OF SENSATION IN GENERAL. 229 



309. Besides the various kinds of sensibility which have been just enume- 

 rated, there are others which are ordinarily associated together along with 

 the sense of material resistance (and its several modifications), and the sense 

 of temperature, under the head of Common Sensation ; but several of them, 

 especially those which originate in the body itself, can scarcely be regarded 

 in this light. Such are the feelings of Hunger^and Thirst; that of Nausea; 

 that of distress resulting from suspended aeration of the blood; that of "sink- 

 ing at the stomach," as it is vulgarly but expressively described, which results 

 from strong mental emotion; that of the venereal excitement, and perhaps 

 some others. Now in regard to all these, it is impossible in the present state 

 of our knowledge to say, whether their peculiarity results from the particular 

 constitution of the nerves that receive and convey them, or only from a modifi- 

 cation in the impressing causes, and in the mode in which they operate. 

 Thus we have no evidence that the nervous fibrils, which convey from the 

 lungs the sense of distress resulting from deficient aeration, may not be of a 

 different character from those which convey from the surface of the air-pas- 

 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 ordinary 

 sensation, whilst the nerves of truly special sensation are not sensible to com- 

 mon 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 anaesthesia is in other respects complete. 



310. 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 nauseating 

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

 sibility 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 chilli- 

 ness 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 sensorium the 

 changes to which that nerve is usually subservient. Thus, congestion in the 

 nerves of common sensation gives rise to feelings of pain or uneasiness ; 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 maybe 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 sensation the feeling 



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