GENERAL STRUCTURE OF THE RETINA. 221 



All the layers of the retina lying between the two limiting 

 layers are composed of the two different elementary parts 

 which have already been named dements of the nerve tissue, 

 and elements of the connective tissue. This is admitted on 

 all hands. The differences of opinion that exist relate to which 

 of the two groups of tissues this or that fibre, this or that cell, 

 belongs. This disagreement, as our researches on the mode of 

 termination of the nerves at the periphery and in the centres 

 show, depends on the extremely embarrassing circumstance 

 that very fine non medullated nerve fibres are indistinguish- 

 able by any absolutely certain characteristic, even where high 

 powers are used, from other kinds of fibres, especially where 

 both kinds are intimately interwoven with each other, as un- 

 doubtedly occurs in many parts of the retina. In order to 

 obtain a starting-point to enable us to determine the distinction 

 between these two kinds of fibres, we shall commence our con- 

 sideration of the minute anatomy of the retina with that of the 

 undoubted nerve fibres which spread out divergingly from the 

 optic nerve, and form the layer of optic-nerve fibres immedi- 

 ately succeeding the membrana limitans interna. With the 

 knowledge thus gained we ^shall proceed to investigate, and 

 endeavour to distinguish, the nerve fibres of other layers, which 

 cannot be shown to be continuous with true nervous elements. 

 We shall then describe the supporting connective tissue in a 

 special section, as well as the modifications of structure pre- 

 sented by the retina at the macula lutea, the fovea centralis, 

 and ora serrata. The vessels of the retina will be elsewhere 

 described. 



THE NERVOUS ELEMENTS OF THE RETINA. 



The optic nerve, at the point where it reaches the external sur- 

 face of the globe of the eye, consists, just as in its whole course 

 through the orbit, (independently of its sheaths, bloodvessels, 

 and lymphatics,) of medullated fibres, which are grouped into 

 fasciculi, and are imbedded in relatively dense connective tissue. 

 On breaking up small portions, we meet, when examined in the 

 fresh condition, and in indifferent fluids, with short pieces of 

 nerve fibres and with rounded and cylindrical masses of nerve 



