307 
CHAPTER III. 
Plate II. — the eye. 
Fig. 1 . The crystalline lens of a fish; it is proportionably larger 
than in other animals, and perfectly spherical. 
Fig. 2. A section of the human eye. It is formed of various 
coats , or membranes, containing pellucid humours of different de¬ 
grees of density, and calculated for collecting the rays of light into 
a focus, upon the nerve situated at the bottom of the eye-hall. 
The external membrane, called sclerotic, is strong and firm, and 
is tne support of the spherical figure of the eye: it is deficient in the 
centre, but that part is supplied by the cornea , which is transparent 
and projects like the segment of a small globe from one of larger 
size. The interior of the sclerotic is lined by the choroid, which is 
covered by a dark mucous secretion, termed pigmentum nigrum, in¬ 
tended to absorb the superfluous rays of light. The choroid is rep¬ 
resented in the plate by the black line. The third and inner mem¬ 
brane, which is marked by the white line, is the retina, the expanded 
optic nerve. 
Within these coats of the eye, are the humours, a, the aqueous 
humour, a thin fluid like water; b, the crystalline lens, of a dense 
texture; c, the vitreous humour, a very delicate gelatinous substance, 
named from its resemblance to melted glass. Thus the crystalline 
is more dense than the vitreous, and the vitreous more dense than 
the aqueous humour: they are all perfectly transparent, and togeth¬ 
er make a compound lens, which refracts the rays of light issuing 
from an object, d, and delineates its figure e, in the focus upon the 
retina, inverted. 
Fig. 3. The lens of the telescope. 
Fig. 4. The crystalline lens, or, as it has been called, the crystal¬ 
line humour , of the eye. 
Fig. 5, 6. A plan of the circular and radiated fibres which the 
iris is supposed to possess; the former contracts, the latter dilates 
the pupil, or aperture formed by the inner margin of the iris. 
Fig. 7. a, a, a, a, the four straight muscles, arising from the bot¬ 
tom of the orbit, where they surround, c, the optic nerve; and are 
inserted by broad, thin tendons at the fore part of the globe of the 
eye into the tunica sclerotica. 
