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HANDBOOK OF PHYSIOLOGY. 



fibres and olfactory cells with which it is connected. These parts to- 

 gether form a sensory end-organ which resembles in many respects the 

 retina. The discovery of its true structure has thrown a flood of light 

 on the architecture of the nerve-centres as a whole. 



The olfactory bulb is not a nerve, but a modification of the brain 

 cortex. A transection shows it to be made up of four layers: 



1st. Peripheral fibres. 



2d. Olfactory glomerules. 



3d. Layer of mitral cells. 



Ependymal epithe- 

 lium. 



Layer of central 

 fibres. 



Cribriform M:im>\ plate of 

 x/ >'> -^ 



Layer of mitral 

 cells. 



Layer of olfactory 

 fibrillse. 



Nasal 

 Epithelium. 



Fig. 393. Principal constituent elements of the olfactory bulb of a mammal. (Van Gehuchten.) 



4th. Layer of granular cells and deep nerve-fibres. 



1st. The first and external layer is composed of the fino nerve-fibrils 

 of the olfactory nerves. They pass through the cribriform plate of the 

 ethmoid and continue on, ending in the olfactory cells. 



2d. The glomerular layer contains numbers of small round bodies 

 whose structure is now known to be nervous. They are made up of the 

 expansions of the olfactory fibres on the one hand and of the "mitral " 

 cells on the other. These are mingled in a close network, but do not 

 anastomose, It was by the study of these bodies in part that the fact of 



