F LACRYMAL ORGANS. 
we will be described in speaking of the con- 
junctiva. In birds there is no example of the 
eyeball being covered over in a similar manner 
bya continuation of the-skin. 
.) In man the upper eyelid is the more de- 
“yeloped and the more moveable. Descending 
n him the lower is found gradually to 
issume the superiority in this respect. In the 
eetacea the upper.and lower eyelids are tumid 
olds of skin enclosing fat, but no tarsus, and 
xe Meibomian glands are entirely wanting. 
Phere is no orbicularis palpebrarum muscle, 
ut muscular fibres proceed from the anterior 
and posterior end of the orbital process of the 
frontal bone to the eyelids in the region of the 
outer and inner canthi. Instead of the levator 
alpebree superioris there is a hollow conical 
muscle which arises from the circumference of 
he foramen opticum and terminates in the eye- 
ids. A similar in four divisions occurs in the 
_ seal.* In the echidna there is a circular eye- 
In birds the lower eyelid is in general 
much larger and more moveable than the 
yper. In the ostrich and some parrots both 
elids are equally moveable. 
In birds the eyelids are closed in death. In 
the gallinaceous birds which I have examined 
_ this appears to be owing to an expansion of 
 @lastic membrane attached to the margin of the 
orbit and interlaced round the eyelids. During 
life the lower eyelid is opened by muscular 
ction by a proper depressor muscle. The 
ipper eyelid retains its levator. There is a 
tarsal cartilage in the lower eyelid; in the 
ipper eyelid there is a less dense fibrous struc- 
ure. The skin of the lower eyelid is naked 
md finer than that of the upper in accordance 
with its greater mobility. 
In chelonian reptiles the lower eyelid is, as 
n birds, the larger and more moveable ; and as 
_ the more moveable the eyelid the finer the 
integument, so in them the lower eyelid is naked 
as in birds. 
Tn lizards the eyelids form a sort of sheet 
stretched before the eyeball with a horizontal 
fissure closed by a circular muscle and opened 
bya levator and depressor. In the chameleon 
the palpebral fissure is represented by a small 
hole opposite the pupil. Tracing the eyelids 
in a general way from batrachian reptiles to 
fishes, we find a gradual depreciation of struc- 
| ture; thus salamanders have two folds of skin, 
‘upper and under, for eyelids, but not sufficient 
to cover the eyeball, whilst the pipa has none. 
_ In fishes generally, the skin before passing over 
he front of the eye becomes finer and forms a 
| slight circular fold, often only well-marked 
me bove Sometimes there is an anterior and a 
sterior semilunar fold, as in the herring, 
lmon, but especially in several of the shark 
It is the Orthagoriscus mola which has 
“circular folds for eyelids, and an orbicular mus- 
ele for closing them. The eyelids are again 
opened by five radiating muscles. 
| *Rapp, Die Cetaceen Zoologisch-anatomisch 
; t, Stuttgardt and Tubingen, 1837, quoted 
oe Miller’s Jahresbericht, p. cxx. in Archiv, 
i 
a 
4 
— 
i 
95 
Among the invertebrata some of the cepha- 
lopoda have large palpebral folds. In the 
octopus “the eyes are small in proportion, and 
the skin is drawn over them so as to cover them 
entirely at the will of the animal.”* There are 
no eyelids in the sepia officinalis, but a con- 
tinuation of the integuments passes over the 
eye. 
rycen and eyelashes occur only in a few 
mammalia. Eyelashes exist in the pachyder- 
mata, ruminants, &c. but are wanting especi- 
ally in small mammals. Meibomian glands 
are commonly found. 
In birds the eyelids sometimes present cilia. 
This is the case only in some birds of prey, in 
some parrots, in the ostrich, &c. but seldom in 
other orders. Very small Meibomian follicles 
are said to exist imhe eyelids of birds. In the 
eyelids of a common fowl at present before me, 
there are small transverse fissures on the mar- 
gin of the lids filled with a sebaceous matter. 
They have the appearance of very small Mei- 
bomian glands, not closed as in the mammifera, 
but open along their whole length. 
In a preceding part of this article I remarked 
on some points of resemblance between the 
iris and eyelids, as regards functions, and sym- 
pathy in the performance of those functions. 
In some of the lower animals there are certain 
eee in which they even approximate in form. 
n connexion, therefore, with the subject of the 
eyelids it will not be out of place to allude to 
that flocculent growth of the uvea hanging 
from the upper margin of the iris over the trans- 
versely elongated pupil in the horse, &c. and 
which appears to serve the purpose, if it may 
be so expressed, of an internal eyelid. An 
analogous but more curiously and highly deve- 
loped structure—a blending as it were of the 
iris with some remains of the structure of the 
eyelids—exists in the eyes of several fishes ; 
among others in the skate. 
The structure alluded to is a digitated exten- 
sion of the whole substance of the upper part of 
the iris, hanging over the pupil, which it is 
large enough entirely tocover. There being no 
intrinsic power of motion in the iris of fishes, 
the mechanism by which this digitated veil is 
drawn up from over the pupil is this :—Where 
the upper part of the ciliary margin of the iris 
is connected with the sclerotica, the latter is 
very flexible and is externally intimately con- 
nected with a fold or rudimentary eyelid which 
the integument forms before passing as con- 
junctiva over the eye. Muscular fibres are 
inserted into this fold at the point of connexion, 
and draw it upwards; the flexible part of the 
sclerotica of course follows this movement and 
the upper part of the iris, the sclerotica, so that 
the digitated veil is drawn up from over the 
pupil. In a young skate which I removed 
from the egg and preserved alive for some 
weeks, I observed that the digitated veil was 
kept down during the day, but was drawn up 
toward evening, and the large black pupil 
exposed. 
* Cuvier, Regne animal, vol. iii. p. 12, Paris, 
