218 NUNNELEY, ON THE RETINA. 
found in the body, and instead of being thought of as simply 
a terminal nervous expansion, it should be regarded as in 
itself a nervous centre; having the fibres of the nerve ex- 
panded it is true in it, and by “which it is connected with the 
brain, but possessed itself of structures and properties 
peculiar to it, and altogether different from those of the 
optic fibres, which are expanded in and make part of it. 
These fibres are for the purpose of conveying to the sensorium 
the impressions excited in the retina, but are not for receiving 
the impression, while the other complex structures of the 
retina receive and appreciate the impressions of external 
objects. 
The inner surface of the retina is not merely superimposed 
upon the vitreous body, but is organically connected with it, 
by a series of clear, perfectly transparent cells ; while the 
outer surface is most intimately united with the cells forming 
the mner layer of the choroid. It is an extremely delicate, 
thin structure, thickest at its junction with the optic nerve, 
and gradually becomes thinner as it reaches the ora serrata, 
where it is not more than a quarter the thickness it possesses 
at the back part of the eye.* 
In the living condition the retina is as nearly transparent 
as possible, and of a slight pmk tinge, but in a very short 
time after death it becomes translucent, then opaque: by 
immersion in water it very soon becomes so; and by spinit, 
heat, and all chemicals which coagulate albumen, it 1s Im- 
mediately rendered quite opaque and white. In the fresh 
eye it is perfectly smooth, but, in consequence of evaporation 
from the eye, it is at the period when human eyes are com- 
monly dissected found in irregular folds. |The pink tinge of 
the fresh retina is due to the blood which it contains. It is 
a very vascular structure, deriving its supply of blood from 
the central artery of the retina, and, not improbably, some 
of its nutriment, though no vessels, from the choroid coat. 
So numerous and complex, so minute and fragile, so 
transparent and changeable, so intimately connected and 
mingled together, so quickly altering after death, and yet, 
at least in man, so difficult to obtain immediately after death, 
so instantly and most importantly altering on the addition 
of almost every agent that may be employed i in assisting Mm 
the examination of other tissues, that it is no wonder almost 
every description of these structures is different from others ; 
* Kolliker says, ‘“ Its thickness is at first 0°1 of a line, but as it extends 
acai it soon diminishes to 0:06 of a line, until ultimately close to the 
anterior border of the retina it is not. more than 0:04 of a line in thick- 
ness.” (‘ KOlliker’s Human Histology,’ by Busk and Huxley, vol. ii, p. 369.) 
