Cuap. II. FORMATION OF THE ORGANS. 587 
in aleohol. In this manner the topography and relations of the different mem- 
branes and layers can be made out quite easily, and, once familiar with the 
general structure of the eye of the Turtle, it is possible to dissect fresh speci- 
mens knowingly, and readily to detect any misplacements or distortions caused by 
the dissection. The outermost coat or covering of the eye is the conjunctiva 
(Pl. 22, fig. 8, a, a'); that portion of it which lies exterior to the boundaries 
of the cornea (c') is a very thick, soft, and flabby membrane (a); but where it 
passes over the cornea it becomes very thin. Here it presses very closely upon the 
cornea, (c',) and is changed in its nature to a very tender and friable membrane, 
(a',) the conjunctiva of the cornea. The sclerotica (4, , 4’, c') totally envelops the 
eye, except where the optic nerve (/') enters. In front of the eye it forms 
the cornea, (c',) a very tough, elastic, and transparent membrane, considerably 
thicker than the conjunctiva (a') which covers it. At the base of the iris, (e’,) 
the cornea suddenly thickens, and, passing backward all over the eye, constitutes 
the sclerotic coat, (4, ',) which is a tough, dense, bluish membrane, much thicker 
than any other of the coats of the eye, and pressing pretty closely upon the cho- 
roidea (¢); but it is not in actual contact with it, being separated from the same 
by a layer of black pigment (d). This pigment layer (d) extends from the 
edge of the cornea backward, over the whole eye, but disappears (d') at the point 
of entrance of the optic nerve (h'). The choroidea (e, ¢)' is a very thin mem- 
brane, of uniform thickness throughout its whole extent, excepting where it passes 
along the optic nerve (/'); there it thickens considerably (ct). At its anterior 
border it begins again to thicken, just where the posterior edge (¢*) of the cil- 
lary processes (e”) terminates, and continues to increase in thickness until it 
reaches a poimt opposite the junction of the cornea (c') and_ sclerotica, (4,) 
where it thins out as it plunges into the aqueous humor. Here it consti- 
tutes the membrana iridis, (e4) a very spongy and loosely fibrous — structure, 
resembling an irregular network. At this age, the iris (e') does not hang 
perpendicularly to the axis of the eye, but projects very obliquely forward, as it 
must necessarily do, because the crystalline lens presses upon it from behind, and 
forces it to slide over its convex surface, or rather that of the membrana pupillaris, 
(z,) when contracting and expanding. Even in the adult, this peculiarity (see fig. 
7) obtains to a slight extent. The ciliary processes (¢’, ¢?) extend from the free 
border of the iris (e') along its posterior surface, where it forms a thick posterior 
lining, and, thmning out rather abruptly about opposite the anterior edge (7) 
of the retina, continues for a short distance backward, and then terminates sud- 
denly in a sharp edge, which may be easily separated from the choroidea, although 
1 In Pl. 22, fig. 8, the letter e? near i} should be e*. 
c=) 
