878 



OF SENSATION, AND THE ORGANS OF THE SENSES. 



if it do not disperse the haze, it is not sufficiently 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 per- 

 sons grow up to adult age, they should endeavor to dispense with the aid of 

 spectacles. 



880. 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 



Fig. 210. 



Fig. 211. 



Distribution of Capillaries in the Vascular 

 layer of the Retina. 



Part of the Retina of a Frog, seen from 

 the outer surface, showing the staff-like 

 bodies of " Jacob's Membrane." 



now proceed, therefore, to consider the share which the Nervous apparatus per- 

 forms 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 netlike plexus. There is considerable diffi- 

 culty, however, in the precise determination of the course of the nerve-fibres in 



Fig. 212. 





Vertical section of the Human Retina and Hyaloid Membrane: h. Hyaloid membrane, h'. Nuclei on its 

 inner surface, c. Layer of transparent cells, connecting the hyaloid and retina, c'. Separate cell enlarged 

 by imbibition of water, n. Gray nervous layer, with its capillaries. 1. Its fibrous lamina. 2. Its vesicular 

 lamina. 1'. Shred of fibrous lamina detached. 2'. Vesicle and nucleus detached, g. Granular layer. 3. 

 Light lamina frequently seen, g'. Detached nucleated particle of the granular layer. TO. Jacob's mem- 

 brane, m'. Appearance of its particles, when detached, m". Its outer surface. Magnified 320 diameters. 



