96 
2.—The conjunctiva.—Semilunar fold, mem- 
brana nictitans or third eyelid —Lacrymal 
caruncle and glandule of Harder. 
The existence of eyelids supposes a con- 
junctiva,—that is, the integument modified 
into a mucous membrane lining the posterior 
surface of the eyelids, covering the front of the 
eyeball, and thus connecting these two parts 
together. In those animals in which there is 
no palpebral covering either in the usual form 
of eyelids, or in that anomalous form in which 
there is no palpebral opening, there is, properly 
speaking, no conjunctiva: at the most, the com- 
mon integument may be softer, thinner, and 
more transparent where it covers the eye. 
The points deserving particular notice in the 
comparative anatomy of the conjunctiva are, 
1. The oculo-palpebral space of the con- 
junctiva. 
2. The membrana nictitans and third eyelid, 
with the glandule of Harder. 
In most animals the oculo-palpebral space of 
the conjunctiva is as it has been described in 
man. on as have been generally regarded 
as being without eyelids and vie lacrymales. 
The eye, indeed, has been commonly described 
as covered by a transparent lamella of epider- 
mis, which is cast with the rest of the epider- 
mis; and this has been often adduced as an ar- 
gument in favour of the extension of the con- 
junctiva over the cornea in other animals. Ser- 
pents have a palpebral covering, a conjunctiva 
and vie lacrymales; but the conformation of 
these parts is quite peculiar. For the first true 
exposition of the point we are indebted to Jules 
Cloquet.* 
In the article Heartnc, Orcan or, at the 
end of the section on the parallel between 
the ear and the eye, it is said, “ A part in 
the composition of the appendages of the 
eye analogous to the membrana tympani is 
only to be conceived by supposing the exist- 
ence of a mediate anchylo-blepharon, that is, 
an irregular membrane stretched between the 
edges of the eyelids, uniting them together, and 
closing in the space lined by the conjunctiva, 
which space would now communicate with the 
exterior only by the lacrymal canalicules and 
nasal duct, in the same way that the tympanic 
psig communicates with the exterior only by 
the Eustachian tube.” The mediate anchylo-ble- 
pharon here supposed is the actual and regular 
structure in serpents, and is the sole cause of 
the apparent anomaly in the conformation of 
their eye repenaem. The structure is this:— 
Around the margin of the orbit the skin ap- 
pears as if it formed a palpebral fold, covered 
with scales, and representing a sort of frame, in 
which is set, as it were, a transparent continua- 
tion of the skin before the front of the eye. But 
this is not the conjunctiva; it is a natural me- 
diate anchylo-blepharon, or a palpebral covering 
without palpebral fissure. It is, however, 
transparent, and was therefore formerly con- 
founded with the cornea. A conjunctiva lines 
the inner surface of this palpebral covering 
* Memoire sur 1’Existence et la Dis 
ition des 
Voies Lacrymales dans les Serpens. i 
aris, 1821, 
LACRYMAL ORGANS. 
without bral fissure, and invests the, 
part of » P bital, cavity, peeps 
flected on the sclerotica, and is from | 
continued over the cornea, closely adheri 
it. oe 
The conjunctiva thus forms a sac fe ut 
Jules Cloquet oculo-palpebral sac of the 
junctiva), confining a X Sg (which may ii 
manner be called oculo-palpebral) into} 
open the excretory ducts of the lacrymal g 
rom it the tears are drawn off by a cane 
tween the jaw and palate bone into the mx 
of which more will be said fartheron. 
Vertical section of the eye of the common viper, to 
rs disposition of the conjunctiva. ( 
a, the eyeball; 5, the optic nerve; ¢, the eye 
d,d, the scales surrounding the eyelid, ar 
which it appears as if at ae e, e, f 
culs-de-sac formed by the oculo-palpebral conj 
tiva on being reflected from the orbital cavity | 
the eyeball; f, cavity of the oculo-palpebral s 
the conjunctiva, 
‘ 
From the above description it will be reat 
understood how it is from the palpebral 
sion of the skin that the epidermis falls 
the skin is said to be cast, and not from th 
junctiva. 2 
Muller* has found the above structure in 
true serpents, even in the amphisbeenz, wh 
eyes are covered with thick skin. I do 
know if it has been observed in ccecilize ¢ 
He has found a similar structure among 
saurian reptiles in the geckoes, even in 
genus phyllurus. He has even found it | 
mammal, spalax typhlus,t the ore of wh 
appear to be covered by the thick hairy s 
underneath which, however, there is a su 
sac of conjunctiva. In the chameleon: 
follow the family of the geckoes, in Cu 
arrangement, there is a near approach t 
same structure, the palpebral covering pre 
ing only a very small palpebral opening © 
site the pupil. _ 
The membrana nictitans and third eyeli 
In the quadrumana, as in man, the conj 
tiva forms at the nasal canthus a simple : 
lunar fold, but larger. In the. other 
fera, including the herbivorous cetacea, and 
* Ueber eine Merkwiirdige Eigenthiimlichk 
Bau der Augen und Threnence e bei 
Geckonen. In Ammon's Zeitschrift, Bd, i. py 
+ Handbuch der Physiologie des Mens 
Zweiten Bandes Zweite Abtheilung, S, 313. 
blentz, 1838. 
4 
