EYE OF THE ENGLISH SPARROW 363 



the membrane (fig. 6, B), but, as the membrane is drawn for- 

 ward to its resting position at the anterior angle of the eye, this 

 plait springs or turns out and thus rubs against the lining of the 

 lids (fig. 6, A). 



The physiological significance of this arrangement is apparent. 

 Harder's gland, opening into the sac between the nictitating 

 membrane and eyeball, keeps the inner surface of this membrane 

 well supplied with its secretion. When the membrane is swept 

 over the front of the eye it copiously bathes its conjunctival 

 surface. At the same time the thin edge allows detritus and 

 excess of fluid which accumulated on this surface to flow on to 

 the anterior surface of the nictitating membrane. The mar- 

 ginal plait, lying flat, allows this to occur more readily. As the 

 membrane returns to its passive position, this scoop-like plait 

 collects this excess of fluid and carries it forward between the 

 lids and the nictitating membrane to the anterior canthus, 

 where it escapes into the lacrimal canals. 



The supporting structures of the nictitating membrane con- 

 sist almost wholly of elastic fibers and connective tissue, inter- 

 spersed with numerous blood-vessels, nerves, and lymph spaces. 

 Numerous pigment cells occur in the middle portion of the mar- 

 ginal plait. Doenecke ('99), Fumagalli ('99), and Ellenberger 

 ('06) have demonstrated the presence of smooth muscle fibers in 

 the nictitating membrane, but I have not seen such tissue in the 

 sparrow. The structures of the nictitating membrane are so 

 arranged and so transparent that when the membrane is over 

 the eye it can only slightly interfere with vision. This trans- 

 parent quality, coupled with the extremely rapid movement with 

 which this membrane is swept across the eye, leaves the field of 

 vision clear practically all the time. 



The tendon from the pyramidalis muscles which pulls the 

 nictitating membrane over the eye is attached to the lower end 

 of the marginal plait in the region of the inferior fornix. The 

 downward and backward pull of this tendon not only moves the 

 membrane, but also flattens the scoop-like marginal plait. 



The upper end of the marginal plait is rather firmly attached 

 to the eyeball, which prevents it being pulled away from that 



