THE CENTRAL NERVOUS SYSTEM 



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most common shape, mitral cells, along with smaller nerve-cells 

 ('granules') and neuroglia ; (4) the nuclear layer, containing 

 numerous small nerve-cells or ' granules ' intermingled with white 

 fibres. The mitral cells give off axons, which pass through the 

 fourth layer, and then as fibres of the olfactory tract to the grey 

 matter of the hippocampal region of the brain. The course of the 

 impulses from the olfactory mucous membrane to the brain is 

 shown in Fig. 341. The olfactory tract, as it runs back, divides 

 into portions called its ' roots.' Of these the lateral is the most 



FIG. 340. NUCLEI AND ROOTS OF CRANIAL NERVES (TOLDT). 

 Lateral view. Motor red, sensory blue. 



important, and it terminates in the hippocampal and uncinate 

 gyri of the same side. Fibres of the olfactory tract are also con- 

 nected either directly or through the relay of another neuron with 

 the opposite side of the brain, especially the opposite uncinate gyrus. 

 The anterior commissure contains numerous fibres, which connect 

 the hippocampal regions of the two sides. Other central connections 

 of the olfactory tract exist, but some are imperfectly known. The 

 name ' rhinencephalon ' is given to the portions of the brain con- 



cerned with the sense of smell. 



Disturbances of smell sensation may 

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