ORGANS OF SENSE. 169 
of the vitreous humor by a series of folds, called ciliary processes, arranged 
in a circular form around the margin of the crystalline lens. The third, or 
innermost tunic, is the retina, the delicate expansion of the optic nerve; 
this extends from the entrance of the nerve, at the back of the eye, all 
around the interior of the choroid membrane, and terminates in front about 
two lines behind the anterior border of the latter ; this is the essential part 
of the organ, being endowed with sensibility to light ; all the other parts may 
be considered as subordinate to it. The humors of the eye are the aqueous, 
the crystalline lens, and the vitreous; these, though of different densities, 
and inclosed in membranous capsules, are all transparent, and exactly fill 
the globe. The aqueous fluid occupies the space between the cornea and 
the lens ; the iris floats and moves in this fluid, and divides the space into 
two chambers, the anterior and posterior. ‘The anterior is between the 
cornea and iris, and the posterior between the iris and the lens and vitreous 
humor ; this space is very small, and both communicate freely through the 
pupil, or the circular aperture in the centre of the iris. All the posterior 
region of the eye is filled by the vitreous humor, a soft, gelatinous, transpa- 
rent mass, composed of the most delicate cellular membrane, the cells filled 
with fluid. In a depression on the fore part of this substance, and behind 
the pupil and iris, is placed the crystalline fluid or humor, of greater density 
than either of the other two, and of the form of a double convex lens. We 
shall next proceed to the examination of these parts of the eye. in greater 
detail. 
1. Tuntca Scterotica. , This, together with the cornea in front, forms 
the external coat or case of the eye, and extends from the optic nerve to 
the circumference of the cornea; it is a dense, opake, fibrous membrane, 
very strong and inelastic, preserving the figure of the organ and protecting 
from injury and pressure the delicate structure within. Near the entrance 
of the optic nerve it is pierced by numerous small foramina, for the short 
ciliary nerves and vessels; more anteriorly, by small oblique canals for the 
long ciliary; and a little behind the transverse axis there are four or six 
larger canals for the exit of veins. The internal surface of the sclerotic 
is in contact with the choroid membrane, and is stained by the brown 
pigment. When washed clean it presents a smooth glossy appearance, 
owing partly to the presence of a very fine serous-like lamina. The thick- 
ness of the sclerotic is greatest posteriorly, about the optic nerve; thence it 
thins out towards the centre. The optic nerve perforates the sclerotic 
about a line and a half internal to the antero-posterior axis. As the nerve 
approaches this point, it is suddenly constricted ; its fibrous sheath of dura 
mater is intimately united to the sclerotic, and the contracted nerve passes 
through a small aperture in the membrane which appears traversed by 
fibres, and has received the name of lamina cribrosa. When the nerve 
is squeezed, a central dark point will be observed in a section, called 
the porus opticus, and showing the position of the cerebral artery of the 
retina. 
2. Tus Cornea forms the anterior fifth of the eyeball, and completes the 
external case, by joining on to the sclerotic; it projects beyond the level 
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