IX.] 



'THE NERVOUS SYSTEM. 



out division (into upper and lower), so that the primitive 

 folds have, as it were, coalesced, entirely shutting off the 

 conjunctival chamber from the exterior, except by means 

 of the communication effected with the nares through the 

 lachrymal canal. 



A third eyelid (called the nictilating membrane) often 

 exists, as e.g. in Birds, attached to the inner side of the front 

 of the orbit. It is moved in various ways, as already noted 

 in describing the muscles of the eye. A minute rudiment of 

 this structure exists in man. 



The form of the eyeball may be much less spherical than 

 in man. Thus it may be much flattened anteriorly, as in 

 Fishes ; or, on the contrary, much elongated, i.e. anteriorly 

 produced, as in Birds, e.g.- the Owls. 



The optic nerves may, after development, entirely or all 

 but entirely abort, as in the Mole ; but this atrophy is very 

 exceptional. 



On the other hand, that primitive condition of the optic 

 nerves in which the fibres do not decussate may persist 

 throughout life, as in the osseous Fishes ; nevertheless, the 

 nerves themselves cross (each one going to the eye of the 

 side of the head opposite to 

 that of its own origin), though 

 they do not form a chiasma. 



Unlike the nasal organ, the 

 optic structure is never single 

 and median except by mon- 

 strosity. 



There may even be the 

 appearance of two superim- 

 posed eyes on each side of 

 the head. This remarkable 

 structure occurs in the Fish 

 Anableps, and is produced 

 by the presence of a hori- 

 zontal opaque tract in the 

 middle of the cornea together 

 with a double perforation of 

 the iris. 



Both eyes may come in the 



adult to be placed on one side of the head, as in the Flat 

 Fishes, such as the Sole and Turbot ; or both eyes may be 

 closely approximated on the upper surface of the head, as 

 in the' Fish thence termed the Star-gazer ( Uranoscopus]. 



FIG. 342. RIGHT SIDE OF THE HEAD 

 OF ONE OF THE PLEURONECTID^E. 



