134 



SENSE OF TOUCH. 



however, in which the impression is made is in each case alike, and 

 equally simple. It is merely necessary, that the substance, which 

 causes it, should be brought in contact with what may be termed the 

 physical part of the organ the cuticle ; the nervous part is seated in 

 the corpus papillare, for if the nerves proceeding to this layer of the 

 skin be cut, the sense is destroyed. In the exercise of touch, each of 

 the layers seems to have its appropriate office: the corium, the inner- 

 most layer, the base on which the others rest, offers the necessary re- 

 sistance, when bodies are applied to the surface; the rete rnucosum is 

 unconcerned in the function : the erectile tissue, on which the papillae 

 are grouped, probably aids them in their appreciation of bodies ; and 

 the epidermis modifies the tactile impression which might become too 

 intense, or be painful, did this anorganic envelope not exist. The de- 

 gree of perfection of the sense is greatly influenced by the state of 

 the cuticle. Where thin as upon the lips, glans penis, clitoris, &c. 

 the sense is very acute ; where thick and hard, it is obtuse ; and where 

 removed as by blistering the contact of bodies gives pain, but does 

 not occasion the appropriate impression of touch. 



Professors Weber 1 and Valentin 2 have shown that the tactile power of 

 the skin is not proportionate to its sensibility. The mammae, for ex- 

 ample, are easily tickled, and susceptible of great pain when irritated ; 

 yet they are moderately endowed with the sense of touch. The differ- 

 ent parts of the skin, too, vary in their tactile power. The left hand, 

 in most persons, is more sensible to temperature than the right, proba- 

 bly owing to the epidermis being thinner from less use. Weber made 

 various experiments for the purpose of determining the relative sensi- 

 bility of different portions of the skin, by touching the surface with 

 the legs of a pair of compasses, the points of which were inserted into 

 pieces of cork. The person's eyes being closed at the time, the legs 

 were brought together so as to be separated by different distances. 

 The following are some of the results of his experiments. 



Lines 



Point of middle finger 



Point of tongue 



Palmar surface of third finger - 1 



Red surface of lips - - 2 



Palmar surface of middle finger - 2 



Dorsal surface of third finger 3 



Tip of the nose - - 3 



Dorsum and edge of tongue 4 



Part of lips covered by skin - 4 



Palm of hand - 5 . 



Skin of cheek ... 5 



Extremity of great toe - - 5 



Hard palate ... 6 



Dorsal surface of fore finger 



Dorsum of hand ... 8 



Mucous membrane of gums 



Lower part of forehead - 



Lower part of occiput 



Back of hand 



Neck, under lower jaw - 



Vertex - 



Skin over patella 



Skin over sacrum 



acromion 



Dorsum of foot - 

 Skin over sternum 

 Skin beneath occiput 

 Skin over spine, in back 

 Middle of the arm 

 thigh 



Lines. 



9 



10 

 12 

 14 

 15 

 15 

 16 

 18 

 18 

 18 

 20 

 24 

 30 

 30 

 30 



Weber found, that the distance between the legs of the compasses 



1 See art. Tastsinn und das Gemeinge/uhl, in Wagner's Handworterbuch der Physiologie, 

 22ste Lieferung. s. 539. Braunschweig, 1849. His earlier experiments are detailed and 

 confirmed 'by Dr. Allen Thomson, in Edinb. Med. and Surg. Journal, for July, 1833. 



9 Lehrbuch der Physiologie des Menschen, ii. 565. Braunschweig, 1844; and Grundriss 

 der Physiologie, s. 331. Braunschweig, 1846. 



