STRUCTURE OF THE VERTEBRATES 309 



sation is incomplete. The axones from the two sides of the 

 retina divide, so that the outer (posterior) half from the left 

 eye and the inner (anterior) half from the right eye pass to the 

 left side of the brain. The condition is similar in the right optic 

 nerve. The complete nerve from the eye meets its mate from 

 the other side just anterior to the pituitary body, and forms an 

 incomplete chiasma. 



Accessory Structures. The eyelids form protective covers for 

 the eyes. The fish either lack or have very poorly developed 

 dorsal and ventral lids; but a few sharks and teleosts have a 

 third lid, the nictitating membrane, which arises on the anterior 

 margin of the eye and moves posteriorly. The anura and most 

 reptiles have all three lids developed; but in the mammals the 

 membranous third lid becomes small, and in the human is left 

 only as a small fold on the inner angle of the eye. The mam- 

 malian lids are fringed with stiff hairs, the lashes of the eye. 

 The epidermal covering of the lids is continued over the inner 

 surface as the conjunctiva. The lids and the surrounding skin 

 are well supplied with muscles. The eyes are bathed by fluids 

 from the lacrimal glands, the excess fluid passing to the nasal 

 passages through the lacrimal ducts. 



E. Auditory Organs 



The ears undergo marked progressive changes in the different 

 classes of vertebrates. Primitively the organ is concerned only 

 with a sense of balance or equilibrium. This is true of the 

 cyclostomes and fish, in which the ear is limited to the inner ear, 

 consisting of the semicircular canals. 



In all vertebrates the auditory apparatus arises as a thicken- 

 ing of the ectodermal covering of the embryo which sinks in as 

 a hollow vesicle, a pore connecting it with the outside. The 

 connection with the exterior remains open in the adults of some 

 elasmobranchs, but closes in all the other groups. The cavity 

 of the inner ear is filled with an endolymphatic fluid, and the 

 cavity in which it rests is similarly filled with perilymph. 



The most primitive ear is found in the lower cyclostomes. 

 these animals having a single canal, flattened on the bottom 

 and rounded dorsally. The tube is lined by epithelial cells with 



