512 THE NERVOUS SYSTEM. 



leading to the interior. Sensibility is most acute in mucous mem- 

 branes supplied by the fifth pair, namely, in the conjunctiva, anterior 

 part of the nares, inside of the lips and cheeks, and the anterior two- 

 thirds of the tongue. It diminishes from without inward, and disap- 

 pears altogether in the internal organs not abundantly supplied with 

 cerebro-spinal nerves. 



Sensations of Temperature. The appreciation of temperature is, in 

 general, most highly developed in parts which have the greatest tactile 

 sensibility. The difference in this respect between the sensitive integu- 

 ment of the face and the comparatively insensible scalp is very marked ; 

 and hot applications may be readily borne by the scalp which would be 

 intolerable upon the face. The extent of surface exposed has also an 

 influence on the effect produced by temperature ; and a moderate degree 

 of warmth or cold applied over a large area is more readily perceived 

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

 pressions of temperature and those of touch are either transmitted by 

 different nerve fibres, or depend on different forms of nervous excite- 

 ment ; since, according to Brown-Sequard,* there are instances in which 

 the two kinds of sensibility are impaired independently of each other. 

 In some forms of paralysis, tactile sensibility may be lost while that 

 of temperature remains ; or, on the other hand, the power of appreci- 

 ating temperature may disappear while impressions of contact are still 

 perceived. 



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

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

 produced by exaggerated mechanical irritation or by excessive heat or 

 cold ; and in most instances, when the intensity of an impression rises 

 above a certain point, the ordinary perceptions 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 tactile 

 sensibility, its qualities of form and surface. But if the pressure be 

 increased beyond a certain degree, or if the integument be wounded, 

 we have no further perception of the physical properties of the foreign 

 body, and are only conscious of the pain which it inflicts. The appre- 

 ciation of cold or warmth, in like manner, is only possible within mod- 

 erate limits ; and when either is so excessive as to produce pain, all 

 accurate notion 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 not essentially different from each other, and marked 

 only by their painful character. 



The sense of pain may be preserved or lost independently of other 

 kinds of sensibility. The anaesthesia produced by ether or chloro- 

 form may be carried to such a point that the capacity for feeling pain 

 is abolished, while tactile sensibility remains ; and in this condition the 



* Physiology and Pathology of the Central Nervous System. Philadelphia, 1860, 

 pp. 84, 98, 125. 



