STRUCTURE OF THE CEREBRUM. 



251 



within the ventricle, and which is prolonged externally into the fimbria 

 (Fi), where its fibres become longitudinal in direction. 



In the dentate gyrus (fascia dentata, fig. 283, 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 is a thick layer of superficial neu- 

 roglia (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 no nerve-cells. Outside the central neuroglia lies the white 





FIG. 284. SECTION ACROSS A PART OF THE OLFACTORY BULB. (Henle.) 



1, 3, bundles of very fine transversely cut nerve-fibres, forming the flattened medullai-y ring, 

 enclosing the central neuroglia 2 : this is the anterior continuation of the olfactory tract ; 

 5, white layer with numerous small cells (granules) ; (i, mitral layer ; 7, layer of olfactory 

 glomeruli, t, tt ; 8, layer of olfactory nerve-fibres, bundles of which are seen at * 

 passing through the cribriform plate of the ethmoid bone. 



or medullary substance, consisting of bundles of longitudinal white 

 fibres. Most externally is a thin superficial layer of neuroglia. 



The olfactory bulb (fig. 284) has a more complicated structure. 

 Dorsally there is a flattened ring of longitudinal white bundles inclos- 



