802 THE POPULAR SCIENCE MONTHLY. 



Now if acquired structural traits are inheritable, the various 

 contrasts above set down are obvious consequences ; for the gra- 

 dations in tactual perceptiveness correspond with the gradations 

 in the tactual exercise of the parts. Save by contact with clothes, 

 which present only broad surfaces having but slight and indefi- 

 nite contrasts, the trunk has but little converse with external 

 bodies, and it has but small discriminative power ; but what dis- 

 criminative power it has is greater on its face than on its back, 

 corresponding to the fact that the chest and abdomen are much 

 more frequently explored by the hands : this difference being 

 probably in part inherited from inferior creatures, for, as we may 

 see in dogs and cats, the belly is far more accessible to feet and 

 tongue than the back. No less obtuse than the back are the mid- 

 dle of the back of the neck, the middle of the forearm, and the 

 middle of the thigh ; and these parts have but rare experiences of 

 irregular foreign bodies. The crown of the head is occasionally 

 felt by the fingers, as also the back of one hand by the fingers of 

 the other ; but neither oj: these surfaces, which are only twice as 

 perceptive as the back, is used with any frequency for touching 

 objects, much less for examining them. The lower part of the 

 forehead, though more perceptive than the crown of the head, in 

 correspondence with a somewhat greater converse with the hands, 

 is less than one third as perceptive as the tip of the nose ; and 

 manifestly, both in virtue of its relative prominence, in virtue of 

 its contacts with things smelt at, and in virtue of its frequent 

 acquaintance with the handkerchief, the tip of the nose has far 

 greater tactual experience. Passing to the inner surfaces of the 

 hands, which, taken as wholes, are more constantly occupied in 

 touching than are the back, breast, thigh, forearm, forehead, or 

 back of the hand, Weber's scale shows that they are much more 

 perceptive, and that the degrees of perceptiveness of different 



If we figure to ourselves the separate sensitive areas which severally yield independent feel- 

 ings, as constituting a network (not, indeed, a network sharply marked out, but probably 

 one such that the ultimate fibrils in each area intrude more or less into adjacent areas, so 

 that the separations are indefinite), it is manifest that when, with exercise, the structure 

 has become further elaborated, and the meshes of the network smaller, there must be a 

 multiplication of fibers communicating with the central nervous system. If two adjacent 

 areas were supplied by branches of one fiber, the touching of either would yield to con- 

 sciousness the same sensation : there could be no discrimination between points touching 

 the two. That there may be discrimination, there must be a distinct connection between 

 each area and the tract of gray matter which receives the impressions. Nay more, there 

 must be, in this central recipient tract, an added number of the separate elements which, 

 by their excitement, yield separate feelings. So that this increased power of tactual dis- 

 crimination implies a peripheral development, a multiplication of fibers in the trunk-nerve, 

 and a complication of the nerve-center. It can scarcely be doubted that analogous changes 

 occur under analogous conditions throughout all parts of the nervous system — not in its 

 sensory appliances only, but in all its higher co-ordinating appliances up to the highest. 





