VISUAL AND OTHER SENSATIONS. 689 



Lastly, without attempting to enter into psychological questions, we may 

 at least say that the birthplace of what we call the " will," is not conter- 

 minous with the motor area ; the will arises from a complex series of events, 

 some of which take place in other regions of the cortex, and probably in 

 other parts of the brain as well. With these parts the motor area has ties 

 concerned not in the carrying out of volition, but in the generation of the 

 will. So that, looking round on all sides, it is obvious, as we have said, that 

 the motor area is a mere link in a complex chain. It is, moreover, a link of 

 such a kind, that while the changes which the breaking of it makes in the 

 daily life of a lowly animal, such as the dog, in whom the experience of the 

 individual adds relatively little to the nervous and psychical storehouse 

 transmitted from his ancestors, can hardly be appreciated by a bystander, 

 those which the breaking of it makes in the daily life of a man, whose brain 

 at any moment is not only a machine fitted for present and future work but 

 a closely packed record of his past life, are obvious not only to the individual 

 himself, but to his fellows. 



ON THE DEVELOPMENT WITHIN THE CENTRAL NERVOUS SYSTEM OF 



VISUAL AND OF SOME OTHER SENSATIONS. 



Visual Sensations. 



579. In the chain of events through which some influence brought to 

 bear on the periphery of a sensory nerve gives rise 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 expres- 

 sion 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 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 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 im- 

 pulses 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 upon the 

 retina ; or in other words, in any one position of the eye only a portion of 

 the external world is visible at the same time. The portion so seen is spoken 

 of as the visual field for that position. 



The image thrown on the retina is an inverted one, so that the top of an 

 actual object is represented by the lower, and the bottom by the upper, part 



44 



