592 THE ORGANS 



The layer of olfactory fibres (Fig. 391, a) consists of a dense plexi- 

 form arrangement of the axones of the above-described olfactory cells. 

 From this layer the axones pass into the layer of olfactory glomeruli 

 where their terminal ramifications mingle with the dendritic terminals 

 of cells lying in the more dorsal layers, to form distinctly outlined 

 spheroidal or oval nerve-fibre nests, the olfactory glomeruli (Fig. 391, 

 og) . The latter mark the ending of neurone system No. I. of the olfac- 

 tory conduction path. (For olfactory tract see pp. 481, 483,) 



The molecular layer contains both small nerve cells and large 

 nerve cells. These send their dendrites into the olfactory glomeruli. 

 The smaller cells belong to Golgi Type II. (seepage 135) and appear 

 to be association neurones between adjacent glomeruli. The axones 

 of the larger cells, the so-called brush cells, become fibres of the olfac- 

 tory tract. 



Of the mitral cells (Fig. 391, me), the main dendrites end in the 

 olfactory glomeruli, while their axones, like those of the brush cells, 

 become fibres of the olfactory tract. 



In addition to the fibres which pass through it (axones of mitral 

 and of brush cells), the granular layer contains numerous nerve cells. 

 Many of these are small and apparently have no axones (amacrine 

 cells). Their longer dendrites pass toward the periphery, their 

 shorter dendrites toward the olfactory tract. Larger multipolar cells, 

 whose axones end in the molecular layer, also occur in the granular 

 layer. 



The layer of longitudinal fibre bundles consists mainly of the 

 centrally directed axones of the mitral and brush cells. These fibres 

 run in distinct bundles separated by neurogHa. Leaving the bulb 

 they form the olfactory tract by means of which they pass to their 

 cerebral terminations. 



The brush cells and mitral cells with their processes thus consti- 

 tute neurone system No. II. of the olfactory conduction path. 



TECHNIC 



(i) Carefully remove the olfactory portion of the nasal mucosa (if human 

 material is not available, material from a rabbit is quite satisfactory). This may 

 be recognized by its distincdy brown color. Fix in Flemming's fluid (technic 8, 

 p. 8), or in Zenker's (technic 10, p. 8). Stain thin vertical sections with hajmat- 

 oxylin-eosin (technic i, p. 20) and mount in balsam. 



(2) For the study of the nerve relations of the olfactory cells material should 

 be treated by the rapid Golgi method (page 36). 



