BIRDS-EYE VIEWS. 



which, though contracting in opposite directions, and mutu- 

 ally antagonistic as far as the nerve is concerned, eventually 

 exert their force in the same direction, and work harmo- 

 niously for a common purpose. When the tendon of the 

 pyramidrtlis is loosened by relaxation of the two muscles, the 

 nictitating membrane is set free, and returns to its hiding- 

 place by virtue of its own elasticity, just as the curtain of a 

 coach window, after being forcibly drawn down, rolls itself 

 up again when the lever that sets a spring in action is 

 moved. 



We understand the mechanism of the nictitating membrane 

 better than we do its use. Birds can wink with this one 

 eyelid alone, as might be expected from its name, wherein 

 they beat mammals, that cannot wink without moving both 

 lids. If we menace a bird's eye with the finger, we see that 

 the nictitating is the first of the lids to rush to its defence. 

 But the membrane is believed to be chiefly subservient to 

 regulating the amount of light to be admitted to the eye. 

 The eagle is, probably, able to soar aloft directly in the sun's 

 rays, by drawing this covering over its eyes. Owls habit- 

 ually sit, in the daytime, with drawn curtains to shut out 

 the glare of light. It is also quite possible that many, or 

 most birds that are rapid flyers, make great use of this mem- 

 brane in guarding against various dangers to which the eye 

 would be exposed in their dashing career. A screen is 

 placed before the eye, Avhich, while not jDreventing sight, as 

 closure of the outer lids would, opposes the entrance of any 

 particles of matter. 



Three lids of the casket that holds the gem have been 

 raised, and yet there is still another covering of the jewel 

 within. A very delicate filmy membrane, not very apparent 

 on ordinary inspection, is laid over the front of the ball, 

 from around whicli it is reflected over on to the inside of the 

 two outer lids. This is the conjunctiva^ so-called because it 

 joins the lids to the ball. It is a highly vascular tissue, Avith 

 numerous tortuous blood-vessels ramifying all through it. 



