252 THE EVOLUTION OF MAN. 



the tear-glands, the secretion of which keeps the surface 

 of the eye smooth and clean. 



Directly under the protective membrane is a delicate 

 darkrred, highly vascular membrane, the vascular mem- 

 brane (choroidea, e), and within this the retina (0), which 

 is a dilatation of the optic nerve (i). This latter is the 

 second brain nerve. It extends from the " centre of sight " 

 (the second brain-bladder) to the eye, penetrates the outer 

 coats of this, and then extends, as the retina, between the 

 vascular membrane (choroidea) and the vitreous body 

 (corpus vitreum). Between the retina and the vascular 

 membrane lies another very delicate membrane, which 

 is commonly, but wrongly, considered as part of the latter. 

 This is the black pigment membrane (pigmentosa, lamina 

 pigTuenti, n), or the " black carpet " (tapetum nigrum). 

 It consists of a single layer of beautiful hexagonal cells 

 accurately joined together and filled with black pigment 

 gi'anules. This pigment membrane lines, not only the inner 

 surface of the actual vascular membrane, but also the pos- 

 terior surface of its anterior muscular prolongation, which, 

 as a circular ring-like membrane, covers the edge of the lens, 

 and prevents the penetration of lateral rays. This is the 

 well-known "rainbow membrane" (iris, /i), which is differently 

 coloured in different persons (blue, gray, brown, etc.). This 

 " rainbow membrane " is the limit towards the front of 

 the vascular membrane. The round hole in the iris is the 

 pupU, through which the rays of light pass into the interior 

 of the eye. Where the iris proceeds from the edge of the 

 actual vascular membrane, the latter is much thickened and 

 forms a beautiful ciliated crown (corona ciliaris, g), which 

 surrounds the edge of the lens with about seventy large, 

 afid mamj smaller rays. 



