CLASSIFICATION OF THE SENSES. 



271 



of nerve-fibers; those that mediate pain cannot by a mere modi- 

 fication of the stimulus give also a sense of pressure. 



3. The only objective manifestation of a nerve impulse that 

 we can study in the nerve itself is the electrical change that 

 accompanies it or perhaps constitutes its essence. This electrical 

 change is qualitatively the same in all kinds of nerve fibers, and this 

 fact agrees with the view that the nerve impulse is qualitatively the 

 same in all fibers. 



So far as the sensory nerve fibers are concerned, the chief ob- 

 jection to this view of the doctrine of specific nerve energies is 

 found perhaps in the difficulty or impossibility of applying it to 

 the explanation of color vision. According to the strict interpreta- 

 tion of the view, each fundamental color sense, being distinct in 

 quality, should be mediated by its ouii set of nerve fibers. When 

 Helmholtz first formulated his theory of color vision he spoke, 

 therefore, of three kinds of nerve fibers, — the red, the green, and 

 the violet, — each when stimulated alone giving its owti specific 

 sensation and not capable of giving any other. The facts accumu- 

 lated regarding color vision, however, seem to show that this view 

 will not hold. One and the same cone, with its connecting fiber, 

 may give rise to any or all of the primary color sensations, and, 

 unless we choose to further subdivide the nerve unit and assume 

 that the separate nerve fibrils of which the axis cylinder is composed 

 constitute the separate conductors for the primary sense qualities, 

 it would seem to be impossible to apply the doctrine of specific 

 energies to this case. Not too much weight should be given per- 

 haps to this objection. For it must be remembered that all of our 

 present theories of color vision are unsatisfactory, and possibly 

 when we attain to the right point of view the facts may not be 

 so difficult to interpret in terms of this theory of specific energies. 



The alternative view proposed in place of the doctrine of 

 specific nerve energies assumes that the nerve impulses may 

 vary in quality as well as in intensity, and that therefore one and 

 the same nerve fiber may arouse different qualities of sensation and 

 have different end effects according to the character of the impulse 

 conveyed. This point of view is not capable of much discussion, 

 since there are no positive facts that support it. It is logically 

 satisfactory in meeting the cases in which the former view seems 

 to be unsatisfactory. It is difficult, however, in our ignorance of 

 the nature of the nerve impulse to imagine in what respects it may 

 possibly differ in character. 



The Weber-Fechner (Psychophysical) Law. — One difficulty 

 that has been encountered in the physiological study of sensory 

 nerves is that the end reaction cannot be measured with exactness. 

 With efferent nerves the end reaction is a contraction or secretion 



