272 THE ANATOMY OF THE HORSE. 



but it speedily becomes opaque. In consistence it is delicate and 

 jelly-like. 



Structure. — Ten distinct layers are described as composing the thick- 

 ness of the retina. These, enumerated from within to without, are as 

 follows : — 



1. The Membrana Limitans Interna. — This, although appearing as a 

 distinct line in a transverse section, is not a distinct stratum, but merely 

 the inner limiting line of a sustentacular framework — the radial fibres 

 of Miiller — which pervades and supports the nervous elements in the 

 other layers of the retina. 



2. The Layer of Nerve Fibres. — This layer results from the radiation 

 of the optic nerve, whose fibres at their point of entrance into the eye- 

 ball lay aside their medullary sheath. 



3. The Layer of Nerve Cells. — This is a single layer of multipolar 

 nerve cells. 



4. The Inner Molecular Layer is a thick stratum of fibres and inter- 

 mediate granular matter. 



5. The Inner Nuclear Layer contains spindle-shaped or bipolar nerve 

 cells with distinct oval nuclei and only a small amount of protoplasm. 

 The inner and outer poles of the cells are continued through the 4th 

 and 6th layers respectively. 



6. The Outer Molecular Layer repeats the structure of the inner 

 molecular layer. 



7. The Outer Nuclear Layer contains spindle-shaped cells with con- 

 spicuous nuclei and a small amount of protoplasm, the poles of the cells 

 being prolonged as in the case of the similar elements in the inner 

 nuclear layer. 



8. The Membrana Limitans Externa. — This is the outer boundary 

 of the sustentacular framework of fibres already mentioned. 



9. The Layer of Rods and Cones, or the bacillary layer, is com- 

 posed of two different kinds of elements. The longer elements, 

 the rods, extend vertically between the 8th and 10th layers; the 

 cones are much shorter than the rods, and do not reach so far as 

 the next layer. 



10. The Pigmented Epithelium. — This is a layer of polygonal pig- 

 mented cells, generally six-sided. 



Directions. — The third eye should be transversely divided with a 

 sharp scalpel, about half an inch behind the junction of the sclerotic 

 and cornea. This should be done with the eye immersed in water. 

 The posterior half, after removal of the vitreous humour, should be 

 used for the better examination of the inner surface of the retina. The 

 lens should be removed for examination from the anterior half. In the 

 eye already used for the display of the retina, that coat should be in 

 part removed, so as to display the vitreous humour with the lens 



