70 PIGMENT EPITHELIUM. [BOOK in. 



the attachment of the retina to the epithelium is much less, and 

 the retina can be more readily removed so as to leave the pigment 

 epithelium adherent to the choroid. 



Urari has an effect on these cells of the pigment epithelium 

 of such a kind that they cease to throw out their processes ; they 

 seem to be paralysed. Hence in the eye of a urarized animal the 

 pigment epithelium readily separates from the retina. 



We may add that in frogs at least, this shifting of the pigment 

 may be seen to be accompanied by a change of form in the inner 

 limbs of the cones. Under the influence of light the inner limb 

 becomes shorter and broader, in fact contracts, and when the 

 influence of the light is removed elongates to its original length. 

 Moreover these changes in the cones may be induced, not only by 

 light falling on the retina but also, through a mechanism not at 

 present fully understood, as the result of stimulation of the skin, 

 by light or otherwise ; in these latter cases the change of form of 

 the cone is not necessarily accompanied by migration of the 

 pigment. 



