CHAP. XIV.] DURATION OF IMPRESSIONS OF TOUCH. 433 



The sensations of tickling, tingling, itching, and many others al- 

 lied to them, are also referrible to the nerves of touch. Respecting 

 tickling, it has been well observed by Weber, that it is most apt to 

 be excited in parts of feeble tactile power. 



Impressions made on the organ of touch, as on the other organs 

 of sense, continue perceptible for a period more or less prolonged 

 after the stimulus has ceased to be applied. The sting of a smart 

 blow does not soon subside ; and even the simple contact of any 

 object, as a ring or an article of clothing, with a part of the skin, if 

 long continued enough, leaves, after its removal, an impression of 

 its presence, which is apt to deceive the individual for a consider- 

 able time. The influence of habit on sensation in general, may be 

 .well illustrated in the case of the nerves of common sensation and 

 of touch. Impressions sufficiently strong in the first instance to 

 arouse the attention, soon become feeble, and in time wholly disre- 

 garded, if continued uniformly or frequently repeated; although the 

 mind can still, at any moment, take cognizance of them by a volun- 

 tary effort. The sensations of heat and cold may, by long habit, 

 in like manner come to be unnoticed, or lightly heeded, within cer- 

 tain bounds. This is a matter of common experience, and may be 

 exemplified in the case of the lower classes of society, among whom 

 the privation of the comforts of warm clothing and lodging, and the 

 absence of the mistaken luxury of over-heated rooms, are compen- 

 sated for by the possession of that diminished susceptibility to cold, 

 under slight exposures, which is so remarkable in those subject, in 

 a moderate degree, to the inclemencies of the seasons. 



Little needs be said of the subjective sensations pertaining to the 

 nerves now under consideration. They are among the best known, 

 and most familiar in the body. The peculiar tingling of a limb 

 " asleep," which commonly depends on pressure on its trunk, may 

 result from morbid changes in the centre ; as may likewise sensa- 

 tions of formication, or the creeping of insects, and those of itching, 

 of heat, of chilliness, etc., and lastly of pain of various kinds. 



Besides the references in the foot-notes and the various treatises on Phy- 

 siology and General Anatomy already cited, we may refer on the subject of the 

 preceding chapter to Rudolphi, Grundriss der Physiologic, band ii. ; Weber, 

 de pulsu, resorptione, auditu, et tactu ; Lips. 1834 ; Breschet et Roussel de 

 Vauzerne, Ann. d. Sciences Nat. 1834, torn. i. ; Schwann, Mikroskop. Unter- 

 suchungen ; Eble, die Lehre von den Haaren; Gurlt, Midler's Archiv. 1836; 

 Van Laer, de structura capillorum humanorum ; Traj. ad Rhen. 1841. 



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