672 MAMMALS 



Wherever in higher (i e., placental) mammals the eye departs from this 

 fundamental sphericity and gains the appearance of having a circum- 

 corneal scleral sulcus, it is owing to the production of a cornea whose 

 radius of curvature is substantially less than that of the sclera ie. g., 

 man). 



The cornea has a 4-5-layered epithelium, and no Bowman's mem- 

 brane; but Descemet's membrane is ordinarily very thick (not, however, 

 in Marmosa). The cornea is usually uniform in thickness throughout, 

 but is thinned peripherally in opossums and perhaps in other small- 

 eyed forms. 



The chorioid is usually about as thick as the sclera — hence, thin in 

 small eyes and thick in large ones. It is heavily pigmented, richly vas- 

 cular, and ordinarily is built quite as in the placentals. In Didelphis, 

 however, a choriocapillaris is present only in the pouch young. During 

 growth to adulthood, pari passu with the maturation of the tapetum 

 formed from the retinal pigment epithelium and the unusual invasion of 

 the outer nuclear layer by retinal capillaries, the choriocapillaris is re- 

 placed by (or becomes) a plexus of plump, thin-walled veins which occupy 

 the same position against the back of the glass membrane. These alter- 

 ations bespeak a turning of the visual cells from the chorioid to the 

 retinal circulation as their source of supplies, owing to the impermeability 

 of the tapetum. In a very few other marsupials (Dasyurus, Thylacinus, 

 possibly Sarcophilus and Petaurus) the chorioid is modified by the pres- 

 ence of a tapetum fibrosum. In Dasyurus viverrinus this nearly fills the 

 chorioid — squeezing the large vessels and the few thin, pigmented lamel- 

 lae out against the sclera — and runs practically from ora to ora, though 

 permitted to reflect light back through the retina only in the superior 

 half of the eyeground, where the retinal pigment epithelium is devoid of 

 pigment. It is probably significant that it is only in Dasyurus, Sarco- 

 philus, Didelphis, and Marmosa (which may once have had a tapetum 

 like its close relative Didelphis) that any retinal vessels are known to 

 occur — necessitated, apparently, by the interference of the tapetum with 

 the nourishment of the retina by the chorioid (see pp. 652-4) . A vestigial 

 conus papillaris may also occur in marsupials (Perameles, Hypsiprymnus, 

 and kangaroos generally) . The supply of the retinal capillary bed, where 

 present, is from paired veins and arteries which radiate over the retina 

 from the disc. In Dasyurus viverrinus each of these veins (and their 

 larger branches) is triangular in cross-section and is embedded in the 

 inner layers of the retina, with its round arterial companion lying on 



