A. LIAUTAKD. 
253 
occular globe. In the ordinary state, the membrana nictitans is 
concealed in the great angle of the eye, its anterior border only be¬ 
ing visible; but if a permanent contraction of the muscles of the eye 
draws it back in its orbit, or when this action is stimulated by a 
foreign body, the membrana nictitans shows itself under the shape 
of a thin lamella, convex on its external face, concave on the in¬ 
ternal, wide at its anterior part and narrow at its posterior, thus 
assuming the shape of a triangle, whose posterior angle is much 
elongated. In the motion described above, the third eyelid glides 
between the two palpebral veli and the globe of the eye, as in 
two grooves represented by the conjunctival fissures; these give 
it its directions ; without them it would look upwards. Except in 
the case of permanent contraction of the muscles, as in tetanus, the 
action of the membrana nictitans is instantaneous, and it soon 
returns in the nasal angle of the eye. 
The angular extremity of the membrana nictitans is united to 
a large adipous cushion situated between the muscles of the eye; 
its projection forward then is only an action entirely mechanical. 
Indeed, in the retraction of the eye in its cavity, there is pressure 
upon that cushion, which has a tendency to change position, and 
pushes forward the membrana nictitans, which rests upon it; its 
action is therefore so much more complete that the contraction of 
the muscles is greater. In this motion the third eyelid carries 
with itself the conjunctival fold which surrounds it. It is this 
same fold with adipous tissue which covers the internal angle of 
the eye in case of contraction ; for its adherent angle would be too 
narrow to reach this result without the existence of this extra 
apparatus. 
The membrana nictitans may be affected with an ulcerative 
inflammation winch may extend to the cornea. 
Differences .—The membrana nictitans is so much more devel¬ 
oped that the number of digits is smaller. In its greatest devel¬ 
opment in solipeds and ruminants, it becomes less voluminous in 
the pig, smaller still in carnivorous. The third eyelid of birds 
must not be taken for it. 
ORBITAL CAVITY. 
In the skeleton this is reduced to an opening situated on the 
