218 INTRODUCTION TO NEUROLOGY 



termed a glomerulus, which also receives one or more dendrites 

 from the olfactory neurons of the second order, or mitral cells. 

 The glomerulus, therefore, contains the first synapse in the ol- 

 factory pathway. The axons of the mitral cells form the ol- 

 factory tract and discharge into the olfactory area, or secondary 

 olfactory nucleus, at the base of the olfactory bulb. These axons 

 give off collateral branches which discharge among very small 

 neurons of the olfactory bulb, the granule cells, whose chief 

 processes are directed peripheralward, to end among dendrites 

 of the mitral cells. 



Attention has already been called (pp. 75 and 91) to the fact 

 that, though smell and taste are both chemically excited senses, 

 the olfactory organs can be excited by much more dilute solu- 

 tions of the stimulating substances than can the gustatory or- 

 gans. The lowering of the threshold for olfactory stimuli has 

 been effected by several means, among which we may mention 

 the following: Whereas in the taste-buds there is a synapse 

 between the specific receptor cells and the peripheral nerve-fiber 

 (Fig. 35, p. 91), there is no such synapse in the olfactory organ, 

 the peripheral receptor cell giving rise directly to the olfactory 

 nerve-fiber (Fig. 104). In the second place, the peripheral 

 gustatory nerve-fiber discharges centrally into several neurons 

 of the primary gustatory center in the medulla oblongata; but 

 many peripheral olfactory fibers enter a single glomerulus, where 

 they are engaged by dendrites from only one or two mitral cells, 

 thus providing for the summation of stimuli in each mitral cell. 

 Again, the collateral discharge from the olfactory tract into the 

 granule cells (which are very numerous) carries the discharge 

 from the mitral cells back again into these cells and thus rein- 

 forces their discharge (see pp. 101, 192). By these and other 

 devices exceedingly feeble peripheral stimuli may give rise to 

 very strong excitations in the olfactory centers. 



The fibers of the olfactory tract reach the olfactory area, or 

 secondary center, by three paths which spread out from the 

 base of the olfactory bulb and are known as the medial, inter- 

 mediate, and lateral olfactory striae (these are shown but not 

 named on Fig. 53, p. 120). The olfactory area has various sub- 

 divisions (Fig. 105), the most important of which are: (1) the 

 lateral olfactory nucleus (or gyrus) which receives the lateral 



