GENERAL SENSIBILITY. 595 



consist of a very thin, external envelope of connective tissue, inclosing, 

 as in the tactile corpuscle, a mass of homogeneous or finely granular 

 substance. The medullated nerve fibre which penetrates the bulb, loses 

 its medullary layer at its entrance, and runs through the central homo- 

 geneous substance, to terminate by a free extremity near its apex. 

 Both the tactile corpuscles and the terminal bulbs are therefore anatomi- 

 cal forms, in which the axis cylinder of the sensitive nerve fibre termi- 

 nates, after divesting itself of its medullary layer. 



The tactile sensibility varies considerably in different regions of the 

 integument. The best method of appreciating this variation is that 

 adopted by Weber and Valentin. It consists in applying to different 

 parts the points of a pair of compasses, tipped with suitable pieces of 

 cork. If these points be applied to the skin when fixed at very short 

 distances apart, the two sensations cannot be accurately distinguished 

 from each other but are blended into one ; and the impression thus pro- 

 duced is that of a single contact. The minimum distance at which the 

 two points can be distinguished by the integument thus becomes a 

 measure of its sensibility at that spot. The observations of Valentin, 1 

 which are the most varied and complete in this respect, give the follow- 

 ing as the limits of distinct perception in different regions : 



DISTANCE AT WHICH TWO POINTS MAY 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 give an absolute measure of tho 

 aculeness of sensibility in the different regions, since the two points 

 might be less easily distinguished from each other in any one region, 

 and yet the absolute amount of sensation produced might be as great 

 as in the surrounding parts; but it undoubtedly affords an accurate 

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

 slight inequalities in the surface of solid bodies. There is every reason 

 to believe that the two qualities of delicacy and acuteness of local sensi- 

 bility correspond with each other in their degree of development in 

 various localities; since the regions where tactile sensibility is most 

 delicate are frequently found to be also those where the amount of 

 sensation is the greatest. A feeble galvanic current msy be perceived 



1 In Todd's Cyclopaedia of Anatomy and Physiology, vol. iv., article on Touch. 



