324 



NORMAL HISTOLOGY. 



stratum. Enclosed within the continuous ring of fibres lies the flat- 

 tened gelatinous or neuroglia zone corresponding to the area of 

 the obliterated former lumen of the tract. Outside the fibre-layer, 

 a sheet of gray substance, extremely thin on the ventral surface, 



represents the cortex 

 FIG. 348. of the convolution. 



The Bulbus Olfac- 

 torius. While the 

 layers present in the 

 tract are continued into 

 the terminal olfactory 

 bulb, the greater devel- 

 opment of the ventral 

 zone considerably mod- 

 ifies the structure of this 

 division of the olfactory 

 lobe. In the bulb the 

 area of the obliter- 

 ated ventricular cav- 

 ity lies eccentrically, 

 closely approaching the 

 dorsal surface, from 

 which it is separated by 

 a thin layer of fibres 

 and the greatly attenu- 

 ated gray cortical 

 stratum, here reduced 

 to a delicate lamina. 

 The ventral layers, 

 on the contrary, are 



greatly developed, and culminate in the nerve-fibres which pass 

 through the cribriform plate as the true olfactory nerves. 



In transverse section, the dorsal portion of the olfactory bulb is 

 occupied by (i) the central neuroglia, the atrophic field representing 

 the obliterated lumen of the lobe ; this area is enclosed by (2) the 

 flattened ring of medullary substance, consisting of closely- 

 placed bundles of longitudinal nerve-fibres. 



Next the ventral portion of the medullary ring lies (3) the stratum 

 granulosum, a thick zone of gray matter containing numerous 

 ganglion-cells of different size. Some of these are small irregular 

 elements, and immediately beneath the ring constitute a dense aggre- 

 gation. The most conspicuous elements of the gray matter, how- 

 ever, are the large pyramidal or mitral cells (30-50 ) which 

 occupy the deeper parts of this stratum. 



Section of portion of human olfactory bulb : i, 3, bundles of 

 transversely-cut nerve-fibres, enclosing central neuroglia (2) ; 

 4, 5, 6, granule layer; 7, zone of olfactory glomeruli (g-) ; 8, 

 layer of olfactory nerve-fibres, bundles (o) of which pass to 

 olfactory mucous membrane. (After ffenle.) 



