724 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



ister to ordinary sensation, whilst the nerves of truly "special" sensation 

 are not sensible to tactile impressions, it is evident that the probability seems 

 in favor of the identity of the fibres which minister to these sensations, with 

 those of the usual sensory character. We shall see that with regard to the 

 sense of Temperature, there is strong evidence that its peculiarity partly de- 

 pends on the specialty of the apparatus by which impressions are received 

 at the peripheral extremities of the tactile nerves, and partly upon some 

 peculiarity in the transmitting fibres. 



592. There are certain external agencies which can excite changes in the 

 Sensorium through several different channels ; the sensation being in each 

 case characteristic of the particular nerve on which the impression is made. 

 Thus pressure, which produces through the nerves of common sensation the 

 feeling of resistance, is well known to occasion, when exerted on the eye, the 

 sensation of light and colors; and, when made with some violence on the 

 ear, to produce "tinnitus aurium." It is not so easy to excite sensations of 

 taste and smell by mechanical irritation ; and yet, as was shown by Dr. 

 Baly, 1 this may readily be accomplished in regard to the former. The sense 

 of nausea may be easily produced, as is familiarly known, by mechanical 

 irritation of the fauces. Electricity still more completely possesses the 

 power of affecting all the sensory nerves with the changes which are peculiar 

 to them ; for, by proper management, an individual may be made conscious 

 at the same time of flashes of light, of distinct sounds, of a phosphoric odor, 

 of a peculiar taste, and of pricking sensations, all excited by the same cause, 

 the effects of which are modified by the respective peculiarities of the in- 

 struments through which it operates. But although there are some stimuli 

 which can produce sensory impressions on all the nerves of sensation, it will 

 be found that those to which any one organ is peculiarly fitted to respond, 

 produce little or no effect upon the rest. Thus the ear cannot distinguish 

 the slightest difference between a luminous and a dark object. A tuning- 

 fork which, when laid upon the ear whilst vibrating, produces a distinct 

 musical tone, excites no other sensation when placed upon the eye than a 

 slight jarring feeling. The most delicate touch cannot distinguish a sub- 

 stance which is sweet to the taste, from one which is bitter; nor can the 

 taste (if the communication between the mouth and the nose be cut off) per- 

 ceive anything peculiar in the most strongly odoriferous bodies. It may 

 hence be inferred that no nerve of special sensation can, by any possibility, 

 take on the function of another. 2 



2. Sense of Touch. 



593. By the sense of Touch, as commonly understood, is meant that modi- 

 fication of the common sensibility of the body, of which the Cutaneous sur- 

 face is the special seat. The Skin is peculiarly adapted for this purpose, not 

 merely by the large amount of sensory nervous fibres which are distributed 

 in its substance, but also by its possession of a papillary apparatus in which 

 these nerves foifcthe most part terminate, or rather commence. The papilla 



1 Translation of Miiller'.s Elements of Physiology, p. 1002, note. 



2 It. does not, however, seem a thing altogether incredible, that impressions power- 

 fully arousing one sense may consentaneously excite another sense, and in evidence 

 of this Nussbaumcr, Monatschrift f. Ohrenheilkund, March, 1873, records the fact, 

 that liot'n In- and his brother can perceive certain colors when the auditory nerve is 

 powerfully excited by certain sounds. In one brother " a " on the piano produces 

 the subject ive impression of dark Prussian blue, in the other brother it produces dark 

 yellow. Some of the colors, though differing in the two brothers, are not produced 

 by any .sound. 





