2l8 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



monotremes it is partly or wholly cartilaginous, but in other mam- 

 mals, and in the lampreys it consists of fibrous tissue. In the stego- 

 cephals and in many reptiles and birds portions of the sclera ossify as 

 a ring of sclerotic bones (figs. 75, 103). 



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 htunor. 



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. 133) 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 pedicle, extends from the ball 

 to the skull. This is replaced in the teleosts by a fibrous band, the 

 tenacultun, 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 Hd, 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 Hd. 

 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 conjunc- 

 tiva, which continues beyond the edge of the Ud as the epidermis. 

 The nictitating membrane appears in some sharks, again in the am- 

 phibia, and receives its highest development in the sauropsida, while 

 in the mammals it is reduced to a rudimentary fold, the plica semi- 

 lunaris, at the inner angle of the eye. 



There are no glands connected with the eyes in cyclostomes or 

 fishes, but in the urodeles a series of glands is developed from the con- 

 junctival lining of the lower lid. In the amphibia they show little 

 differentiation, but in all sauropsida (glands are lacking from a few 

 reptiles — crocodile tears are non-existent) they become divided into 



