394 FUNCTION OF THE EETINA AND CHOROID. 



ical Magazine, September, 1840, 1 have given proofs that the essential 

 condition of the chemical activity of a ray of light is its being thus ab- 

 sorbed. As an illustration may be given the well-known result, that if 

 chlorine and hydrogen be exposed to the sun, they unite with a violent 

 explosion, but, under the same circumstances, oxygen and hydrogen will 

 utterly refuse to unite, no matter how long the period of exposure may 

 be, nor what the brilliancy of the light ; and the difference in the two 

 cases is merely this, that the chlorine, being of a yellowish color, can ab- 

 sorb the violet light, and therefore be influenced by it ; but the oxygen, 

 being uncolored, can not. For photographic effects, as well as calorific, 

 the essential condition is absorption. A medium like the retina, which 

 is without absorbing action, permits rays to pass through it without any 

 kind of effect, but a surface like the black pigment, which receives them 

 all equally, whatever their color may be, and absorbs them all equally, 

 is equally affected by them all. 



The impression arising from the disturbed condition of the retinal 

 Function of the vesicles is carried by the optic tubules to the chiasm of the 



two chief lay- ^wo nerves. Apart from the general facts elsewhere pre- 



ers of the reti- -,.1 , . />ITT 



naandthecho- sented by physiology, the existence of a bund spot at the 

 entrance of the optic nerve, where there is a necessary ab- 

 sence of vesicular structure, is a clear proof of the insensibility of the 

 tubular structure to the influence of light. Considering, therefore, the 

 retina as typically composed of three layers, one of tubules, one of vesi- 

 cles, and one of granules, and these in health being perfectly transparent, 

 the luminous beams pass through them just as they do through the at- 

 mosphere, without exerting the slightest effect ; and as, when those rays 

 strike the opaque surface of the earth, or are absorbed by the sea, heat is 

 disengaged and effects ensue, so likewise, when they have reached the 

 black pigment, the changes I have been designating arise. The vesicu- 

 lar layer undergoes rapid metamorphosis, the effect of that change is 

 transmitted by the tubular layer, and in the granular the germs are con- 

 stantly arising from which the waste of the middle layer is repaired. So, 

 therefore, the tubular layer is for conduction, the vesicular layer for waste, 

 the granular layer for repair ; and now appears the significance of the 

 construction and proximity of the choroid coat, for the waste of the ve- 

 sicular layer can not occur save under the oxidizing influence of the ar- 

 terial blood, nor can the nutrition of the granular layer be accomplished 

 except under the same condition. Moreover, the resulting products of 

 waste require to be quickly removed, and it is not possible to conceive 

 the construction of an arrangement better adapted for this triple object 

 than that which the choroid presents. On the old view of the nature of 

 vision, the construction of the choroid seems to be without significance. 

 The analogy between the mechanism of the retina and that of the 



