GENEKAL SENSIBILITY. 597 



solid support, and the foreign body placed upon it, its weight is then 

 appreciated solely by the amount of pressure which it causes. The 

 sensation of muscular contraction is itself a result, so far as we can 

 judge, of the physical impression produced upon the sensitive nerve 

 fibres in the muscular tissue ; and there is nothing to indicate that it 

 differs essentially from that caused by pressure upon the nerves of the 

 integument. 



Sensations of Temperature. The appreciation of temperature is also 

 most highly developed, as a general rule, in those parts which have the 

 greatest share of tactile sensibility. The difference in this respect be- 

 tween the integument of the face and that of the scalp is very marked ; 

 since hot applications may be readily borne upon the scalp, which would 

 be nearly or quite intolerable upon the face. The extent of surface ex- 

 posed to a given temperature has also an influence upon the effect pro- 

 duced, and a moderate degree of either warmth or cold applied over a 

 considerable portion of the skin is much more readily perceived than 

 if confined to a limited region. There is evidence that the impres- 

 sions of temperature and those of touch, if transmitted by the same 

 fibres, depend upon two different forms of nervous excitation, or are 

 received by different peripheral nervous structures ; since abundant 

 instances have been observed in which one of these two kinds of sensi- 

 bility was impaired independently of the other. In various forms of 

 paralysis, tactile sensibility may be lost while that of temperature re- 

 mains ; or, on the other hand, the power of appreciating temperatures 

 may disappear while impressions of contact continue to be perceived. 1 



Sensations of Pain. The sense of pain is different in character from 

 that caused by tactile impressions or by variations in temperature. It 

 is caused by any exaggerated mechanical irritation or by the application 

 of excessive heat or cold ; but in all these instances, when the intensity 

 of the impression rises above a certain point, the ordinary perceptions 

 produced by it disappear, and that of pain takes their place. Thus if 

 the blade of a knife or the point of a needle be placed gently in contact 

 with the skin, we perceive, by means of tactile sensibility, the character 

 and form of its surface. But if the pressure be increased beyond a cer- 

 tain degree, or if the integument be actually wounded, we obtain no pre- 

 cise information of the physical qualities of the foreign body, and are 

 only conscious of the pain which results. The appreciation of cold or 

 warmth, in like manner, is only possible within moderate limits ; and 

 when the variations are so excessive as to produce pain, all accurate 

 perception of the degree of temperature is lost. The contact of a red- 

 hot iron and that of one much below the freezing point of water produce 

 sensations which are not essentially different from each other, and which 

 are marked only by their painful character. 



It is not known whether the sensation of pain be confined to nerve 



1 Brown-Sequard, Physiology and Pathology of the Central Nervous System. 

 Philadelphia, 1860, pp. 84, 98, 125. 



