VISION 339 



mented cells are connected with each of the half-million or more fibers 

 of each optic nerve. Many details of the structure of the rods and the 

 cones are known, for knowledge of which the reader is referred to the 

 special literature. At present, however, these facts of structure throw 

 little or no light on the real nature of vision. We can see the rods and 

 cones and the neurones beneath them, but as to their actions we know 

 nothing. 



The f ovea centralis is the center of the macula lutea, and is the spot 

 on which the visual image has to be focused whenever the object is to 

 be clearly seen. This focusing is accomplished by several sorts of 

 complex movements. 



The Various Adjustments of the Eyes. As has been just suggested, the 

 fovea centralis, a spot only a few millimeters in diameter at the inner 

 end of the visual axis (not of the optical axis) of each eye, is the only part 



FIG. 193 



The extrinsic eye-muscles, and the eye-balls' axes of rotation. (Gariel.) 



of the whole retina at which vision is quite distinct. In other words, the 

 eyes must be continually so directed toward any point of an object 

 which is to be clearly seen that the rays of light therefrom shall fall 

 exactly on this narrow spot, the fovea. There must therefore be some 

 adequate arrangement for quickly and reflexly directing the eyes in 

 exactly the right direction if vision is to be useful. Moreover, the intensity 

 of the light entering the eyes and the distance of the objects looked at 

 vary greatly and there is required some means of adjustment to these 

 conditions also. The former sort of adjustments are made by move- 

 ments of the head and of the eyes within the head, and the latter kind by 

 means of the accommodating mechanism proper (the iris, ciliary muscle, 

 etc.), assisted sometimes by the muscles of the eyelid. 



HEAD-MOVEMENTS for visual purposes scarcely need detailed descrip- 

 tion. The possible movements of the head upon the axis at the top of 



