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VII.—On the Physiological Action of Light. By JAMes Dewar and JoHN Gray 
M‘Kenpricx, M.D. (Part I. Plates X., XI.) 
(Read 21st April, 5th May, and 2d June 1873.—Received, 6th February 1874.) 
I. INTRODUCTION. 
Sensory nerves are divided into two classes, those of general sensibility and 
those of special sense. The nerves of general sensibility are distributed to the 
skin, muscles, or viscera, and convey influences to the brain which give rise to 
sensations of touch, heat, &c., or to those vague sensations, not definitely 
localised, which we include under the name of the muscular sense. The nerves 
of special sense are endowed with special and individual physiological properties. 
When a nerve of this order is irritated in any way, either by mechanical, chemi- 
cal, or electrical stimuli, an influence is conveyed to the brain which gives rise 
to the same kind of sensation as that produced by the normal stiraulus on the 
terminal organ. For example, pressure on the eyeball, as shown by Newton 
and Youn«, electrical stimulation by a continuous current, as demonstrated by 
Prarr,* HeLtmyorrz,t Rirrer,{ PurKinJe,§ Du Bors-ReyMmonp,|| and ScHELSKE,1 
produce many of the phenomena of vision, including not only the perception of 
light, but the perception of various colours and tints. But while this is the case, 
it is equally certain that each terminal organ responds to its normal stimulus. 
Thus the retina, though capable of stimulation by pressure or electricity, is spe- 
cially fitted by its histological] structure for the reception of those minute vibra- 
tions of the ether which constitute light. But while the terminal organ is capable 
of receiving a most delicate action of the normal stimulus, the nerve in connec- 
tion with it is not so affected. For example, although the retina is affected by 
light, the optic nerve is not so, as may be proved by Marriorre’s** well-known 
experiment, by which it may be demonstrated that when the image of an 
external object falls on the entrance of the optic nerve, there is no correspond- 
ing sensation. ‘The nerve is thus insensible to the normal stimulus of the sense 
organ, the retina. The current in the nerve cannot be excited by the direct 
_ * Thierische Elektricitat, 1795, p. 142. 
t Physiologische optik, p. 203. 
{+ Beitrage zur naheren kenntniss der Galvanismus und der Resultate seiner Untersuchung. Jena, 
1805, p. 159, et seq. 
§ Beobachtungen iiber Versuche zur Physiologie der Sinne. Band i. 
|| Thiersche Elektricitaét. Band i. p. 353. 
‘| Zur Farben-empfindung. Graefes Archives ix. p. 49. 
** Phil. Trans. 1668, p. 668; 1670, p. 1023. 
VOL. XXVII. PART I. 
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