CIT. LVI.] 



THE RETINA 



771 



serrata, but is really represented by the uvea to the very margin 

 of the pupil. The nerve-cells in the retina remind us that the optic, 

 like the olfactory nerve, is not 

 a mere nerve, but an outgrowth 

 of the brain. 



In the centre of the retina 

 is a round yellowish elevated 

 spot, about 2* T of an inch (1 mm.) 

 in diameter, having a depression 

 in the centre, called after its 

 discoverer the macula lutea or 

 yellow spot of Scemmering. The 

 depression in its centre is called 

 the fovea centralis. About -^ of 

 an inch (2 '5 mm.) to the inner 

 side of the yellow spot, is the 

 point (optic disc or white spot) at 

 which the optic nerve leaves the 

 eyeball. The optic nerve-fibres 

 are the axons of the nerve-cells 

 of the retina; the dendrons of 

 these cells ultimately communi- 

 cate with the visual nerve-epi- 

 thelium (rods and cones). 



The optic nerve passes back- 



Fio. 571. A section of the retina, choroid, and part 

 of the sclerotic, moderately magnified ; a, 

 Membrana limitans interna ; b, nerve-fibre layer 

 traversed by Miiller's sustentacular fibres ; c, 

 ganglion cell layer ; d, internal molecular layer ; 

 e, internal nuclear layer ; /, t xternal molecular 

 layer; g, external nuclear layer; h, membrana 

 limitans externa, running along the lower part 

 of i, the layer of rods and cones ; k, pigment 

 cell layer ; I, m, internal and external vascular 



portions of the choroid, the first containing 

 capillaries, the second larger blood-vessels, cut 

 in transverse section ; n, sclerotic. (W. Pye.) 



wards to the ventral surface of 

 the cerebrum enclosed in pro- 

 longations of the membranes, 

 which cover the brain. This ex- 

 ternal sheath at the exit of the 

 nerve from 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 superarachnoid 

 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 surrounded by the myelin sheath, but 

 do not possess the ordinary external nerve sheath. In the retina 

 itself they have no myelin sheaths. 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 retftia consists of certain elements arranged in ten layers 

 from within outwards (figs. 571, 572, 573). 



