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THE SIGNS OF LIFE [LECT. 



I shall do my best to talk in simple language, but I know 

 that we shall often be forced to fall back upon technical terms. 

 The ideal way would be, in first place, to have written a mono- 

 graph in the dry and freely technical language of a Philoso- 

 phical Transactions paper ; in second place, to retell the same 

 story here in the simple words and homely metaphors by which 

 we ordinarily attempt to tell each other what we are thinking 

 about. It will not be possible to me to strictly follow this ideal 

 way, but I shall do my best to justify narrative by reference to 

 duly accredited publications ; and as to technicalities, well, perhaps 

 some degree of physiological technicality may be excused in the 

 Physiological Laboratory of the University of London. 



Every one here is no doubt acquainted with the physiology 

 of vision, in so far as it is given in the text-books. You have 

 studied the eyeball as an optical instrument, containing a living 

 sensitive surface that receives the focussed (or unfocussed) radi- 

 ations of light. You have studied the course and the central 

 cortical terminus of the nerve fibres leading from retina to 

 brain. You know that the thing as seen by you is your 

 subjective picture, aroused by the objective retinal pattern of 

 the objective field of vision. You have proceeded to the closer 

 study of the objective retinal change from which the subjective 

 visual sensation takes origin, and have learned that the objective 

 effects of light acting upon the retina of an excised but sur- 

 viving eyeball are : a bleaching of its visual purple, a movement 

 of expansion of its pigment cells, a movement of contraction of 

 its cones, and, as in every case of physico-chemical change, an 

 accompanying electrical change. 



One more preliminary consideration. Our own retina, that 

 arouses in our own brain the detailed images of complicated 

 objects when detailed images are focussed upon it, gives rise 

 to diffuse sensations of light and to the fantastic appearances 

 called phosphenes when it is stirred into action by direct 

 mechanical or electrical stimuli. Mechanical pressure of the 

 eyeball, or an accidental blow upon it, or an electrical current 

 traversing it, anything, in short, that suddenly disturbs the retina, 

 elicits in consciousness the specific subjective symptoms of a 

 suddenly disturbed retina, formless or fantastic, a blaze of 



