MAMMALS 



433 



Fig. 541. — The Posterior Segment of the Globe of the Platypus. 



ch, choroid ; /, orbital fat ; r, retina ; s, scleral cartilage ; sc, fibrous 

 sclera ( X 126) (O'Day). 



scopic examination), Kolmer (1925-36), Franz (1934), Gresser and 

 Noback (1935) and O'Day (1938-52) on the echidna (Figs. 538 to 540). 



THE GLOBE OF THE EYE is roughly Spherical, the sclera, as in most 

 Reptiles, having a well-formed cartilaginous cup extending forwards 

 to a little behind iTachyglossus) or to the level of the ora (Zaglossus) 

 or to the level of the ciliary processes {Ornithorhynchus) ; it is perforated 

 to allow the transmission of the optic nerve, 

 and the intra-ocular vessels and nerves (Fig. 

 541). This is the only instance of a scleral 

 cartilage among the Mammals.^ The corneal 

 epithelium of the echidna is heavily kera- 

 tinized like that of other ant-eaters (the 

 armadillo, Xnarthia, and the aard-vark, 

 Orycteropus), possibly as a protection against 

 the formic acid with which ants defend 

 themselves. As in aquatic Vertebrates 

 generally, the corneal epithelium of the 

 platypus is thick and Bowman's membrane 

 is absent. The anterior ends of the ciliary 

 processes are connected by a shelf-like 

 structure running circumferentially around 

 the globe (the Sims of Virchow, 1886 ; the 



^ With the exception of the cartilaginous nodules in Notoryctes, p. 438. 



S.O.— VOL. T. 28 



Fig. 542. — The Ciliary Pro- 

 cesses of Echidna 



C, ciliary body ; CS, 

 ciliary shelf ; I, iris ; S, 

 sphincter muscle (after 

 Franz, 1911). 



