STRUCTURE OF THE OLFACTORY TRACT AND BULB 205 



themselves amongst the pyramidal cells of the more superficial layers, 

 with processes from which they are probably continuous. 



In the hippocampal region a remarkable difference of structure 

 manifests itself (fig. 240). The superficial layer of neuroglia and the 

 white stratum, which overlies it as a thin band in other parts of the 

 cortex, are in this region both very strongly marked (s, 6), the neuroglia 

 layer having a very distinctly reticular aspect, and being in part beset 

 with small cells. All the rest of the thickness of the grey matter 

 appears to be mainly composed of, or at least to contain, long conical 

 cells (3, 4), the distal processes or apices of which are prolonged into 

 fibres which lose themselves in the superficial layer of neuroglia. The 

 pyramidal cells rest upon the white centre, here known as the alveus (i ), 

 which is the part of the hippocampus seen within the ventricle, and 

 which is prolonged externally into the fimbria (Fi), where its fibres be- 

 come longitudinal in direction. 



In the dentate gyrus (fascia dentata, fig. 240, Fd) the pyramidal 

 cells are arranged in an irregularly radiating manner, occupying the 

 centre of the convolution , and surrounded by a ring of closely packed 

 small cells (*), external to these being the relatively thick layer of 

 superficial neuroglia (7). 



The olfactory tract is an outgrowth of the brain which was ori- 

 ginally hollow, and remains so in many animals ; but in man the 

 cavity has become obliterated, and the centre is occupied by neuroglia, 

 containing, however, no nerve-cells (fig. 241). Outside the central 



FIG. 241. SECTION ACROSS THE OLFACTORY TRACT. 



neuroglia lies the white or medullary substance, consisting of bundles 

 of longitudinal white fibres. Most externally is a thin superficial 

 layer of neuroglia. 



The olfactory bulb (fig. 242) has a more complicated structure. 

 Dorsally there is a flattened ring of longitudinal white bundles enclosing 

 neuroglia (i, 2, 3), as in the olfactory tract, but below this ring a 

 number of layers are superadded as follows : 



1. A granule layer (fig. 242, 4), characterised by the presence of a 



