THE SENSE OF SIGHT. 



639 



182. 



vessels, it results from the sudden spreading out or ex- 

 pansion of the optic nerve, of whose terminal fibres, appa- 

 rently deprived of their external white substance, together 

 with nerve-cells, it is essentially composed. 



Exactly in the centre of the retina, and at a point thus 

 corresponding to the axis of the 

 eye in which the sense of vision 

 is most perfect, is a round yellow- 

 ish elevated spot, about -r of 

 an inch in diameter, having a 

 minute aperture at its summit, 

 and called after its discoverer the 

 yellow spot of Swmmering. It is 

 not covered by the fibrous part 

 of the retina, but a layer of 

 closely-set cells passes over it, 

 and in its centre is a minute 

 depression called fovea centralis. 

 About T \j- of an inch to the 

 inner side of the yellow spot, and 

 consequently of the axis of the 

 eye, is the point at which the 

 optic nerve spreads out its fibres 

 to form the retina. This is the 



* Fig. 182. Vertical section of a small part of the retina (after Kolliker), 

 -". A, entire section of a small part of the retina ; B, two cones re- 

 presented separately in their connection with the fibres of Miiller and 

 other structures ; C, two rods represented separately in their connection 

 with the granules, fibres of Miiller and the nerve-cells ; I, columnar 

 layer ; , in A and C. the rods, in B, the terminal part of the cone ; &, 

 cones ; 2, granular layer ; c, outer layer of nuclei (striated corpuscles of 

 Henle) ; d, inner layer of nuclei ; /, inter-nuclear layer ; 3, nervous 

 layer ; g, fine molecular substance outside h. the nerve-cells ; k, nerve 

 fibres ; 2, membrana limitans ; c, inner ends of the fibres of Miiller 

 resting on the limiting membrane. 



