OF SENSATION IN GENERAL. 391 



sages the sense of the contact of a foreign body. But as we know that all 

 the trunks, along which these peculiar impressions travel, do minister to ordi- 

 nary sensation, whilst the nerves of truly special sensation are not sensible 

 to common impressions, it is evident that the probability is in favour of the 

 identity of the fibres, which minister to these sensations, with those of the 

 usual sensory character. For the sense of temperature, however, it is not by 

 any means certain that a special set of fibres does not exist ; for many cases 

 are on record, in which it has been lost, whilst the ordinary sense of tact 

 remained ; and it is sometimes preserved, when the anesthesia is in other 

 respects complete. 



516. 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 pro- 

 duced 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 nau- 

 seating substance with the tongue or fauces. The various phases of common 

 sensibility often originate thus ; and it is an additional evidence in favour of 

 the distinctness of the fibres which convey the impressions of temperature, 

 that these are frequently affected, a person being sensible of heat or of chil- 

 liness in some part of his body, without any real alteration of its temperature, 

 whilst there is no corresponding affection of the tactual sensations. 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 senso- 

 rium the changes to which that nerve is usually subservient. Thus, conges- 

 tion in the nerves of common sensation gives rise to feelings of pain or un- 

 easiness ; but when occurring in the retina and optic nerve it produces flashes 

 of light; and in the auditory nerve it occasions "a noise in the ears." It 

 may be observed, also, of some external causes, that they may excite changes 

 in the sensorium through several different channels ; and that in each case the 

 sensation is characteristic of the particular nerve, on which the impression is 

 made. Thus pressure, which produces through the nerves of common sensa- 

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

 eye, the sensation of light and colours ; 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 Dr. Baly* has shown, 

 it may readily be accomplished in regard to the former. The sense of nau- 

 sea may be easily produced, as is familiarly known, by mechanical irritation 

 of the fauces. The stimulus of Electricity still more completely possesses 

 the power of affecting all the sensory nerves, with the changes which are pe- 

 culiar to them; for, by proper management, an individual may be made con- 

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

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

 cause, the effects of which are modified, according to the respective peculi- 

 arities of the instruments through which it operates. But although there are 

 some stimuli which can produce sensory impressions on all the nerves of 



* Translation of Muller's Physiology, p. 1062, note. 



