III. THE SPECIAL SENSES. 



A. Vision. 



The Physiology of Vision. — The eye is the organ by means of which 

 certain vibrations of the lnminiferons ether are enabled to aifeet our conscious- 

 ness, producing the sensation which we call " light." Hence the essential part 

 of an organ of vision is a substance or an apparatus which, on the one hand, 

 is of a nature to be stimulated by waves of light, and, on the other, is so con- 

 nected with a nerve that its activity causes nerve-impulses to be transmitted to 

 the nerve-centres. Any animal in which a portion of the ectoderm is thus 

 differentiated and connected may be said to possess an eye — i. e. an organ 

 through which the animal may consciously or unconsciously react to the exist- 

 ence of light an Mind it. 1 But the human eye, as well as that of all the higher 

 animals, not only informs us of the existence of light, but enables us to form 

 correct ideas of the direction from which the light comes and of the form, color, 

 and distance of the luminous body. To accomplish this result the substance 

 sensitive to light must form a part of a complicated piece of apparatus capable 

 of very varied adjustments. The eye is, in other words, an optical instrument, 

 and its description, like that of all optical instruments, includes a consideration 

 of its mechanical adjustments and of its refracting media. 



Mechanical Movements. — The first point to be observed in studying the 

 movements of the eye is that they are essentially those of a ball-and-socket 

 joint, the globe of the eye revolving freely in the socket formed by the capsule 

 of Tenon through a horizontal angle of almost 88° and a vertical angle of about 

 80°. The centre of rotation of the eye (which is not, however, an absolutely 

 fixed point) does not coincide with the centre of the eyeball, but lies a little 

 behind it. It is rather farther forward in hypermetropic than in myopic eyes. 

 The movements of the eye, especially those in a horizontal direction, are sup- 

 plemented by the movements of the head upon the shoulder-. The combined 

 eye and head movements are in mosl persons sufficiently extensive to enable 

 the individual, without any movement of the body, to receive upon the lateral 

 portion of the retina the image of an object directly behind his back. The 

 rotation of the eye in the socket is of course easiest and most extensive when 

 the eyeball has an approximately spherical shape, as in the normal or emme- 

 tropic eye. Winn the antero-posterior diameter is very much longer than those 



1 In certain of the lower orders of animals no local differentiations seem to have occurred, 

 and the whole surface of the body appears to be obscurely sensitive to light. See Nagel : Der 

 lAchtxinn augenloser Thiere, Jena, 1896. 

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