412 



ON SENSATION, AND THE ORGANS OF THE SENSES. 



powerful. In general it is better to employ a glass which somewhat under- 

 compensates the eye, than one which is of a curvature at all too high ; since, 

 with the advance of years in elderly persons, a progressive increase in power 

 is required ; and, as young persons grow up to adult age, they should endeavour 

 to dispense with the aid of spectacles. 



539. Many other interesting inquiries, respecting the action of the eye as 

 an optical instrument, suggest themselves to the physical philosopher ; but the 

 foregoing are the chief in which the Physiologist is concerned ; and we shall 

 now proceed, therefore, to consider the share, which the Retina and Optic 



Fig. 175. 



Fig. 176. 



Part of the Retina of a Frog, seen 

 from the outer surface. Magnified 300 

 times. 



Distribution of Capillaries in Vascu- 

 lar layer of Retina. 



[Fig. 177. 



Nerve perform in the phenomena of vision. The optic nerve, at its entrance 

 into the eye, divides itself into numerous small fasciculi of ultimate fibrils; 



and these appear to spread themselves out, 

 and to inosculate with each other by an ex- 

 change of fibrils, so as to form a net-like 

 plexus. There is considerable difficulty, 

 however, in the precise determination of the 

 course of the nerve-fibres in the Retina; on 

 account of their minute size, and the absence 

 of their distinctive characters. According to 

 Mr. Bowman, the tubular membrane and the 

 white substance of Schwann are deficient; 

 and only the central part of the nerve-fibre, or 

 axis-cylinder, is continued into this expansion. 

 The plexus of nerve-fibres comes into relation 

 with a plexus of capillary vessels, very minute- 

 ly distributed ; and also with a layer of cells, 

 so closely resembling those of the cortical sub- 

 stance of the brain, that there can be no rea- 

 sonable doubt of their correspondence in func- 

 tion. This layer of cells, constitutes the in- 

 ternal layer of the true retina. We have here, 

 then, all the elements of an apparatus for the 

 origination of changes in the nervous trunks, 

 in a fully displayed form ; and it can scarcely 

 be doubted that the essential parts of the same 

 structures exist in the papillae of the cutaneous 

 and other sensory surfaces. The true Retina 

 is covered externally by a very peculiar in- 

 vestment, the Membrane of Jacob, which 



A portion of the Retina of an Infant, 

 with its vessels injected and niagnilird 

 _>5 diameters. An outline of ihe natural 

 si/e of this piece is seen just below the 

 main cut.] 



