Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 BEFRACTILE MEDIA AND PIGMENT. 87 tion of a cutaneous nerve which is sensitive only to gradations of temperature. Although the sensation of light is the function of the nerve centre, the rods and cones at the end of the optic nerve fibres are the elements which convert the external movement of the ether waves into an excitation of the optic nerve fibres adequate for the production of the sensation of light. For the perception of an image refractile apparatuses in front of the terminal exp
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 BEFRACTILE MEDIA AND PIGMENT. 87 tion of a cutaneous nerve which is sensitive only to gradations of temperature. Although the sensation of light is the function of the nerve centre, the rods and cones at the end of the optic nerve fibres are the elements which convert the external movement of the ether waves into an excitation of the optic nerve fibres adequate for the production of the sensation of light. For the perception of an image refractile apparatuses in front of the terminal expansion of the optic nerve (retina) are necessary; and further, the elements of the latter imist be sufficiently isolated to admit of the stimuli set up in them being carried as separate movements to the nerve centre. Instead of a general sensation of light a complex sensation made up of many separate perceptions is produced, which corre- spond in position and quality with the parts of the exciting source. For the refraction of the light convex and often lens- shaped thickenings of the body covering (cor- nea, corneal lens) through which the rays pass into the eye, are developed ; refractile bodies are also found behind the cornea (lens, crystalline cone). The rays diverging from the various parts of the source of the light are, by means of the refractile media, collected and brought to corresponding foci on the retina or peripheral expansion of the optic nerve, which consists of the rod-shaped ends of the nerve fibres and some more or less complicated ganglionic structures. Lately, in consequence of the discovery of the visual purple * in the outer segments of the rods, it has been attempted to reduce the excitation of the end apparatus of the optic nerve to a photo-chemical process taking place in the retina. The fact that the diffuse pigment (visual purple) of the outer segments of the rods is bleached by the f In addition to the older works of Krohn, H. Miiller, M. Schultze,
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