Page:American Journal of Psychology Volume 21.djvu/555

Rh 1,307,254, assuming that the innervation is the same for both sides. Ingbert also counted the fibres in the ventral roots of the same cord and found the total number, for both sides, to be 407,400. This would make the ratio of sensory to motor greater than 3:1 and at once suggests James's figure of a "funnel" with the big end representing sense processes and the small end movements.

Of still greater interest from a psychological point of view is Ingbert' s table showing the number of fibres for each spinal segment. The curve for these figures shows in a striking manner that the number of fibres and the area of the nerves representing sensation from the more mobile and voluntarily controlled parts of the organism, particularly the arms, are very much greater in proportion to mass than they are for the less controlled organs. While there are no data demonstrating it in detail, roughly and in general, at least, one may say that sensory innervation is directly proportional to mobility and voluntary control. This position is supported by the experiments of Van Biervliet on "Le toucher et le sens musculaire." (L'Année Psy., 13, 1906, pp. 114-121.) Van Biervliet found that sensibility to touch is directly proportional to the mobility of the organ and that sensitivity increases with practice and skill in control.

Further light is thrown on these anatomical facts by Head's recent study of peripheral sensibility. ("The Consequences of Injury to the Peripheral Nerves in Man." Brain, 1905, pp. 116-338, and "The Afferent Nervous System from a New Point of View. " Ibid., pp. 99-115.) Head distinguishes three forms of peripheral sensibility:

I. "Deep sensibility, capable of answering to pressure and to the movements of parts, and even capable of producing pain under the influence of excessive pressure, or when the joint is injured. The fibres, subserving this form of sensation, run mainly with the motor nerves, and are not destroyed by division of all the sensory nerves to the skin." (p. 111.) "Deep sensation is not materially affected by the destruction of all the nerves to the skin, and it must reach the central nervous system by fibres that run in other channels than the so-called sensory nerves." (p. 215.)

II. "Protopathic sensibility, capable of responding to powerful cutaneous stimuli, and to the extremes of heat and cold. This is the great reflex system producing a rapid, widely diffused response, unaccompanied by any definite appreciation of the locality of the spot stimulated." (p. 111.)

III. "Epicritic sensibility, by which we gain the power of cutaneous localization, of the discrimination of two points, and of the finer grades of temperature, called cool and warm." (p. 111.)