EYE OF THE ENGLISH SPARROW 371 



section through a pyramid. Its broad origin lies under the 

 insertion of the inferior obhque and inferior rectus muscles. 

 From this region it extends upward, lying close to the eyeball, 

 and gradually narrowing until it terminates m a narrow flat 

 tendon just anterior to the optic nerve. This tendon now 

 turns backward, passes through the pully-like loop of the quad- 

 ratus, just above the optic nerve entrance, and extends down- 

 ward and backward, having almost completely encircled the 

 optic nerve. After passing through the loop of the quadratus 

 the pyramidal tendon lies in a groove, formed partly by the 

 sclera and partly by the adjacent connective tissue. It is thus 

 prevented from shifting its position, enabling it to pull always 

 in the same direction. It thus extends around to the front side 

 of the eye where it is attached to the lower movable part of the 

 nictitating membrane. This insertion is mainly to the marginal 

 plait of this membrane where it spreads out in a fan-like manner. 

 Some strands of the tendon extend farther forward arid are 

 inserted into the lower part of the membrane. The united con- 

 raction of the pyramidalis and quadratus muscles is necessary 

 to prevent the tendon of the pyramidalis from pulling down on 

 the optic nerve and injuring it. This arrangement of muscles 

 furnishes a greater amplitude of movement of the nictitating 

 membrane than could be secured by the action of a single muscle. 

 Both of these muscles are innervated by the same nerve. 



By various experiments I have found that the sparrow can 

 seldom be induced to close its true lids, even when the cornea is 

 touched with an instrument. In fact, the true lids are appar- 

 ently closed only in sleep or death and, in this bird, the nicti- 

 tating membrane performs the function of the upper and lower 

 lids of mamals. 



Numerous observations show that the movements of the nic- 

 tating membrane of each eye are more or less independent. 

 They are often moved simultaneously, but one frequently moves 

 across the eye while the other remains quiet. 



It was further seen that the brightness of the light had a 

 marked influence on the frequency of movement. When the 

 sparrow was kept in the darker portion of the room, the nrnn- 



