488 SPECIAL SENSES. [BOOK in. 



tiatioii of the simple uniform superficial cell into a number of cells, 

 each of which was more susceptible to particular stimuli than its 

 fellows. Thus one cell, or rather one group of cells, would become 

 eminently susceptible to the influence of light : in them the impact 

 of rays of light would give rise to nervous impulses more readily 

 than in the other groups ; another group would develope a sensi- 

 tiveness to waves of sound, and so on. In this way the primary 

 homogeneous bodily surface would be differentiated into a series of 

 seme-organs, disposed and arranged among ectodermic cells, the 

 purpose of the latter being simply protective, and therefore not 

 demanding the existence of any direct connection with the central 

 nervous system. Similar but less highly marked differentiations 

 would be established in the endings of the afferent nerves connect- 

 ing the central nervous system with the internal surfaces and parts 

 of the body. 



Moreover it is obvious that the sensory impulses transmitted to 

 the central nervous system by these differentiated sense-organs will 

 probably be themselves largely differentiated. Just as the impulses 

 which pass along a motor nerve differ according to the nature of the 

 stimulus which is applied to the nerve (whether, for instance, the 

 stimulus be a single induction-shock, or several shocks repeated 

 slowly, or several shocks repeated rapidly, and so on, the effect on 

 the muscle being in each case a different one), so also and even to a 

 much greater degree do the impulses generated by light in a visual 

 sense-organ in all probability differ from those generated by simple 

 pressure in a tactile sense-organ. 



And since these various sensory impulses have much work to 

 perform on arriving at the central nervous system, in the way of 

 influencing the multitudinous molecular operations going on in the 

 central cells, and of affecting consciousness, this differentiation of 

 sensory organs and sensory impulses will naturally be accompanied 

 by a corresponding differentiation of those central cells which the 

 impulses first reach on arriving at the central organ. Those cells, 

 for instance, of the central nervous system, which first receive the 

 particular nervous impulses coming from the visual sense-organs, 

 will be set apart for the task of so modifying and preparing those 

 impulses as to adapt them in the best possible way for the work 

 which they have to do. Hence each peripheral sense-organ will be 

 united by means of its nerve with a corresponding central sense- 

 organ, the former being able to affect various parts of the central 

 nervous system only through the medium of the latter. And we 

 have evidence, at least as far as relates to all the central nervous 

 operations in which consciousness is concerned, that such central 

 sense-organs do really exist. For of the total characters which 

 belong to an affection of consciousness by means of any of the 

 sense-organs, i.e. which belong to any special sensations, we find 

 that while some are gained during the rise of the sensory impulses 

 in the peripheral sense-organ, others first appear in the central 



