BIRDS 



403 



Fig. 49;"). — The Ring of Scleral 

 Ossicles of the Right Eye of 

 THE Goshawk, Astch PALcvBARirs 



d, dorsal ; r, ventral ; /;. nasal ; 

 t, temporal (after Franz). 



Fig. 496. — The Cartilaginous Cup 

 IN THE Posterior Part of the 

 Globe of the Hawk. 



strengthened by a firm cartilaginous cup which occupies the inner half 

 of the thick fibrous sclera, while the waist-like constriction is maintained 

 by a ring of imbricating scleral ossicles made up of membranous bones 

 overlapping the anterior edge of the cartilaginous cup (Figs. 495 and 

 497). These ossicles, described by Malpighi (1697) in the eye of the 

 eagle, vary in number from 10 to 18, the commonest being 15 (Dabelow, 

 1926-27), and while they are formed of compact bone in small eyes, in 

 large and particularly in tubular eyes they contain air-spaces as do 

 many of the bones of the bird's skeleton (Lemmrich, 1931) ; it is this 

 ring of bone w^hich essentially determines and maintains the configura- 

 tion of the intermediate segment and therefore of the entire eye. 



Incorporated in the posterior cartilaginous cup a ring- or horse-shoe-shaped 

 bone may be found, the os opticus or ossicle of GEiiMiNGEK (1852) surrounding 

 the optic nerve-head in one or several pieces ; like the anterior scleral ossicles 

 it is highly cancellous in texture. Tiemeier (1950) found it present in 219 out of 



Magjjie 



Fig. 497. — The Ciliary Region of the Chicken. 

 Showing the imbricated scleral ossicles beneath ( X 84) (Norman Ashton). 



