1448 A MANUAL OF ANATOMY 



cortex. It is therefore composed chiefly of grey matter, and is covered 

 superficially by a thin layer of white matter, called the alveus, which is con- 

 tinuous with the fimbria. The hippocampus major is composed of the follow- 

 ing layers, named in order from the ventricular surface outwards : (i) the 

 alveus, composed of white matter, and covered by the ventricular ependyma ; 

 (2) neurogliar layer, consisting of neuroglia fibres and cells ; (3) pyramidal 

 layer, composed of large pyramidal cells ; (4) stratum radiatum, which is the 

 outer part of the pyramidal layer, and is composed of the dendrites of the 

 apical parts of the pyramidal cells, being thereby rendered striated in appear- 

 ance ; (5) stratum laciniosum, composed of the ramifications of the foregoing 

 apical dendrites, intimately intermixed ; (6) stratum granulosum, composed 

 of many small cells ; and (7) the involuted medullary lamina, consisting of 

 white fibres. 



Olfactory Tract and Olfactory Bulb.— These are developed as a 

 hollow outgrowth from the anterior cerebral vesicle, more par- 

 ticularly from the part of it which ultimately gives rise to the lateral 

 ventricle, and is known as the telencephalon. In many animals 

 the central cavity persists, and maintains its connection with the 

 lateral ventricle ; but in man the cavity disappears, though traces 

 of its ependymal lining remain. External to the vestigial epen- 

 dyma there is a layer of white matter, and, superficial to this, there 

 is a layer of grey matter. In the olfactory tract the layer of grey 

 matter is very thin over the ventral or inferior aspect, but over 

 the dorsal or superior aspect it is fairly thick. In the bulb the 

 reverse is the case, the grey matter being thick over the ventral 

 aspect, where it receives the olfactory filaments, but thin over the 

 dorsal aspect. 



Structure of the Ventral Grey Matter of the Olfactory Bulb. — 

 The grey matter consists of three layers, namely (i) the nerve- 

 fibre layer, (2) the glomerular layer, and (3) the granular layer. 



The nerve-fibre layer is the most superficial layer, and is com- 

 posed of olfactory nerve- fibres. These fibres are non-medullated, 

 and arise as the axons of the olfactory cells of the olfactory 

 mucous membrane of the nasal fossa. Having passed through the 

 foramina of one half of the cribriform plate of the ethmoid bone, 

 they enter the grey matter on the ventral aspect of the bulb, where 

 they break up and form arborizations. These intermingle with 

 the arborizations formed by the dendrites of the mitral cells, to 

 be presently described. 



The glomerular layer is composed of round bodies or glomeruli, 

 which are formed by the interlacements between the arborizations 

 of the olfactory nerve-fibres and those of the dendrites of the 

 mitral cells. 



The granular layer lies next to the layer of white matter, and is 

 chiefly characterized by the presence of large mitral cells. These 

 are pyramidal, and one dendrite from each cell passes into the 

 glomerular layer, where it gives rise to a glomerulus in the manner 

 just described in connection with the glomerular layer. Other 

 dendrites intermingle with those of adjacent mitral cells. The 

 axon of each mitral cell enters the white layer of the bulb, and 

 passes along the olfactory tract to the cerebrum. 



