SENSATION. Ill 



operating through this channel upon the blood-vessels is seen in the 

 act of blushing, and also in the pallor that often accompany them. 



Dr. Carpenter thinks that as the sympathetic system is not deve- 

 loped in proportion to the predominant activity of the functions of 

 organic life, but in proportion to the development of the higher 

 division of the nervous system, its office is not to preside over the 

 former, but to bring them into relation with the latter; so that the 

 actions of the organs of vegetative life are not dependent upon it, 

 but influenced by it, in accordance with the operations of the system 

 of animal life. 



SENSATION. 



By this term is meant iJie perception of an impression. It is 

 with the brain alone that the mind possesses the relation necessary 

 for the production of sensation. Hence the brain is often called the 

 sensoriu7n. Sensations are of two kinds, external and internal. 

 By the first are meant those that arise from impressions made upon 

 the external surface of the body, as the sense of sight, touch, or 

 hearing. The internal are such as occur within the body, and 

 arise from some alteration in the function of the part, for the time 

 being. Hunger and thirst are internal sensations. 



With regard to all sensations, it must be remembered that the 

 change of which the mind is informed, is not the change which 

 occurs at the peripheral extremity of the nerves, but the change 

 communicated to the sensorium ; in other words, sensation does not 

 occur at the point impressed, but in the brain. Hence it happens 

 that sensations often occur from impressions upon a nerve some- 

 where in its course. This is of frequent occurrence in the senses 

 of sight and hearing, flashes of light being seen, and ringing sounds 

 being heard, when no external stimuli could have produced such 

 impressions. In such cases they not unfrequently arise from im- 

 pressions made on these nerves in their course from special gan- 

 glia to their peripheral termination. This variety of sensations is 

 termed subjective,, to distinguish them from objective,, in which the 

 stimuli are derived from without. The most common cause of these 

 subjective sensations is congestion or inflammation in the course of 

 the nerve. 



Whenever an impression is made upon a nerve in its course, the 

 mind instinctively refers it, not to the point impressed, but to the 

 ordinary termination of the nerve upon the periphery of the body, 

 even although these terminations should not exist, or should be in- 

 capable of receiving impressions. Thus, after amputations, the 

 patients are often troubled with sensations which they refer to the 

 removed extremities ; and in like manner, after the Taliacotian ope- 

 ration, all sensations produced by touching the nose are referred to 



