216 



SENSE OF SIGHT. 



membrane. It completely lines the sclerotic ; and has, consequently, 

 the same shape and extent. Behind, it is perforated by the optic nerve ; 

 before, it has the iris united with it ; and within, it is lined by the 

 retina, which does not, however, adhere to it, the black pigment sepa- 

 rating them from each other. It is chiefly composed of the ciliary 

 vessels and nerves, and consists of two distinct laminae, to the innermost 

 of which Ruysch the son gave the name membrana Ruyschiana. 

 In fishes these laminse are very perceptible, being separated from each 

 other by a substance, which M. Cuvier considers to be glandular. The 

 choroid is impregnated and lined by a dark-coloured mucous pigment, 

 stratum pigmenti, pigmentum nigrum. In some cases, as in the albino, 

 Fig 87 this substance, which is exhaled from 



the choroid, is light-coloured, ap- 

 proximating to white. Leopold 

 Gmelin 1 conceives that it approaches 

 the nature of indigo ; Dr. Young, 2 

 regards it as a mucous substance, 

 united to a quantity of carbonaceous 

 matter, upon which its colour de- 

 pends ; and Berzelius, 3 from his 

 chemical investigations, considers it 

 to consist chiefly of carbon and iron ; 

 Pigmentum Nigmm. but Professor Jacob thinks it obvi- 



A. choroid epithelium, with the ceils filled ouslv an animal principle sui generis, 



with pigment, except at a. where the nuclei are V , -, r j 



visible The irregularity of the pigment-cells its elements being OXygen, hydrogen, 



is seen. b. Grains of pigment. ^QV^ATI or>rl m+mrron "Hv ArkinVm 



B. Pigment-cells from the substance of the Carbon, ana nitrogen. UY. ADJOnn 

 choroid. A detached nucleus is seen. Magnified found 100 DartS. in a dry State, leave, 

 320 diameters. . .^ ' . . . ,, ' 



when incinerated, 4-46 of a calx 



consisting of chloride of calcium, carbonate of lime, phosphate of lime, 

 and peroxide of iron. Mr. Thomas Wharton Jones has examined the 

 layer of black pigment on the inner surface of the choroid microscopi- 

 cally. He states that it possesses 

 organization, and constitutes a 

 real membrane pigmental mem- 

 brane consisting of very minute 

 flat bodies of an hexagonal form, 

 joined together at their edges. 4 

 It is generally considered to con- 

 sist of pigment cells, which form a 

 kind of pavement, and are some- 

 what of a polyhedral shape; 

 lying in a very regular manner, 

 with some intercellular substance 

 between them. 



On the outer side of the bottom 



1. Terminatins 

 2. Foramen of S 

 Crystalline lens. 



Retina. 



anteriorly in a scalloped border, 

 immering. 3. Zonula ciliaris. 4. 



1 Dissert. Sistens Indagationem Chemicam Pigment! Nigri Oculorum Taurorum, Getting., 

 1812. 



2 Medical Literature, p. 521, Lond., 1813. 3 Medico-Chirurg. Trans., iii. 225. 



* Art. Eye, by Dr. Jacob, in Cyclop, of Anat. and Physiol., Part x. p. 181, for June, 1837. 



