Physiology of the special senses / by M. Greenwood, junr.

  • Greenwood, Major, 1880-1949.
Date:
1910
Catalogue details

Licence: Public Domain Mark

Credit: Physiology of the special senses / by M. Greenwood, junr. Source: Wellcome Collection.

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    rapidly alternating or oscillating stimuli, They do not coincide with pressure points, than which they are at least four times more numerous. The belief that cutaneous pain was induced by direct stimulation of nerve fibres, is contradicted by the fact that it can be elicited by the employment of stimuli too weak to act as ordinary nerve excitants, Further, weak mechanical stimuli, just above the liminal value, provoke a delayed sen- sation of pain, the delay being greater than can be accounted by the time required for the conduction of an impulse to the spinal cord, and greater than the interval which separates stimulus and response when the former is applied directly to a nerve fibre. Although the interpretation of these experiments is more dubious than some think, they are not inconsistent with the hypothesis of specific organs for cutaneous pain. V. Frey suggests that the free nerve endings of the skin are the organs in question, and the following confirmatory evidence is adduced :— (1) Diminution of the surface area stimulated does not diminish the effectiveness of the stimulus. | (2) The liminal value of an electrical stimulus is lower than that of any other form. | (3) Corrosives produce, first of all, a sensation of pain. These facts suggest that the pain end organs are more superficial than those of touch or temperature. The only nervous elements definitely superficial to the end bulbs supposed to be associated with touch are the free intra- epithelial nerve endings, hence they should be the required organs. 3 It has also been asserted that, in the cornea, the only receptive elements are the intra-epithelial nerve endings, and that from the cornea painful sensations alone can be elicited. V. Frey suggests that the actual excitant might be fluid squeezed from the cells into the inter-cellular spaces. Such fluid not being sufficiently concentrated owing to the im- permeability of the cell wall for certain salts, its low con- centration may render it a chemical stimulus for the nerve endings, Apart from other objections which can be raised,