HAI.LOWEI.L DAVIS 



6l 



sensory nerve impulse, although it becomes more apparent if the stimulus to the 

 nerve is near threshold and if the nerve impulse sets off nervous reactions 

 beyond the first synapse. 



In the sense organs the amount of energy in the original stimulus may be very 

 small, and of the same order of magnitude as the work done in exciting the 

 sensory nerve fibers. We know that each sense organ provides a differential 

 sensitivity to a particular form of energy, and usually the absolute sensitivity at 

 threshold is very great. As long as sufficient energy is available in the stimulus, 

 as in the stretch receptors of our muscles, the only question seems to be just how 

 the energy is applied to the nerve in a way that will excite without causing in- 



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Fig. I. The cochlea of the guinea pig is exposed l)y opening the bulla. Various positions 

 lor intracochlear electrodes are shown. The wires are attached to the edge of the bone with 

 dental cement. The jwsitions It and Iv are shown in the cross section drawing in figure 3. 



jury. The case becomes rather more interesting, however, if it can be shown that 

 the amount of energy available in the stimulus is probably inadequate to do the 

 physical work necessary to excite a nerve impulse. In the case of the eye, the ear 

 and probably the organs of chemical sense, the stimuli near threshold are so 

 weak that an additional second-order trigger action of some sort can safely be 

 inferred. By this, we mean that a mechanism of some sort must be interposed 

 between the physical or chemical stimulus and the nerve fiber to amplify the 

 energy of the stimulus to make possible stimulation of the nerve. We shall 

 examine the inner ear with reference to this possibility as an example of a 

 particularly delicate trigger mechanism. 



