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, cf. Boll 

 Sit zungsberi elite cler Akad. Berlin, 1876 and 1877, also E \vald and Kiihne. 



FIG. 84. Diagram of the auditory labyrinth. .1. of a 

 fish. II. of a bird. III. of a mammal (after Wal- 

 deyer). V, utricle with the three semicircular canals ; 

 S, saccule ; US, alveus communis ; C, cochlea ; L, la- 

 gena ; R, aqueductus vestibuli ; Cr, canalis reuniens. 



