some fibres of the medial olfactory stria disappear in the substance of this 

 gyrus. 



(6) The anterior perforated substance must be considered with the rhinenceph- 

 alon, but, like the limen insuhe, it can only be considered as belonging in part to 

 this division of the brain. It comprises the basal region between the optic chiasma 

 and tract and the olfactory trigone. I'sually the posterior parolfactory sulcus 

 (fissura jrrima of the embryo) is sufficiently evident to more or less distinctly separate 

 it from the latter. A few fibres from the middle stria are known to disappear within 

 its depths, and. as mentioned above, many fibres from the lateral stria also pass into 

 it. The intermediate oljtichrri/ xtriii is always much the weakest of the three stria-. 

 and in many specimens is apparently absent. The fibres of this stria run almost 

 st might backwards and plunge directly into the anterior perforated substance, where 

 some of them are known to terminate, while others continue into the uncus. On 

 embryological grounds, the subcallosal gyrus and the anterior perforated substance 

 are classed as the posterior division of the 'olfactory lobe' or anterior part of the 

 rhiiiencephalon. 



The olfactory hulli and tract arise as a hollow outgrowth from the lower and anterior part 

 of the anterior of the three primary veefelei. It is a tubular structure at first, ami in man\ 



of the mammals tin- cavity maintains throughout life as the olfactory ventricli. In man lin- 

 earity becomes occluded and tlie ependyma and gelatinous substance which surround it become 

 tlie grey core of the bulb and tract of the adult. 



Tin- grey substance persists and developes chiefly in the bulb, and in fact produces it as such. 

 tt is much thicker on the ventral surf arc of tin- hull) than on the dorsal surface, and in srrtinti .shows 

 definite layers. From within outwards, the principal of these layers arc (1) the layer of large 

 cells whose shape suggests their name, tnilrnl IT//.S; (2) large den d rites of the mitral cells project 

 towards the BUrfaoe "i.l I here break up into numerous telodendria which copiously interdigitate 

 with like telodendria of the entering fibres of the olfactory nerve, thus forming rounded, much 

 I. -muled glomeruli and the layer containing these, the i/l<n< riilnr lni/< r; (.'( I the superlicial layer, or 

 ory IIII/IT. (-.insists of the fibres of the olfactory nerve which form a dense interlacement with 

 each other on the ventral surface of the bulb before they pass into its interior. I lie dorsal 

 surface of the bulb becomes formed almost wholly of the fibres which arise as axones of tin- 

 mitral cells and pass backwards to form the olfactory tract, and tlience to their localities of termina- 

 tion, chiefly hy way of the three stria-. Along the dorsal aspect of the olfactory tract the gela- 

 tinous substance of the core may show through as a grey ridge. 



The so-called limbic lobe (a name introduced by Hroca in 1S7M consists of the 

 structures which may be considered as comprising the posterior portion of the 

 rhinencephalon. It takes part in both the medial and tentorial surfaces of the 

 hemisphere. Seen from the medial surface, it forms an irregular elliptical figure 



