THE SENSES. 511 



points can be distinguished thus indicates the grade of sensibility at 

 that spot. The observations of Valentin* give the following as the 

 limits of distinct perception in different regions : 



DISTANCE AT WHICH Two POINTS MAT BE SEPARATELY DISTINGUISHED. 



At the tip of tongue 1.00 millimetre. 



" palmar surface of tips of fingers . . 1.50 " 



" " of second phalanges . 3.24 " 



" of first phalanges . 3.44 " 



" dorsum of tongue .... 5.22 ." 



" dorsal surface of fingers . . . 8.12 " 



" cheek 9.46 " 



" back of hand 14.50 " 



" skin of throat 17.27 " 



" dorsum of foot 26.10 " 



" front of sternum 33.07 " 



" middle of back 50.43 



This method does not necessarily measure the acuteness of sensi- 

 bility, since the two points might be less easily distinguished from each 

 other in any one region, and yet the intensity of the sensation produced 

 might be as great as in the surrounding parts ; but it affords an esti- 

 mate of the delicacy of tactile sensation, by which we distinguish 

 slight inequalities of surface in foreign bodies. There is reason, how- 

 ever, to believe that the two qualities correspond with each other in 

 development in various localities ; and tactile sensibility is frequently 

 found to be most delicate where the amount of sensation is also greatest. 

 A feeble galvanic current may be perceived at the tips of the fingers, 

 though it may produce no impression on the limbs or trunk ; and one 

 too faint to be distinguished by the fingers may be perceptible at the 

 tip of the tongue. 



Certain parts of the body, furthermore, are especially suitable for 

 organs of touch, not only from their acute sensibility, but also on 

 account of their conformation and mobility. In man, the hands are 

 the most favorably constructed for this purpose, owing to the varied 

 movement of the fingers, by which they may be applied to surfaces of 

 any form, and brought successively in contact with all their parts. 



In some animals, the long bristles on the lips are used for this pur- 

 pose, each bristle being connected at its base with a nervous papilla ; 

 and in the elephant the end of the nose, developed into a flexible and 

 sensitive proboscis, is the principal organ of touch. This function, 

 therefore, may be performed by any part of the body where the acces- 

 sory organs are sufficiently developed. 



In the head and face, the sensibility of the skin is dependent on the 

 branches of the fifth pair. In the body and limbs it is due to the sen- 

 sitive fibres of the spinal nerves. It exists, to a considerable extent, in 

 the mucous membranes of the mouth and nose, and other passages 



* Todd's Cyclopaedia of Anatomy and Physiology, vol. iv., article Touch. 



