237 



into one fasciculus, that may strike a single point of the nervous membrane 

 exclusively calculated to ieel its impression. An outer, membranous, 

 hard, and consistent covering supports all its parts. Within the first mem- 

 brane called the sclerotic, lies the choroid, adarkish coat, which lines the 

 inside of the sclerotic, and forms the eye into a real camera obscura*. 

 At the anterior part of the globe, there is a circular opening in the sclerotic 

 in which the transparent cornea is inserted. At about the distance of the 

 twelfth part of an inch from this convex segment, received in the anterior 

 aperture of the sclerotica, lies the iris, a membranous partition placed 

 perpendicularly, and containing a round opening, (the pupil) which di- 

 lates or contracts, according to the state of dilatation or contraction of 

 the iris. 



At the distance of about half a line from the back part of the iris, to- 

 wards the union of the anterior fourth of the globe of the eye with the 

 posterior three fourths, opposite to the opening of the pupil, there is 

 situated a lenticular body, enclosed in a membranous capsule, immo- 

 veably fixed in a situation by adhering to the capsule of the vitreous hu- 

 mour. 



Behind the crystalline lens, the posterior three fourths of the cavity of 

 the eye contain a viscid transparent humour, enclosed in the cells of a 

 remarkably fine capsule, called hyaloid. This vitreous humour forms 

 about two-thirds of a sphere from which the anterior segment had been 

 taken out; the pulpous expansion of the optic nerve, the retina, is spread 

 out on its surface, so as to be concentrical to the choroid and sclerotic 

 coats. 



The eye-ball being nearly spherical, the length of its different diameters 

 differs but little. The diameter of the eye, from the fore to the back 

 part, is between ten and eleven lines : the transverse and vertical diame- 

 ters are somewhat shorter. Within the space measured by the diameter 

 from the fore to the back part, there are situated, taking them in their 

 order from the fore part, the cornea, the aqueous humour contained in 

 the anterior chamber, the iris and its central opening or pupil ; the aque- 

 ous humour of the posterior chamber ; the crystalline lens, surrounded 

 by the ciliary processes ; then, the vitreous humour in its capsule, and 

 behind those transparent parts of the eye, through which the luminous 

 rays pass, in approaching to a perpendicular, are the retina which re- 

 ceive the impression, the choroid whose black point absorbs the rays 

 that pass through the thin and transparent retina, and the sclerotic 

 in which there is an opening for the passage of the optic nerve to the 

 globe of the eye. 



The cornea, contained in the anterior aperture of the sclerotica, like 

 the glass of a watch case within its frame, is about the third of a line in 



* GMKLIN (Schweigger's Journ. x. 507.) made an interesting 1 set of experiments, in 

 order to determine the composition of the black pigment which lines the choroid coat 

 of the eye. 



"Its colour, "he informs us, " is blackish brown, is tasteless, and adheres to the 

 tongue like clay ; is insoluble in water, alcohol, ether, oils, lime-water, and distilled 

 vinegar. It dissolves in potash and ammonia when assisted by heat, and is again preci- 

 pitated by acids. Sulphuric acid dissolves it, and changes its colour to redish-brown. 

 When distilled, it yields water, a brown oil, and carbonate of ammonia. It gives out, 

 at the same time, carburetted hydrogen, carbonic oxyde, azotic and oxygen gasses. 

 The coal remaining in the retort, consists almost entirely of charcoal." Copland, 



