SEC. 8. ON THE DEVELOPMENT WITHIN THE CENTRAL 

 NERVOUS SYSTEM OF VISUAL AND OF SOME OTHER 

 SENSATIONS. 



Visual Sensations. 



666. 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 reaching 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 con- 

 nected 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 attention 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 by 

 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 position of the eye, the rays of 

 light proceeding from a portion only of the external world fall 



