CHAPTEE VII 



RETINAL EXCITATION AND VISUAL STIMULATION 



CONTENTS. 1. Histological structure of retina. 2. Direct and indirect vision 

 (blind spot, retinal elements the receptors of light -stimuli). 3. Objective 

 phenomena of retinal stimulation (visual purple, migration of pigment, contraction 

 of epithelial cells and inner segment of cones, electromotive phenomena). 4. 

 Colour visron ; effects produced by different kinds of luminous radiation ; limits 

 of their visibility. 5. Visual acuity ; phases of visual sensations ; positive and 

 negative after-images. 6. Retinal adaptation to light and darkness ; sensibility 

 of central and peripheral regions of retina to day vision and twilight vision 

 (photopia and scotopia). 7. Achromatic and chromatic perceptions in relation to 

 intensity of light stimulus and retinal adaptation to light and darkness. 8. 

 Duplicity theory of functions of rods and cones. 9. Colour mixtures ; comple- 

 mentary colours. 10. Colour contrast ; successive and simultaneous. 11. 

 Theories of achromatic and chromatic vision. 12. Colour blindness ; partial and 

 total. Bibliography. 



THE function of the eye, as the peripheral organ of vision, depends 

 on the Retina, i.e. that portion of the inner coat of the eye-ball 

 which is formed by the terminal expansion of the optic nerve. 

 Light is its adequate stimulus. Very weak luminous stimuli, as 

 those that come from the stars, suffice to excite the retina, 

 whereas the strongest rays of the sun are ineffective as light- 

 stimuli when they act directly upon cells and nerve-fibres in 

 general. It is accordingly necessary to assume in the end-organ 

 of the optic nerve the presence of a special apparatus (comparable 

 to some extent with a sensitive photographic plate) containing 

 photochemical substances which are capable of liberating potential 

 energy, and thus of producing more effect than the ether- vibrations 

 that act as stimulus. 



The first problem in discussing the physiological functions of 

 the retina is to ascertain which of the numerous elements of the 

 retinal tissue are sensitive to light and capable, in consequence 

 of the luminous vibrations of the ether, of liberating the energy 

 required to throw the complex neural apparatus of vision into 

 activity. At the outset, therefore, we have to study the histology 

 of the retina. 



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