THE SENSE OF SMELL 



543 



FIG. 8. Three records with double oscillograph system showing discharge from the oral and 

 aboral regions of the rabbit's olfactory bulb. In each record the upper tracing is from the oral region 

 and the lower from the aboral. The signal line shows increasing odor concentration. With acetone 

 (top record) the discharge is confined to the oral region, with paraflFin oil (bottom record) to the 

 aboral and with amyl acetate (middle record) discharge occurs in both regions. [From Adrian (9).] 



just great enough to e.xcite and there the specific 

 excitation will always show itself. 



TEMPORAL DIFFERENTI.\TION OF RESPONSE. Adrian (lo) 



suggests that at the beginning and end of each respira- 

 tion the concentration of odorous substance is near 

 threshold values. Physical and chemical properties of 

 the substance will therefore determine the time course 

 of the response. The integrated outline or envelope 

 of the response as .seen in oscillographic records will 

 thus have a characteristic contour and a particular 

 smell might be identified from this outline. Volatility 

 and solubility in water both favor a rapid rise and 

 decline of the discharge, with little pensistence be- 

 tween one inspiration and the next. Thus the re- 

 sponse to amyl acetate has a shorter latency and a 

 more abrupt rise and fall than the longer latency 

 responses to pentane. Increasing concentrations affect 

 the areal differentiation but have no effect on the 

 temporal pattern. Patterns of temporal integration 

 have been recorded i^y the more elaborate techniques 

 of Mozell & Pfaffmann (72) in determining the 

 relative sensitivity of different parts of the mucosa 

 and bulb to amyl acetate and heptane (fig. 10). 



Mozell (71) has determined the neural response 

 curve of the integrated spike discharge from four 

 points on the olfactory bulb as a function of concen- 

 tration of amyl acetate, heptane, ethyl ether and 

 benzene. Discharge strength and duration increased 

 approximately as a negatively accelerated function of 

 concentration. The curves reached their a.symptotes 

 in about i to 1.5 log units of physical concentration. 

 By contrast, the asymptote in other senses is not 

 reached until the intensities have been increased 

 several thousand times, or bv 4 to 6 log units. It is 



suggested that this may account for the relatively 

 narrow range of suiyective odor intensity discrimina- 

 tion. This study confirms the existence of a relatively 

 gross anteroposterior spatial differentiation of re- 

 sponsive zones within the bulb for different substances. 

 Thus smells seem to be distinguished by a combina- 

 tion of detailed pattern and general region of excita- 

 tion (lo). Hainer et al. (52), in discussing an informa- 

 tion theory of olfaction, also emphasize the iinportance 

 of threshold phenomena in the conveyance of essen- 

 tial olfactorv information. 



CENTR.'^L CONNECTIONS OF OLF.ACTORY BULB 



Olfactory functions were originally ascribed to 

 many deep parts of the temporal lobe, including the 

 hippocampal gyrus and hippocampal formation, and 

 to certain regions of the frontal lobe, including the 

 cingulate area. Much of the early work in this field 

 can be seen in an English translation of certain works 

 of Ramon y Cajal (78). Experimental determination 

 of the sites of termination of the olfactory tract has 

 indicated a much more restricted distribution of these 

 fibers. Reviews by Brodal (27), Allison (15) and 

 Pribram & Kruger (77) have discussed the morpho- 

 logical aspects of this problem. 



Efferent Pathways From Olfactory Bulb 



The majority of the axons of the mitral cells run 

 caudally to be collected on the lateral and inferior 

 aspects of the olfactory peduncle, forming the lateral 

 olfactory tract or stria. In addition to the superficial 

 pathways, there is a centrally placed group of delicate 



