866 THE NERVOUS SYSTEM 



definite gyrus. In front this is separated from the superior frontal gyrus by the 

 anterior parolfactory sulcus, and from the subcallosal gyrus behind by the deeper 

 posterior parolfactory sulcus (fig. 679). It is continuous above into the gyrus cin- 

 guli of the limbic lobe, a portion of the posterior part of the rhinencephalon. 



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. 680). 



(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 terminahs. It 

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

 continuous with the gyrus cinguli, and ventrally it goes over lateralward and 

 posteriorly into that portion of the anterior perforated substance known as the 

 diagonal hand of Broca, and in this way it extends into the uncus. Mesially, it 

 approaches its fellow of the opposite side so closely that the groove separating 

 the two is known as the median subcallosal sulcus of Retzius. Some fibres of the 

 medial olfactory stria disappear in the substance of the subcallosal gyrus. 



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

 cephalon, but, like the limen insulse, 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. Usually the posterior parolfactory 

 sulcus (fissura prima of the embryo) is sufficiently evident to more or less 

 distinctly separate it from the latter. Its postero-lateral area is occupied by the 

 diagonal band of Broca. A few fibres from the medial stria are known to dis- 

 appear within its depths, and, as mentioned above, many fibres from the lateral 

 stria also pass into it. The intermediate olfactory stria is always much the weakest 

 of the three striae, and in many specimens is apparently absent. The fibres of this 

 stria run almost straight backward and plunge directly into the anterior area of 

 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 with the posterior division of the ' olfactory ' lobe or anterior division of the rhinen- 

 cephalon. 



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

 of the anterior of the three primary vesicles. It is a tubular structure at first, and in many of 

 the mammals the cavity maintains throughout life as the olfactory ventricle. In man the cavity 

 becomes occluded and the ependyma and gelatinous substance which surround it become 

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



The grey substance persists and develops chiefly in the bulb, and in fact produces it as such. 

 It is much thicker on the inferior surface of the bulb than on the superior surface, and in section 

 shows definite layers. From within outward, the principal of these layers are — (1) the layer of 

 large cells whose shape suggests their name, mitral cells; (2) large dendrites of the mitral cells 

 project toward the inferior surface of the bulb and there lareak up into numerous telodendria 

 which copiousl}' form synapses with like telodendria of the entering fibres of the olfactory nerve, 

 thus forming rounded, much tangled glomeruli and the layer containing these, the glomerular 

 layer; (3) the superficial layer, or olfactory layer, consists of the fibres of the olfactory nerve 

 which form a dense interlacement with each other on the inferior surface of the bulb before they 

 pass into its interior. The superior surface of the bulb becomes formed almost wholly of the 

 fibres which arise as axones of the mitral cells and pass backward to form the olfactory tract, and 

 thence to their localities of termination, chiefly by way of the three striae. Along the dorsal, 

 covered, aspect of the olfactory tract the gelatinous substance of the core may show through as 

 a grey ridge. 



The posterior division of the rhinencephalon or the so-called limbic lobe (a 

 name introduced by Broca in 1878) takes part in both the medial and tentorial 

 surfaces of the hemisphere (fig. 681). Seen from the medial surface, it forms an 

 irregular elliptical figure which encloses the corpus callosum and the extremities 

 of which approach each other at the anterior perforated substance, where they 

 arc continuous with tii(! structures of the anterior division of the rhinencephalon. 

 The figure is bounded externally by the sulcus cinguli above, ])y the subparietal 

 sulcus (postlimbic sulcus) and the anterior limb of the calcarine fissure behind, 

 and by the collateral fi.ssurc below. These respectively separate it from the 

 frontal, parietal, occipital, and temporal lobes. It comprises the following] 



