IVIAMVIALS 457 



is impervious to tears and presumably camiot mediate an adequate respiratory 

 exchange. 



The Choroid. The layers of the choroid in the placental eye 

 correspond with those of man (Fig. 578), i the choriocapillaris being 

 usually thin, exceptionally so in the Sciuridse (squirrels) and Gliridae 

 (dormice) ; exceptions to this are aquatic types (Pinnipedes, Cetaceans) 

 Avherein the choroid is unusually thick. One interesting and variable 

 feature, however, is the tapetum lucidum,^ an adaptation acquired 

 by certain nocturnal animals to improve vision in dim illumination. 

 Optically the tapetum acts as a mirror which, lying behind the rods 

 and cones, reflects the incident light so that it traverses the visual 

 elements twice, thus increasing differences in apparent brightness. 



The tapetum of Placentals was first adequately described by Briicke 

 (1845) and thereafter the subject has received much study ; its 

 histological characteristics were fully elucidated by Sattler (1876) 

 while its ophthalmoscopic variations were beautifully illustrated by 

 Johnson ( 1901 ) (Plates XIV and XV). It lies in the upper posterior part 

 of the fundus with a preference for the temporal side which is used for 

 forward vision. Ophthalmoscopically it appears as a bright area in 

 the fundus, usually of triangular shape with its base horizontal just 

 above the optic disc, sometimes lying entirely above this structure 

 (horse), sometimes including it (cat) ; it varies, however, considerably 

 in extent, being unusually large in the Cetaceans (dolphins and 

 whales), while in the PinniiDcdes (seals) it occupies the entire posterior 

 area of the fundus up to the equator and beyond on the temporal side. 

 In the tapetal area pigment is lacking in the retinal epithelium to allow 

 the transmission of light, and, lying between the choroidal layer of 

 vessels and the choriocapillaris, it is traversed by small vessels to 

 supply the latter, visible ophthalmoscopically as stellate dark dots on 

 the bright background — the " stars " of Winslow. The tapetum does 

 not appear ophthalmoscopically in the puppy until some weeks after 

 birth (Usher. 1924). 



Histologically two types of tapetum are found, both completely 

 different in origin and structure — the tapetum fibrosum and the 

 tapetum cellulosum (Figs. 579 and 580). 



The TAPETUM FIBROSUM develops from the thin layer of elastic 

 fibres found normally in the inner layer of small vessels of the choroid 

 (Sattler, 1876). It is composed of dense fibrous tissue the fibres of 

 which are closely woven together so that the entire structure glistens 

 like a piece of fresh tendon. Among Placentals such a tapetum is 

 typically foiuid in the Ungulates, among which it is almost universal 



^ The unique structure of the choroid of the larger bats will be noted subsequently, 

 p. 459. 



^ Tapetum lucidum, bright carpet. 



