niYSIOLOGICAL INTERPKETA'l IONS 79 



synaptic junctions, differing in histological structure, electrical prop- 

 erties, and perhaps also in chemical reactivity. These afferent fibers 

 may come from widely separated regions with diverse functions, and 

 the impulses delivered may differ in intensity and temporal rhythm. 

 Bodian's description ('37, '4'2) of axon endings on Mauthner's cell 

 of the medulla oblongata shows four main types of synaptic contact 

 which vary from 0.5 to 7/i in extent, with a wide variety of arrange- 

 ments. There are between four and five hundred of these endings on a 

 single cell, and the presumption is legitimate that these diverse 

 structures are correlated with significant differences in electrical and 

 chemical properties, including the timing of the pulses of transmis- 

 sion. It has been suggested that some of the influences transmitted 

 across the synaptic junctions are excitatory and that others are in- 

 hibitory. Synaptic junctions on dendrites are in some cases struc- 

 turally different from those on the axon hillock or axon, and they 

 may be activated from different sources. Some observers believe that 

 excitation of dendrites is excitatory and of axons is inhibitory, a 

 supposition supported with physiological evidence by Gesell and 

 Hansen ('45, p. 156). In their theory of the electronic mechanism of 

 activation and inhibition, these functions are viewed as basically 

 similar, activation being associated with an increasing, and inhibition 

 with a decreasing, intensity of the electronic current. The connec- 

 tions of horizontal cells of the retina as described by Polyak ('41, p. 

 385) suggest to him a different inhibitory apparatus. The horizontal 

 cells may exert an inhibitory influence upon the synapses between 

 the rods and cones and the bipolar cells, that is, the synapses of the 

 horizontal cells may function as "countersynapses" to the photore- 

 ceptor-bipolar synapses. 



Whatever may be the mechanism employed in central inhibition, 

 it is clear that in some parts of the brain excitatory functions pre- 

 dominate, in other parts inhibitory functions. Noteworthy examples 

 of the latter are (1) the head of the caudate nucleus (Fulton, '43, p. 

 456) ; (2) a region in the reticular formation of the medulla oblongata 

 explored by Magoun ('44) ; and (3) certain specific zones of the cere- 

 bral cortex (areas 4^, ^s, 19s, and some others) which are known as 

 "suppressor bands." In all these cases, excitatory and inhibitory 

 fields are intimately related physiologically in such a way as to secure 

 appropriate balance of activation and inhibition of the members of 

 synergic systems of muscles in proper sequence. 



The role of general inhibition in the patterning of behavior has 



