494 THE EYE IN EVOLUTION 



The tears are drained away by the lacrimal passacjes. Since the 

 lacrimal gland was originally situated at the nasal end of the lower lid, 

 the lacrimal passages are always located in this region. These passages 

 are built on the same general plan throughout the Vertebrates and 

 only minor modifications exist (Walzberg, 1876 ; Lichal, 1915 ; 

 Rochat and Benjamins, 1916 ; Sundwall, 1916). The puncta usually 

 open on the inner surface of the lid, not on the margin as in man. The 

 rabbit has one (inferior) canaliculus (Monesi, 1906 ; Rochat and 

 Benjamins, 1916 ; Zaboj -Bruckner, 1924). The sac is rudimentary or 

 lacking in most domestic animals. In some (such as the rat) the naso- 

 lacrimal duct is small and inconspicuous. In others (such as the 

 guinea-pig) it is wide with a well-developed ciliated epithelium and 

 surrounded by a rich venous plexus ; in others again (such as the 

 horse) it is relatively narrow (1 to 2 mm.) with several dilatations 

 (1 to 2 cm.) throughout its length (Kelemen, 1950 ; and others). The 

 passages are completely lacking in aquatic types (the Pinnipedes, the 

 Mustelidse, the hippopotamus, and the Cetaceans) and the elephant 

 (Sardemann, 1884). 



The two lacrimal puncta separate a portion of the lower lid to form the 

 caruncle. Since it is isolated from the margin of the lower lid by the develop- 

 ment of the canaliculus, the caruncle is absent in those animals which have no 

 lacrimal apparatus (Bromann and Ask, 1910). Frequently its cutaneous origin 

 is emphasized by its continuity with the lid -margin (calf and dog) ; it may be 

 deeply pigmented (Fey, 1914), and contains tubular muc<jus glands (Caprino, 

 1955). 



Harder' s gland (1694), an acino-tubular gland the primary function 

 of which is to lubricate the nictitating membrane, lies on the nasal 

 side of the orbit ; sometimes it is very large extending to a variable 

 extent over the posterior aspect of the globe (particularly in the mouse). 

 According to Miessner (1900) it is absent in the deer, among the lower 

 monkeys it is rudimentary (Giacomini, 1887), and in the Anthropoids 

 and man it is represented only by a transitory fcetal structure in the 

 infero -lateral fornix (Loewenthal, 1910).i The gland of Harder 

 secretes a sebaceous (Wendt, 1877) or a mucous material (pig, dog, 

 sheep ; Virchow, 1910) which it pours into the conjunctival sac by 

 two ducts. 



The extra-ocular muscles comprise four recti, two obliques and 

 (usually) a retractor bulbi muscle. The recti are arranged as in man ; 

 the mammalian superior oblique differs from that of lower Vertebrates in 

 the migration of its origin to the apex of the orbit, the reflected tendon 

 being designed to retain the original direction of action (Poole, 1905) 

 (Fig, 2!;3). This mode of development is emphasized in some animals 



' H:; :■ /;ti's gland is also absent in Cyclostomes, Fishes and aquatic Amphibians. 



