PROJECTION OF " TOUCH." 



941 



nervous system of the sensory channels for a patch of the palm is 

 greater than for an equal patch from the shoulder. It has been pointed 

 out that in the limb the amount of sensory root overlap of distribution 

 corresponds broadly with the degree of " local sign." But in other parts 

 there are striking exceptions to this, e.g. the tongue ; though there again 

 the area of embouchment of the sensory channels into the grey matter 

 is very large (descending root of fifth cranial). The paths of the 

 spinal columns, including the tactual, embouch into and diffuse in 

 the spinal grey matter. It is the wide spatial extent of the deep (central) 

 connections of tactual paths, after their entrance into the cord and brain, 

 that probably form an essential character of the physiological mechanism, 

 correlated with the development of " local sign." 



That the determination of the average liminal distance is by central 

 as well as by peripheral factors, is shown not only by the rapid effect of 



Fig. 369. — Map of the skin distributions of the seventh, eighth, ninth, and tenth 

 afferent nerve roots of the frog. The test used in determining the distribu- 

 tions was occurrence or absence of response (movement) of the "spinal" 

 animal to mechanical stimuli applied to the skin when all but one of the 

 roots in question had been severed. — Method of remaining resthesia. 



local " education," and by the local reference of tactual nerve fibres 

 stimulated in their course, but also bv the small increase of the distance 

 when the skin is stretched even to twice its unstretched length (see 

 above, p. 937), and by its general increase under mental fatigue. 



Of two lines seen under like conditions, a longer may be regarded 

 as a greater stimulus than a shorter. A surface of 2 cms. in contact 

 with the skin may be considered, cccteris paribus, a greater stimulus 

 than a surface of but 1 cm. That Weber's law holds for comparison 

 of pressures applied to skin, does not imply that it will hold for com- 

 parison of areas applied to the skin. The examination of the scope of 

 the " law " in the latter direction has been less followed than in the 

 former in its application to " touch." Fechner concluded that the law 

 is inapplicable for areas of contact. 1 



Projection of "touch." — Sense perceptions inform at first hand 

 of alterations in the condition of sensifacient organs. Since these 

 alterations are, under normal circumstances, habitually occasioned by 

 external objects, we habitually consider many of them as qualities of 



1 "Elem. d. Psychophysik." 



