The third eyelid, or membrana nicti tans of birds, is a thin semi-transparent fold of the conjunctiva ; which, in the state of rest, lies in the inner corner of the eye, with its loose edge nearly verti cal, but can be drawn out so as to cover the whole front of the globe. By this, ac cording to envier, the eagle is enabled to look at the sun.
It is capable of being expanded over the globe of the eye by the combined ac tion of two very singular muscles, which are attached towards the back of the scle rotica. One of these, which is called from its shape the quadratus, arises from the upper and back part of the scleroti Ca ; its fibres descend in a parallel course towards the optic nerve, and terminate in a semicircular margin, formed by a ten don of a very singular construction ; for it has no insertion, but constitutes a cy lindrical canal. The second muscle, which is called the pyrainidalis, arises from the lower and back part of the sclerotica to wards the nose. It gives rise to a long tendinous chord, which runs through the canal of the quadratus, as iw a pulley. Having thus arrived at the exterior part of the eyeball, it runs in a cellular sheath of„ the sclerotica along the under part of the eye to the lower portion of the loose edge of the membrana nictitans, in which it is inserted.
By the united action of these two mus cles, the third eyelid will be drawn Ito wards the outer angle of the eye, so as to cover the front of the globe ; and its own elasticity will restore it to its former situ ation.
Two kinds of eyes, very dissimilar in their structure, are found in insects : one sort in small and simple, (stemmata ;) the others, which are large, seem to consist of an aggregation of smaller eyes ; for their general convexity is divided into an immense number of small hexagonal con vex surfaces, which may be considered as so many distinct corner. The first kind is found in different numbers in most of the aptera, as also in the larva of many winged insects. When these undergo the last or complete metamorphosis, and receive their wings, they gain at the same time the large compound eyes. Several genera of winged insects and ap tera (as the larger species of monoculi,) have stemmata, besides their compound eyes.
The internal structure has hitherto been investigated only in the large poly edrous eyes. The back of the cornea (which is the part divided in front into the hexagonal surfaces, called in French, facettes) is covered with a dark pigment : behind this are numerous white bodies, of an hexagonal prismatic shape, and equal in number to that of the facettes of the cornea. A second coloured mem brane covers these, and appears to receive the expansion of the optic nerve.
Further investigation is, however, re quired, in order to shew how these eyes enable the insect to see ; and to deter mine the distinction between two such very different organs.