THE EYE AS Atf OPTICAL INSTKUMENT. 221 



remarkable proof of the way in which we observe, or fail 

 to observe, the impressions made on our senses, that these 

 muscce volitantes often appear some-thing quite new and 

 disquieting to persons whose sight is beginning to suffer 

 from any cause ; although, of course, there must have been 

 the same conditions long before. 



A knowledge of the way in which the eye is developed 

 in man and other vertebrates explains these irregularities 

 in the structure of the lens and the vitreous body. Both 

 are produced by 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 oft*. 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 the existence of certain inequalities 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 un- 

 perceived. This 'blind spot' will therefore produce a corre- 

 sponding 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. E 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.' 



