138 



HUMAN PHYSIOLOGY 



FIG. 31. The 

 "crazy bone." 

 The ulnar nerve 

 is exposed to 

 rather frequent 

 shocks near 

 the point of 

 the elbow (c). 

 Impulses run- 

 ning up to the 

 brain as a result 

 of such acci- 

 dents produce 

 sensory effects 

 which are much 

 the same as 

 though they 

 had traversed 

 the whole length 

 of the hand. 



that is more easily tested. If impulses 

 are always the same then, no matter what 

 means may have been employed to start 

 them, invariable effects will be produced 

 by the stimulation of a selected path. If 

 all the fibers in the optic nerve have such 

 connections with the brain as to arouse 

 sensations of light then we may excite the 

 optic nerve in an unusual way and still 

 obtain the only kind of sensation proper 

 to this collection of fibers. This seems 

 to be very nearly true. 



We have testimony to the fact that cut- 

 ting this great nerve does not give pain 

 but the sensation of a bright flash. Pres- 

 sure on the eyeball will produce spots or 

 circles of color. If it is objected that 

 there is also the sense of pressure it can 

 be replied that a different nerve from the 

 optic is known to be concerned in this 

 reaction. Electric stimulation of the end- 

 ings of the olfactory nerve is said to cause 

 an odor. 



One result of the alleged uniformity of 

 nerve-impulses is that we have no way to 

 tell whether they come from the distant 

 endings of a nerve or from some point in- 

 termediate between these endings and the 

 centers. This is simply illustrated by the 

 common mishap of striking the elbow a 

 sharp rap. The nerve that passes over 

 the bone at that joint is mechanically ex- 

 cited and the impulses which ascend it 

 have the same effect as though they had 

 come all the way from the fingers. We 

 say that the fingers tingle because we are 

 more used to sensations caused through 



of the fibers from their receptors in the shaded region 



