352 MH'ltuSCOPIC ANATOMY OF THE ORGAXS. 



tions at the papilla nervi optici. This layer is thickest at the 

 place of entrance of the optic nerve (Fig. 264). It contains 

 only naked axis cylinders. The great majority of these are 

 centripetal fibres, which are derived from the cells of the adja- 

 cent layer (H) of the retina. It is highly probable that there 

 are a few (n) centrifugal fibres (Cajal), which are processes of 

 ganglion cells situated in the brain. The greater number of 

 these fibres form by their end arborizations a pericellular 

 network around the cells which lie in the outer part of the 

 inner reticular layer i. e., around the parareticular cells 

 Some of them, on the contrary, end freely after penetrating 

 to the more external layers of the retina (Dogiel). 



The membrana limitans interna (K\ which forms the 

 innermost layer of the retina, is a product of the supporting 

 cells of Miiller (supporting fibres, radial fibres). 



These supporting cells of Miiller are somewhat similar to the 

 ependymal cells of the embryonic spinal cord. They are ele- 

 ments of an epithelial nature (of ectodermal origin), and consist 

 of elongated cells which extend through the whole thickness 

 of the retina. The inner end of the cell is widened into a cone- 

 shaped body, which shows a fibrous structure (radial fibre cone). 

 In consequence of the fusion of these conical bodies, a mem- 

 brane is formed, the membrana limitans interna. From this 

 place the supporting cells extend toward the outer surface. In 

 both reticular layers delicate fibres are given off in all direc- 

 tions. At the level of the outer reticular layer each cell pre- 

 sents an ellipsoidal nucleus. In the outer ganglionic and outer 

 granular layers the cells show numerous cup-like depressions 

 on their surfaces, caused by pressure exerted by other kinds of 

 cells. At the bases of the rods and cones is found the mem- 

 brana limitans externa, which is formed by a membranous 

 widening of the supporting fibres. From its surface there run 

 fine processes, which form the so-called fibre-baskets, which 

 surround the bases of the rods and cones. 



In the supporting tissue of the retina there are, in addition 

 to the Miiller's fibres, neuroytta cells (spider cells), which occur 

 abundantly in the optic nerve. 



