Compamtvve Anatomy of tlie Mammalian Eye. 475 



known to be the case when the hyaloid vessels are directly continued 

 into the retina, where they produce two vascular layers. B. From 

 special retinal vessels cumulating in the art. centralis. This mode is 

 restricted to the Mammalia and some of the Snakes. 



The vessels of the falciform process of the fishes and the central 

 hyaline artery, wherever this occurs, are essentially the same. The 

 falciform process and the pecten are analogous, but not homologous, 

 structures. In Eeptiles and Birds the hyaloid artery is superseded by 

 a new development, viz., the Pectinal system. In some of the lower 

 Mammalia both systems actually occur side by side, but both are 

 rendered unnecessary by the development of a third system of supply, 

 viz., special retinal vessels, which ultimately culminate in the posses- 

 sion of an art. and vena centralis retinae. 



Some of the normal conditions ohservahle in certain animals closely resemble 

 those tvhich ive find in Man as congenital defects or vestigial relics. 1. 

 INIembrana nictitans. A fully developed nictitating membrane active 

 enough to sweep the whole cornea, exists only in the XJngulata, and 

 not even throughout this order. In the Carnivora and Marsupials it 

 is much less developed, whilst throughout the Primates, Rodents, Eden- 

 tata, and Echidna it is still more reduced, and, with rare exceptions, 

 entirely without movement. The primary use of this third lid, viz., 

 that of cleaning the corneal surface, is lost within the class of the 

 ^Mammalia, and seems to serve chiefly to protect the eye in the animals 

 which graze and poke their heads down into the long and sharp grass. 

 2. The retractor muscle of the eyeball is of frequent occurrence, chiefly 

 in Marsupials, Edentates, Eodents, and Ungulates, i.e., in the lower 

 orders of Mamnials. 3. Opaque nerve fibres. All stages of opacity 

 occur congenitally in Man, and are to be found normally throughout 

 the Mammalia. Opaque nerve fibres are most marked in some of 

 the Eodents and Marsupials. 4. Physiological cup and congenital 

 discoloration of the disc frequently occur in Man. An appearance 

 similar to the physiological cup occurs in all the Felidse, and in a con- 

 siderable number of the other Carnivora ; also in the Flying Squirrels 

 and some of the other Rodents. White and grey discs occur normally 

 in a number of animals widely separated in classification, such as the 

 Skunk, Rhinoceros, Porcupine, Armadillo, and Echidna. 5. Structures 

 protruding from the disc into the corpus vitreum. A. Persistent 

 hyaline artery. This congenital defect in Man is found as a normal 

 condition in nearly all the Ruminants and in a large number of 

 Rodents. B. Vestiges of a pecten. In some of the Rodents, more 

 especially in all the Agoutis, a button-shaped vascular pigmented 

 rudimentary pecten protrudes from the disc into the vitreous. It is 

 remarkable to find in the Mammalia a relic of this Sauropsidan organ. 

 In a number of Marsupials vascular protuberances from the disc into 

 the vitreous occur in diff'erent forms. 6. Colobomata. The papillary 



