MAMIVIALS 491 



THE OCULAR ADNEXA 



The conjunctivce of many Mammals show large papillae (horse) or 

 follicles (ox, dog, pig, rabbit) which are not present in the physiological 

 state in man (Bruch, 1853 ; Morano, 1873 : Miimi, 1935). There is 

 usually an accumulation of pigment, especially near the limbus, but fre- 

 quently continued into the cornea, contained in branched contractile 

 cells. The transition from the conjunctival to the corneal epithelium 

 is usually gradual, but in some animals (horse) it is abrupt (Zietzsch- 

 mann, 1904). Variations occur in the conjimctival glands ; thus sweat 

 glands are seen in the bulbar conjunctiva of the pig. the goat and the 

 ox. Small diverticuli filled with epithelial cells somewhat resembling 

 epithelial cell-nests forming tubular depressions near the limbus were 

 first described in the pig as the glands of Manz (Manz, 1859 ; Stromeyer, 

 1859), vestigial traces of which may be seen in man. Their function is 

 uncertain ; according to Aurell and Kornerup (1949) they are the 

 remnants of accessory lacrimal glands which develop in the pig in 

 embryonic life, sorae rimes persisting in the form of epithelial buds and 

 sometimes as tubules with poorly developed lumina. 



In the typical Placental, three eyelids are present — an upper, a 

 lower, and a nictitating membrane (or third eyelid) ; the aquatic 

 Placentals, however, form an exception.^ Of the tlu-ee, the upper lid, 

 as in Selacliians, is the more fully developed and with few exceptions 

 (elephant, deer, hippopotamus, mouse) descends more than the lower 

 ascends — a reverse of the action seen in most lower Vertebrates wherein 

 the lower lid is the more mobile. ^ 



It i.s interesting that Mammalia is the only class wherein spontaneous 

 shutting and ojDening of the lids or blinking is highly developed ; although 

 sometimes slow, particularly in primitive fomis, the blink-movements are usvially 

 very rapid, and except in types with completely lateral eyes, the blink 

 reflexes of both eyes respond when one is threatened or touched. 



The upper lid always has a stiffening tarsal plate, the lower 

 sometimes; it is usually comprised of dense fibrous tissue but is occasion- 

 ally cartilaginous (in the hedgehog, bat and leopard, Anelli, 1936). 

 Embedded in the tarsi and opening on the lid-margin are tarsal 

 (meibomian) glands providing an oily secretion ; in view of the fact 

 that they evolve from the glands of hair-follicles it is understandable 

 that they are found only in Mammals. In Mammals the tarsal glands 

 are usually smaller than in man : they are absent in aquatic types, 

 replaced by Zeis"s glands in the elephant, and by sebaceous glands on 

 the caruncle in the camel (Richiardi, 1877). At the external angle they 



1 p. 501. 



^ The lower lid is the more mobile in Amphibian.s, Reptile.? (except Anolis alligator 

 wherein both are equally mobile, and ? Crocodilians) and, with few exceptions, Birds. 



