EYES. 203 
perfectly transparent, through which light passes to reach the lens. 
The cornea, covered externally by the conjunctiva, the modified 
epidermis of the front of the eye, consists of connective tissue; the 
sclera is usually white. In most of the lower vertebrates and in the mono- 
tremes it is partly or wholly cartilaginous, but in other mammls and in 
the lampreys it consists of fibrous tissue. In the stegocephals and in 
many reptiles and birds portions of the sclera ossify as a ring of scle- 
rotic bones (p. 67). 
Sclerotic bones are lacking in snakes, plesiosaurs and crocodiles. In the 
sturgeon and many teleosts two or more dermal bones develop upon the sclera, but 
neither these nor the calcifications to be found in some sharks‘and teleosts are to 
be confused with true sclerotics. 
Between cornea and lens is a cavity which is partially divided by 
the iris into anterior and posterior chambers which connect with 
each other through the pupil. These are filled with a refracting fluid, 
the aqueous humor. 
The parts so far described form the eye-ball (bulbus oculi) which 
is more or less freely movable in its socket in the side of the head. It 
is moved by the six muscles (p. 128) which are constantly present. 
Others may occur here and there. Thus in the amphibia a distinct 
muscle (retractor bulbi) is developed from the external rectus to 
pull the ball back into the socket, while portions of the jaw muscles 
may be set apart as elevators and depressors of the ball. In the elasmo- 
branchs a cartilaginous rod, the optic pedicel, extends from the ball 
to the skull. This is replaced in the teleosts by a fibrous band, the 
tenaculum, but its equivalent is not found in the higher groups. 
Among the accessory parts of the eye are the lids, of which there 
may be three, the upper and the lower lids so familiar in the higher 
vertebrates and the third lid, the nictitating membrane, a transparent 
sheet which may be drawn horizontally across the front of the eye 
from the inner (anterior) angle of the eye or from beneath the lower lid. 
All three lids are folds of the skin. The upper and lower are poorly 
developed in the ichthyopsida, but appear in the amniotes. They 
are lined on the side next the eye by a continuation of the conjunctiva, 
which continues beyond the edge of the lid as the epidermis. The 
nictitating membrane appears in some sharks, again in the amphibia, 
and receives its highest development in the sauropsida, while in the 
mammals it is reduced to a rudimentary fold, the plica semilunaris, 
at the inner angle of the eye. : 
