SEC. 4. ON THE DEVELOPMENT WITHIN THE CEN- 

 TRAL NERVOUS SYSTEM OF VISUAL AND OF 

 SOME OTHER SENSATIONS. 



Visual Sensations. 



494. In the chain of events through which some influence 

 brought to bear on the periphery of a sensory nerve gives rise to 

 a sensation, we are able, with more or less success, to distinguish 

 between those events which are determined by the changes at 

 the periphery and those which are the expression of changes 

 induced in the central nervous system. Thus when certain 

 rays of light proceeding from an object and falling upon the eye 

 give rise to visual perception of the object, two sets of events 

 happen ; the rays of light, by help of the mechanisms of the eye, 

 partly dioptric, partly nervous, give rise to certain changes in the 

 fibres of the optic nerve, which we may call visual impulses ; and 

 these visual impulses reaehing the brain along the optic nerve 

 give rise to visual sensations and so to visual perception of the 

 object. We shall later on, under the heading of " the senses," 

 deal chiefly with the peripheral events, and have now to consider 

 some points connected with the central events, to learn what we 

 know concerning how the various sensory impulses travelling 

 along the several kinds of sensory nerves behave within the 

 central nervous system. In doing so we shall have from time 

 to time to refer to peripheral events, but only occasionally, and 

 never in any great detail. It will be convenient to begin with 

 the special sense of sight, and we must first briefly call atten- 

 tion to a few points which we shall have to study in fuller 

 detail hereafter. 



The eye is so constructed that images of external objects are 

 brought to a focus on the retina, the stimulation of which In 

 light starts the visual impulses along the fibres of the optic 

 nerve ; and the distinctness with which, by means of the visual 

 sensations arising out of these visual impulses, we perceive 

 external objects is dependent on the sharpness of the retinal 

 images. The eye is further so constructed that, in any posi- 



781 



