218 PRINCIPLES OF PHYSIOLOGY 



ized form of nervous epithelium, which, in its 

 turn, is continued into neurones, and these 

 neurones ultimately, with probably inter- 

 mediary neurones, end in neurones in the 

 cerebrum. This is well seen in the retina, 

 where the specialized receptive epithelium 

 forms the remarkable layer of rods and cones 

 (Jacob's membrane) ; the rods and cones and 

 other structures of the retina are supported by 

 modified epithelium, forming structures called 

 the Mullerian fibres ; the layers of granules 

 in the retina and the layer of large multipolar 

 nerve cells constitute the neurones. From 

 these latter arise the fibres of the optic nerve, 

 which, as already pointed out, carry impulses 

 to the corpora quadrigemina, and thence to 

 he cerebrum. 



127. The nerve of special sense is normally 

 stimulated by the end organ, but it may be 

 stimulated in other ways, as by pressure or 

 electric shock. Thus pressure on the eyeball 

 will give rise to dazzling impressions of light 

 (phosgenes). But the law is that in whatever 

 way the fibres of the nerve of special sense is 

 stimulated, the sensation is always of the 



