Gil LVI1I.] 



THE RETINA 



815 



nerve-cells of the retina; the dendrons of these cells ultimately 

 communicate with the visual nerve-epithelium (rods and cones). 



The optic nerve passes backwards to 

 the ventral surface of the brain enclosed 

 in prolongations of the membranes, which 

 cover the brain. This external 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 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 

 retince. The number of fibres in the optic 

 nerve is said to be upwards of 500,000. 



The retina consists of certain ele- 

 ments arranged in ten layers from within 

 outwards (figs. 513, 514, 515). 



1. Membrana limitans internet. This 

 so-called membrane in contact with the 

 vitreous humour is formed by the junction 

 laterally of the bases of the sustentacular 

 or supporting fibres of Midler, which bear 

 the same relation to the retina as the 

 neuroglia does to the brain. The char- 

 acter of these fibres may be seen in 

 fig. 514. 



2. Optic nerve-fibres. This layer is of very varying thickness in 

 different parts of the retina : it consists of non-medullated fibres 

 which interlace, and most of which are the axons of the large 

 nerve-cells forming the next layer. The fibres are supported by the 

 sustentacular fibres. They are less and less numerous anteriorly, 

 and end at the ora serrata. They all converge towards the optic 

 disc, where they leave the retina as the optic nerve. 



3. Layer of ganglion cells. This consists of large multipolar nerve- 

 cells with large and round nuclei, forming either a single layer, or in 

 some parts of the retina, especially near the macula lutea, where this 

 layer is very thick, it consists of several strata of nerve- cells. They 

 are arranged with their single axis-cylinder processes inwards. 

 These pass into and are continuous with the layer of optic nerve- 



FIG. 513. Diagrammatic section of 

 human retina (M. Schultze). 1, Mem- 

 brana limitans interna; 2, layer of 

 optic nerve-fibres ; 3, layer of optic 

 nerve-cells ; 4, inner synapse or mole- 

 cular layer; 5, inner nuclear or 

 bipolar layer; 6, outer synapse or 

 molecular layer; 7, outer nuclear 

 layer; 8, membrana limitans ex- 

 terna ; 9, layer of rods and cones ; 

 10, layer of pigment cells. 



