ae 
+4 
LACRYMAL ORGANS. 
cerns only the true cetacea, this fold is deve- 
_ loped into the membrana nictitans, which is 
disposed vertically within the horizontal eyelids 
__ at the nasal canthus, and is capable of being 
“pushed more or less towards the temporal can- 
thus, over the front of the eyeball. 
___ The membrana nictitans derives firmness 
_ from a thin plate of cartilage, which has some- 
$s asort of pedicle passing backwards by 
* 
he inner side of the eyeball. In the sheep, for 
xamp!e, the cartilage of the membrana nicti- 
ns is of the shape of the letter T. The cross 
p forms the margin of the membrane; and the 
eg, closely embraced by the glandule of Har- 
er, extends backwards between the eyeball 
ind inner wall of the orbit. -The cartilage of 
1 Membrana nictitans, unlike the tarsal car- 
lages, is true cartilage, with nucleated cor- 
In the elephant it is said there is a muscular 
rangement for carrying the membrana nictitans 
jutwards over the front of the eyeball. I think 
_ Ihave observed in the rabbit that the mem- 
_ brana nictitans receives part of the expansion of 
e levator of the upper eyelid. The membrana 
ctitans has not, however, like the third eyelid 
ds, any proper muscular apparatus; and 
an it is moveable, the motion is produced by 
eyeball, on its being retracted deeper into 
‘orbit by the retractor muscle, which dis- 
es and presses forwards the cartilaginous 
pedicle above described. 
_ The structure called semilunar fold in man, 
d membrana nictitans in quadruped mam- 
mifera, has attained its greatest development 
in birds, in which it is called the third 
It is transparent and capable of 
ing the whole front of the eyeball. En- 
ed in the conjunctival reduplication there is 
a fibro-cartilaginous structure, very thin and 
membranous. Of a triangular form, the third 
; elid has its free margin oblique from above 
downwards, and from without inwards. In the 
State of repose it is retracted and folded verti- 
) cally in the nasal angle of the eye. The third 
slid is drawn over the front of the eye by a 
very peculiar mechanism consisting of two 
Thuscles, the slender tendon of one of which 
funs through an elongated loop in the broad 
free end of the other. This muscular apparatus 
is supplied by the nervus abducens. 
lt! 
a a ae stad me 
The guadratus is a broad thin trapezoidal 
scle. It arises from the upper and posterior 
part of the eyeball, behind the prominence of 
ats largest circumference. From this point its 
fibres, which form a thin but broad fleshy belly, 
cend towards the optic nerve, converging 
Somewhat. It then terminates abruptly in a 
nee tendinous margin, close to the upper part 
of the optic nerve. In this free tendinons mar- 
in, which is considerably narrower than the 
n, there is an elongated loop or canal, in 
hich the tendon of the other muscle plays. 
_ Pyramidalis muscle-—The fleshy part of 
his muscle is comparatively smaller. It arises 
»y a broad curved base from the lower part of 
jhe eyeball, opposite the preceding. In its 
}scent towards the optic nerve, the muscle be- 
jomes contracted, and at last ends in a slender 
VOL. III. 
i 
97 
tendon on the nasal side of the optic nerve- 
The tendon immediately enters the pulley-canal 
in the extreme margin of the quadratus, and in 
traversing it turns round the upper part of the 
optic nerve. Thus changing its original direc- 
tion, it passes downwards on the temporal side 
of the optic nerve to the lower part of the eye- 
ball, round the prominent circumference of 
which it turns to get to the front of it, 
when it immediately enters the lower angle 
of the third eyelid. Having entered, it di- 
vides into two parts, one of which expands 
and runs between the layers of conjunctiva, 
forming the third eyelid, to the nasal angle of 
the eye ; the other passes along and forms the 
pretty firm free margin of the eyelid in question. 
Having traversed the whole margin of this and 
arrived at the upper part of the eyeball, it is 
inserted into the sclerotica just at the middle of 
the line whence the guadratus derives its origin. 
It is by this arrangement that the superior angle 
of the third eyelid is attached to the sclerotica, 
and consequently rendered immoveable. 
By its own elasticity the third eyelid remains 
retracted at the nasal angle of the eye. In this 
state its tendinous margin is relaxed ; but when 
the two muscles just described contract, the 
tendon of the pyramidalis is drawn straight, 
and the third eyelid is thus stretched over the 
front of the eye. 
“ From this curious disposition of the 
muscles,” says Dr. Porterfield,* ‘it is easy to 
conceive, how this internal eyelid is extended 
over the cornea far enough to cover all the 
pupil, though the muscles themselves are con- 
tained in a small space. Every body knows, 
that the contraction of all muscles is only in a 
certain given proportion to their length; and 
therefore that the eyelid might be drawn far 
enough over the cornea, nature was obliged to 
make use of a long muscle, which could not 
be contained in so small a place as the orbit, 
without being bent or inflected ; and therefore 
the one muscle is bent upwards near the optic 
nerve, making an acute angle, where it passes 
through the perforated end of the other 
muscle, by which means its action is greatly 
increased. But its action is yet more increased 
by the contraction of the square muscle itself, 
which must draw the cord or tendon of the py- 
ramidal muscle which passes through it, through 
a space double of what it moves itself; and 
thus the membrana nictitans is extended far 
enough to cover the whole cornea though its 
muscles are contained in a small space.” 
In Owls Nitzsch discovered a small bone on 
the lower surface of the bony ring of the 
sclerotica, ossiculum tuberculare, for the support 
of the long tendon of the pyramidalis. In par- 
rots the third eyelid is small. 
Among reptiles there is in chelonia and 
lizards a third eyelid much the same as in 
birds, but smaller and less moveable. It is 
moved only by a single muscle analogous to 
the pyramidalis of birds. “ An allied struc- 
ture,” says Miiller, “ is a spectacle-like trans- 
parent part in the lower eyelid of some lizards, 
* ‘On the Eye, vol. i. p. 34. Edinburgh, 1759, 
H 
