252 THE SPECIAL SENSES. 



ducting nerve fibers. With regard to this latter question the 

 opinions of physiologists still differ. Most physiologists, perhaps, 

 adopt the view that the specific reaction in consciousness is due to 

 the central ending, that, in other words, the different sensory 

 parts of the cortex give different kinds or qualities of consciousness, 

 while the sensory nerve fibers are simply conductors of nerve im- 

 pulses, which, however much they may differ in intensity, are 

 qualitatively the same in all nerve fibers. According to this view, 

 it would result, as du Bois-Reymond expressed it, that, if the 

 auditory nerve fibers were attached to the visual center and the 

 optic fibers to the auditory center, we would see the thunder and 

 hear the lightning. Each typical sense-organ from this standpoint 

 consists of three essential parts: the central ending, which deter- 

 mines the quality of the sensation; the peripheral end-organ, 

 retina, cochlea, etc., which determines whether or not any given 

 form of stimulus shall be effective and which in most cases is con- 

 structed so as to be responsive to a special form of stimulus desig- 

 nated as its adequate stimulus; and of connecting neurons whose 

 only function is to conduct the nerve impulses originating in the 

 end-organ. The fact, therefore, that the light waves can stimulate 

 the rods and cones of the retina, but are an inadequate stimulus 

 probably to the hair cells of the cochlea or the taste buds of the 

 tongue, is due to a peculiarity in structure of the rods and cones; 

 but the fact that the impulses conducted by the optic fibers arouse 

 a peculiar modality of sensation is not due to any peculiarity in 

 structure in these fibers or in the rods and cones, but to a charac- 

 teristic structure of the optic centers. The positive experimental 

 evidence for the correctness of this view is not conclusive, but, on 

 the whole, is impressive. Such facts as the following may be noted : 



1. When sensory nerve fibers are stimulated otherwise than 

 through their end-organs each reacts, if it reacts at all, according 

 to its specific energy, that is, it produces its own quality of sensa- 

 tion. When the optic nerve is cut, for instance, the mechanical 

 stimulus causes a flash of light ; when the chorda tympani is stimu- 

 lated in the tympanic cavity by mechanical, electrical, or chemical 

 stimuli sensations of taste are aroused. 



2. Mechanical pressure upon the peripheral nerves distributed to 

 the skin may cause a loss of some of the cutaneous senses in certain 

 areas of the skin with a retention of others. Thus the senses of 

 pressure and temperature may be lost and that of pain retained, 

 or pain may be lost and pressure retained. Such facts agree with 

 the view that each sense has its own set of nerve fibers ; those that 

 mediate pain can not by a mere modification of the stimulus give 

 also a sense of pressure. 



3. The only objective manifestation of a nerve impulse that we 



