CH. LV.l 



THE RETINA. 



749 



(optic disc or white spot) at which the optic nerve enters the 

 eyeball, and begins to spread out its fibres into the retina. 



The optic nerve passes forwards from the ventral surface of 

 the cerebrum towards the orbit enclosed in prolongations of the 

 membranes, which cover the brain. This external sheath at the 

 entrance of the nerve into the eyeball becomes continuous with 

 the sclerotic, which at this part is perforated by holes to allow 

 of the passage of the optic nerve fibres, the perforated part being 

 the lamina cribrosa. The pia mater here becomes incomplete, 

 and the subarachnoid and the 

 superarachuoid spaces become 

 continuous. The pia mater sends 

 in processes into the nerve to 

 support the fibres. The fibres 

 of the nerve themselves are 

 exceedingly fine, and are sur- 

 rounded by the myelin sheath, 

 but do not possess the ordinary 

 external nerve sheath. As they 

 pass into the retina they lose 

 their myelin sheaths and- proceed 

 as axis cylinders. Neuroglia 

 supports the nerve fibres in the 

 optic nerve trunk. In the centre 

 of the nerve is a small artery, 

 the arteria centralis retinae. The 

 number of fibres in the optic 

 nerve is said to be upwards of 

 500,000. The axis cylinders 

 pass on to the retina, turning 

 over the edges of the porus 

 opticus, to be distributed on the 

 inner surface of the retina, as 

 far as the ora serrata, as the 

 layer of optic nerve-fibres. 



The retina consists of certain elements arranged in ten layers 

 from within outwards (figs. 569, 570, 571). 



1. Memf/rana limitans interna: This so-called membrane in 

 contact with the vitreous humour is formed by the junction 

 laterally of the bases of the wstentacular or supporting filtres of 

 Miillrr, which bear the same relation to the retina as the 

 neuroglia does to the brain. The character of these fibres may 

 be seen in fig. 570. 



2. Optic nervf fibres. This layer is of very varying thickness 



Fig. 570. Diagram showing the susten- 

 tacular fibres of the retina ; /, flbre- 

 baxket above the external limiting 

 membrane ; m, nucleus of the fibre ; 

 r, base of the fibre. (From McKen- 

 drick, after SUihr.) 



