SENSITIVE SYSTEM. 379 



Magnitude, Figure, and Diameter. — The magnitude of the globe 

 varies something in different individuals. Its figure is a compound 

 of two spheres of different diameters, united in front by aneUiptical 

 line. The small sphere, or rather hemisphere, projects in front from 

 the large one, and is transparent; the large sphere, the less imper- 

 fect one, is flattened posteriorly and prominent around its sides, 

 but is opaque : the former, from its apparent resemblance to horn, 

 has been named the cornea, or, to distinguish it from the latter 

 (which is sometimes called the cornea opacaj, the cornea liicida 

 vel transparens. The diameter of the globe, according to Girard, 

 exceeds its axis by about a line-and-a-half (trois millimetres): this 

 is the reverse of the relative dimensions of the human eye, of 

 which the axis exceeds the diameter by nearly a line. 



Constituent Parts. — The visual organ is composed — \st, 

 of certain membranous parts, mostly opaque, called coats, in- 

 vesting and protecting the contents of the globe; 2dlij, of 

 a series of diaphanous parts, or refracting agents; "ddly, of a 

 nervous lining, which receives a representation of the object seen, 

 and transmits the impression along the optic nerve to the sen- 

 sor! um. 



THE COATS OR MEMBRANES, 



Are, the sclerotic, choroid, retina, cornea, and iris. The first 

 three of these membranes forms the opaque case containing the 

 transparent parts, and are, properly speaking, i\\Q coats of the 

 eye, being concentrically arranged one within another, like the 

 layers of an onion. The cornea is the sight of the eye, the watch- 

 glass, as it were, fitted into this case ; and the iris is the coloured 

 partition internally between the light and dark compartments of 

 the organ, with a hole through its centre by which they com- 

 municate. 



Sclerotic Coat. 



The TUNICA SCLEROTICA (so named from its hardness or 

 firmness) composes the white part of the globe, extending from 

 the insertion of the optic nerve to the cornea, and forming about 

 four-fifths of the entire superficies. Posteriorly, it receives the 

 insertion of the fleshy fibres of the retractor : in front, it is strength- 

 ened by the implantation of the tendons of the four recti, which 

 have been (erroneously) supposed to spread over the part covered 

 by conjunctiva, producing its shiny whiteness, whence the ima- 

 ginary expansion has been named the tunica albuginea ; but, in 

 point of fact, no such expansion exists, the remarkable whiteness 

 of the part being caused by the transparent glossiness of the con- 

 junctiva itself In other places the surface of the sclerotica is 



