424 THE EYE IN EVOLUTION 



movements of the iids there is a more equable distribution of labour 

 than is seen in Amphibians and other Sauropsidans (Bartels and 

 Demiler, 1921) : the lower is usually the more active of the two, but 

 the upjDer lid also plays a considerable part. Except in parrots, the 

 more active lower lid is provided with a fibrous tarsal plate composed 

 of fibro -elastic tissue without cartilage (Naglieri, 1932). The nictitating 

 membrane is well developed with a feather-like epithelium (Kajikawa, 

 1923 , Kolmer, 1923-30 ; AnelH, 1935) ; it sweeps over the globe 



Fig. 534. — The Orbits and Brain of the English Sparrow, Passeb 



domestivus. 



c, optic chiasma ; e, external rectus ; g, gasserian ganglion ; /;, liarderian 

 gland ; in, inferior rectus ; io, inferior obliqvie ; ir, internal rectus ; /, lacrimal 

 gland ; m, medulla ; o, optic nerve ; ol, optic lobe (midbrain) ; p, pituitary ; 

 3, third cranial (oculomotor) nerve, supplying the superior, internal, and inferior 

 recti and the inferior oblique ; 4, fourth cranial (trochlear) nerve, supplying 

 the superior oblic^ue ; 5, fifth cranial (trigeminal) nerve, several of the branches 

 of which carry fibres to the eye and adnexa ; 6, sixth cranial (abducens) 

 nerve, supplying external rectus (Gordon Walls ; drawni from Wood and 

 Slonaker). 



from the nasal canthus controlled by a pyramidalis muscle attached to 

 the posterior surface of the sclera, the optic nerve being protected by 

 lacing the tendon tlu'ough the well-developed quadratus (bursalis) 

 muscle (Figs. 531-2). It is probable that these two muscles are homo- 

 logous with the retractor bulbi of Crocodilians (Wedin, 1953). The 

 nictitans is very transparent and has no fibrous or cartilaginous basis ; 

 it is probable that it can cover the eye without affecting vision greatly, 

 and in fact many believe that it is drawn over the cornea habitually 

 as a p '-^ctive goggle during rapid flight. 



In • in diving birds (diving ducks ; auks, Alcidse ; and the loon, Gavia) 



the nict i membrane has a central clear window which, being highly refractile, 



