196 RECENT PROGRESS OF THE THEORY OF VISION. 



an invagination of the integument of the embryo. A dimple 

 is first formed, this deepens to a round pit, and then expands 

 until its orifice becomes relatively minute, when it is finally 

 closed and the pit becomes completely shut off. The cells of 

 the scarf-skin which line this hollow form the crystalline lens, 

 the true skin beneath them becomes its capsule, and the loose 

 tissue which underlies the skin is developed into the vitreous 

 humour. The mark where the neck of the fossa was sealed is 

 still to be recognised as one of the ' entoptic images ' of many 

 adult eyes. 



The last defect of the human eye which must be noticed is 

 p IG 3 2 the existence of certain in- 



equalities of the surface 

 which receives the optical 

 image. Not far from the 

 centre of the field of vision 

 there is a break in the 

 retina, where the optic- 

 nerve enters. Here there 

 is nothing but nerve fibres 

 and blood-vessels ; and, as 

 the cones are absent, any 

 rays of light which fall on 

 the optic nerve itself are 

 unperceived. This ' blind 

 spot' will therefore pro- 

 duce a corresponding gap 

 in the field of vision, where nothing will be visible. Fig. 32 

 shows the posterior half of the globe of a right eye which has 

 been cut across. R is the retina with its branching blood-vessels. 

 The point from which these diverge is that at which the optic 

 nerve enters. To the reader's left is seen the ' yellow spot.' 



Now the gap caused by the presence of the optic nerve is no 

 slight one. It is about 6 in horizontal and 8 in vertical 

 dimension. Its inner border is about 12 horizontally distant 

 from the ' temporal ' or external side of the centre of distinct 



