44 PHYSIOLOGY CHAP. 



contact but also the sign of the place at which it occurs, which 

 he calls local colour, to be used in consciousness as a local sign. 

 The local colour of the excitations aroused in the skin is gradually 

 differentiated as between one place and another by phylogenesis 

 and by exercise. So long as the difference is slight it is not 

 perceived in consciousness, and the two simultaneous impressions 

 from adjacent points of the skin may fuse into one. But when 

 the difference in local colour increases, because the two impressions 

 arise from more widely separated points, both are appreciated. 

 With exercise and attention it becomes possible to perceive differ- 

 ences in local colour that are not habitually noticed. This explains 

 why the sensory cutaneous areas may be educated by practice. 



This hypothesis is not a scientific explanation; it merely 

 substitutes metaphor for fact, with a view to making it more 

 acceptable. On the other hand, it is open to a grave objection : 

 what has been said above shows that recognition of the place from 

 which a sensation of contact arises is a function of the perceptive 

 centre, and depends, as Johannes Mliller showed and as is con- 

 firmed by later researches, on its specific energy. The nerves 

 merely transmit an excitation or nervous vibration which is 

 common to all the sensations ; they do not transmit any quality, 

 colour, or sign of recognition from the part touched. 



Bernstein formulated an ingenious hypothesis to account for 

 the phenomena observed on applying Weber's compasses to the 

 skin. He held that when the excitation aroused in the skin by 

 contact reaches the cortical centre it spreads more or less widely, 

 as occurs in the periphery with sensations of pain. On the 

 neurone theory this central spread of excitations coming from 

 the periphery is a natural consequence of the fact assumed by 

 Ramon y Cajal that each sensory fibre terminates at the centre 

 in an arborescence ; but even on Golgi's theory of the diffuse 

 fibrillary network, which serves as a vehicle of central com- 

 munication, it may be admitted that nervous activity spreads 

 more or less widely through the meshes of the network according 

 to the intensity of the stimulus. When two adjacent points of 

 the cutaneous surface are touched, the two excitations on reaching 

 the central surface spread and summate into a single excitation, 

 which culminates in a point equidistant from the points of 

 arrival from the two fibres (or groups of fibres) stimulated. In 

 this case, therefore, there is only a single sensation of contact. 

 When, on the contrary, the two points of contact are farther 

 apart, the two excitations on reaching the centre do not summate, 

 but two distinct apices are formed, which correspond with the 

 points of arrival of the stimuli from the two fibres (or groups of 

 fibres) stimulated. In this case, therefore, both contacts are 

 distinctly perceived. Bernstein's theory is clearly illustrated by 

 the geometric diagram (Fig. 23). 



