326 THE BIOLOGY OF BIRDS 



unknown function, which do not seem to occur in other 

 animals. 



The high degree of visual acuteness in birds is essentially 

 due to the delicacy and dense arrangement of the elements 

 in the restricted centre of the fovea or foveas. 



As regards eyelids, birds have three. The upper and 

 lower eyelid meet when the bird is sleeping, but in ordinai7 

 cases they do not move nearly as much as man's do. In 

 our case the upper eyelid is frequently drawn down over 

 the front of the eye, moistening and cleaning the surface 

 of the eyeball, and overlapping at the outer corner the lower 

 eyelid which comes up a little way to meet it. In birds the 

 lower eyelid comes far up. Mr. Beebe notes that in the 

 Great Horned Owls the action of the two lids is much the 

 same as in man. The upper and lower lids are fused in 

 the very young bird until near the time of hatching or for 

 some time after. 



Of great interest is the third eyelid, which made its 

 appearance in reptiles, and is well-developed in the crocodi- 

 lians. It is a very important cleaning and protective struc- 

 ture in birds, and is always being flicked down over the surface 

 from the anterior upper corner where it lies. It is pulled 

 down by two special muscles with delicate tendons, and 

 jerks back by its own elasticity when the muscles relax. 

 As it is semi-transparent, the bird can to some extent see 

 through it when it is drawn down for a considerable time, 

 as in an owl which has ventured abroad in the daytime. 

 The old story of the eagle renewing its sight by looking at 

 the sun may have reference to the fact that the bird draws 

 down its third eyelid when looking up into the bright 

 sky, and discloses its keen eye when it turns again from the 

 glare. 



This third eyelid is present in most mammals and has 

 an eye-cleaning function. It is absent in whales and 

 dolphins where its role is discharged by the continual washing 

 of the front of the eye with water. It is vestigial in man 

 and monkeys, where the upper eyelid has become much 

 more mobile than it is in lower mammals. In man it is 



