CLASSIFICATION OF THE SENSE ORGANS 731 



farther and have rather generally concluded that the specific quality of 

 a sensation in consciousness is dependent upon the center and not upon 

 the conducting path nor upon its receiving organ. The second pos- 

 sibiUty may, of course, be justly ruled out, because nerve fibers are con- 

 ductile elements and nothing more than that. The third possibility 

 cannot be excluded so easily, because the sense-organs exhibit particu- 

 lar structural details which- render them especially adapted for the 

 reception of single kinds of energy manifestations. On the other hand, it 

 is evident that this specificity of the end-organ could not be of any avail, 

 were it not for the fact that the center possesses a similar specificity. 

 . Besides, since the latter is commonly regarded as the controlling factor 

 of all nervous processes, it may also be considered as the determining 

 agent of these impressions in consciousness. Consequently, the end- 

 organ merely enables a particular stimulus to enter, and determines 

 whether or no the latter should become effective centrally. Thus, if 

 it were possible to cross the optic and auditory nerves, so as to connect 

 the retina with the auditory center and the organ of Corti with the 

 visual center, we would hear pictures and see sounds. 



The experimental evidence which might be mentioned in favor of 

 the doctrine of specific nerve energy, includes such positive data as 

 the following: 



(a) An impression in consciousness is often obtained without the help of the 

 sense-organ and conducting path. For example, subjective sensations of light are 

 frequently gotten in a perfectly dark room and when the eyes are closed. Ringing 

 in the ears is another common subjective phenomenon. Central causes must also 

 be held responsible for the multitude of concepts obtained during dreams and in 

 consequence of pathological conditions, such as arise during hysteria and states of 

 excessive sensitiveness of the nervous system. 



(6) The different sense-organs may also be activated by stimuli other than 

 those ordinarily received by them. Thus, sensations of light may also be produced 

 by exerting a slight pressure upon the eyeball or by passing an electrical current 

 through it. Quite similarly, an electrical current directed through the tongue, 

 evokes sensations of taste, but sensations of touch, when applied to the skin. A 

 tuning fork made to vibrate in the vicinity of the ears, gives rise to sound sensations, 

 and when allowed to beat against the skin, to the pecuhar tactile impression 

 known as tickling. 



(c) Very similar effects may be obtained by the stimulation of the nerve fibers 

 leading away from these sense-organs. Thus, the cutting of the optic nerve evokes 

 flashes of light, while the excitation of the chorda tympani during its course through 

 the tympanic cavity gives rise to sensations of taste upon the tip of the tongue. 

 In many cases, these diverse sensations are elicited in consequence of tumors, 

 exudations, and hyperemic conditions in the course of sensory nerves. 



(d) Sensations may be dissociated. A degree of pressure may be brought to 

 bear upon the skin which destroys the sensation of touch and temperature, but not 

 that of pain. This preservation of one particular cutaneous impression to the ex- 

 clusion of others, is frequently observed in the beginning stages of degeneration and 

 regeneration following the division of peripheral nerves. This dissociation may 

 also be effected by chemical means, for example, the taste of bitter may be destroyed 

 by cocain and that of sweet by gymnemna sylvestris. 



The Modality of a Sensation.— It is a matter of common experience 

 that the different sensory impressions are not referred to the brain at 



