Or SENSATION IN GENERAL. 853 



pheral extremities of the tactile nerves, rather than upon any peculiarity in the 

 transmitting fibres ( 866). 



857. There are certain external causes 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 m'ade. 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 sensa- 

 tion of light and colors ; and, when made with some violence on the ear, to pro- 

 duce tinnitus aurium. It is not so easy to excite sensations of taste and smell, 

 by mechanical irritation ; and yet, as Dr. Baly 1 has shown, this may readily be 

 accomplished in regard to the former. The sense of nausea may be easily pro- 

 duced, 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 indi- 

 vidual 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 instruments 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 dis- 

 tinguish 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 substance 

 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) perceive 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. 



858. But whilst there is evidence of the peculiar aptitudes of the different 

 Sensory nerves, to receive and convey impressions of particular kinds, yet there 

 can be no doubt that their special endowments are in great degree dependent 

 upon those of the central organs in which they terminate. For with regard to 

 all kinds of Sensation, it is to be remembered that the change of which the 

 Mind is informed is not the change at the peripheral extremities of the nerves, 

 but the change communicated to the Sensorium ; hence it results that external 

 agencies can give rise to no kind of sensation which cannot also be produced 

 by internal causes, exciting changes in the condition of the nerves in their 

 course. This very frequently happens in regard to the senses of sight and 

 hearing; flashes of light being seen, and ringing sounds in the ears being heard, 

 when no external stimulus has produced such impressions. The production of 

 odorous and gustative sensations from internal causes is perhaps less common ; 

 but the sense of nausea is more frequently excited in this manner, than by the 

 direct contact of the nauseating substance with the tongue or fauces. The 

 various phases of common sensibility often originate thus; and the sense of 

 temperature is frequently affected without any corresponding affection of the 

 tactile sensations, a person being sensible of heat or of chilliness in some part 

 of his body, without any real alteration of its temperature. The most common 

 of the internal causes of these subjective sensations (as they have been termed, 

 in contradistinction to the objective, which result from a real material object) 

 is congestion or inflammation ; and it is interesting to remark that this cause, 

 operating through each nerve, produces in the sensorium the changes to which 



1 Translation of Muller's "Elements of Physiology," p. 1062, note. 



