88 Anatomy of the Nervous System 



At the anterior end of each hemisphere is the olfactory 

 bulb, which is connected by the olfactory tract with masses 

 of gray matter, both pallial and subpallial {i.e., belonging to 

 the basal ganglia). All of these, together with the olfactory 

 centres in the diencephalon, make up the rhinencephalon or 

 olfactory brain. 



The olfactory portions of the pallium are the older phylo- 

 genetically and are given the name archipallium,^ while the 

 non-olfactory portions are called the neopallium. It is the 

 neopallium which has expanded in the mammals, as mentioned 

 above, and in the higher mammals it expands a great deal 

 more, crowding down the other parts until they are almost 

 completely concealed by it, and becoming very much con- 

 voluted. 



A consideration of the rhinencephalon starts naturally 

 with the olfactory bulb (fig. 4, PI. XXV.), which receives the 

 axons of the olfactory nerve cells lying in the nasal mucous 

 membrane. Penetrating the anterior and ventral surfaces 

 of the bulb, these unmyelinated fibres terminate in little, 

 densely-matted knots called glomeruli, each of which contains 

 the terminal branches of several fibres and also one or more 

 dendrites from cells of the second order, the mitral cells. The 

 axons of the mitral cells form the olfactory tract, which 

 conducts the impulses back to the cerebral hemisphere proper 

 and end there in the secondary olfactory area. Just after 

 leaving the cell body, however, the axon gives ofT collaterals 

 which come into relation with the granule cells, small neurons 

 present in the olfactory bulb in great numbers. The axons 

 of the latter extend back to the mitral cells and discharge 

 among their dendrites. This arrangement and the glomeruli 

 are both to be regarded as mechanisms whereby strong central 

 impressions may be produced by weak peripheral stimuli. 



structure of pallial origin dorsal to the latter, so that the name septum 

 should not be applied to the paraterminal body. (See Figs. 7 and 8.) 

 ^See, however, footnote on p. 97. 



