46 PHYSIOLOGY CHAP. 



In reply to the first question, Bernstein assumes that functional 

 exercise increases central resistance to the spread of the excitations. 

 But this contradicts the generally accepted view that exercise 

 renders the paths of transmission less resistant to nervous excita- 

 tions. It seerns more reasonable to assume that the spread of the 

 excitations is increasingly limited, as the inhibitory powers of the 

 centres become developed. 



In attempting the solution of the second difficulty, it is not 

 enough to invoke the varying number of the tactile spots in various 

 regions of the skin, because as Kiesow showed (see pp. 38-39) the 

 extreme differences observed are never greater than 1 to 6-35, 

 while the maximal diameters of the so-called tactile circles vary 

 between 1 and 67 mm. To explain this fact on Bernstein's 

 theory it is necessary to assume a subsidiary 

 hypothesis, according to which the central 

 spread of excitations arising from the regions 

 in which the power of localisation is less 

 well developed must be enormously in excess 

 of the spread of excitations from regions in 

 which the localising power is more highly de- 

 veloped. The improbability of this surmise is 

 obvious. 



And, in reply to the third point, as the 

 hypothesis of the transmission of local signs 

 from the periphery to the centre cannot be 

 accepted, we are forced to assume that the 

 24. Aristotle's ex- power of localising contacts at the periphery is 

 there ^an iiiusion'of P ure ty an( ^ simply a consequence of the general 

 touching two separate law of the excentric projection of sensations. 

 ani e m?dSn t g h e e rs i . nde ' But this is merely the statement of a fact, not 



its scientific explanation. 



In regard to this law, again, it is a matter of controversy 

 whether the empirical or the nativistic theory should be applied to 

 it. Aristotle's well-known experiment favours the former. When 

 the index and middle fingers are crossed (Fig. 24) and an object 

 is placed between the finger-tips, there is an illusory sensation 

 of touching two distinct objects. The illusion is so strong that 

 it does not vanish when controlled by sight, and it increases if 

 the object is rolled between the fingers. Obviously this depends 

 on the fact that the sensitive skin -surfaces are in an unusual 

 position, owing to the crossing of the fingers. When the two 

 fingers are in their normal position, we cannot touch an object 

 simultaneously with the outer edges of the tips of the fore and 

 middle fingers ; two objects are required to produce the double 

 sensation. Hence the illusion of two objects on crossing the fingers 

 depends on the experience already impressed on our brain, which 

 has become the general rule of our perceptions, i.e. of the un- 



