644 BIRDS 



(Fig. 107, p. 270) overlap the scleral cartilage externally (Fig. 191), 

 and extend forward nearly to the limbus. Their number ranges from ten 

 to eighteen, except for rare instances among diving birds, where the 

 basic number has been increased by anterior and posterior fragmentation. 

 Fifteen is the commonest number and perhaps the 'original' one. In a 

 summary of 460 species of birds, Lemmrich found the ossicle numbers 

 to be distributed as follows : 



No. of ossicles: 10 11 12 13 14 15 16 17 18 

 No. of species: 1 18 26 57 138 182 31 3 4 



Though the ossicle of Gemminger, despite its thinness, usually or al- 

 ways contains marrow spaces, such are present in the anterior ossicles 

 only where these are largest and thickest (owls, frogmouths, hawks, 

 eagles, etc.). The fact that the bony ring is made up of separate pieces 

 probably has no physiological significance, but Lemmrich has pointed 

 out that the ring could not otherwise grow with the eyeball. 



The cornea is usually relatively small in area, and especially so in 

 underwater swimmers; but it becomes larger in globose eyes and very 

 large and strongly arched in nocturnal forms. It is ordinarily somewhat 

 thinner at its apex than at the periphery, but in large eyes the thickness 

 of the cornea tends toward uniformity everywhere. In spite of its cus- 

 tomary eccentric position, it is almost always circular in outline and 

 neatly fills the lid opening. Histologically, the avian cornea is quite like 

 that of man, though a Bowman's membrane is not always differentiated. 



The corneal surface is kept especially well polished by the action of 

 the nictitating membrane with its lining of papillose cells. The nictitans 

 also cleans off the inner surfaces of the other lids, and keeps them from 

 smearing the cornea, in those birds in which the lids close just after the 

 nictitans in a 'blink' (e.g., pigeon). The nictitans has a marginal pleat 

 which slides easily under the lids in the 'going' direction, but scrapes 

 them on its way back. In many birds or most, the lids close only in sleep 

 and the nictitans alone blinks. In consequence, the upper and lower lids 

 have a largely unstriated, slow-acting musculature.* 



The chorioid is relatively thick — more so than in mammals and much 

 thicker than in reptiles. It is thickest in the fundus. Its distinct vessels 

 appear to be mostly arteries, and these lie close to the choriocapillaris 

 which they supply. Between them and the thin, pigmented 'lamina fusca' 

 (applied loosely to the sclera) lies a thick region which in prepared slides 



*In altricial birds the lids are closed for a time after hatching, but in all birds they are wide 

 open before hatching, not fused edge-to-edge as in fetal mammals. 



