MAI^HVIALS 



493 



lower forms (Insectivora, Chiroptera, Edentata and Rodentia) and in 

 Primates ; in these with few excejDtions it is immobile (Law, 1905 ; 

 Anelli, 1935). In one monkey {Macacus speciosus) it is capable of 

 slight movement (Jolmson, 1901). and in the aard-vark, Orycterojius, 

 it is freely motile over the keratinized cornea, probably acting as an 

 added protection against the formic acid ejected by the ants on which 

 it feeds. In the C'arnivora, ajiart from the Mustelidse, it is more fully 

 developed, but in the skunk, with its proptosed eyes, it is altogether 

 lacking. In a few Carnivores it is larger (cat, giant panda, bear, deer), 

 wliile in Ungulates it is most highly differentiated ; in these it is 

 sufficiently large to be swept passively but rapidly right across the 

 cornea when the globe is retracted and it is probable that it serves a 

 valuable function in these animals by giving protection to the eyes 

 from long grasses when they graze. 



Among Placentals the nictitating membrane has a basis of hyaline cartilage 

 in most domestic animals (horse, donkey, ox, dog, wolf, pig, goat, cat, hare, 

 etc.) ; in the rabbit (as in Birds) its basis is merely cellular parenchymatous 

 tissue (Naglieri, 1932). Acinous glands resembling the lacrimal gland in structure 

 are also present (Anelli, 1935) ; muscular fibres are vestigial. 



Most Placentals possess two orbital glands. A lacrimal gland 

 secreting a watery fluid is situated in the upper temporal quadrant ; 

 as is usually the case among Vertebrates it is associated with the more 

 mobile lid, in this class, the upper. We have seen that in terrestrial 

 Amphibians in which the gland first ap2:>ears in order to maintain the 

 watery environment of their ancestors for the protection of the cornea, 

 it is situated at the medial canthus in 

 association with the lower lid; in Reptiles 

 and Birds it migrates to the outer canthus 

 still maintaining the same association with 

 the lower lid ; in Mammals it appears at 

 the lateral angle beneath the upper lid 

 (Lor, 1898) (Fig. G49). The structure of the 

 gland varies : it is tubular in man, but 

 is alveolar in some Mammals (horse, pig, 

 ox ; Mobilio, 1912-13) ; in some animals it 

 empties by a single duct (Rodents). 

 Sirenians,^ the pronghorn, Antilocapra 

 americana, and the mouse family are said to 

 lack a lacrimal gland - ; in the pig its 

 secretion is mucoid rather than watery, and 

 in Cetaceans it is oily.^ 



1 p. 502. 



* A lacrimal gland is also lacking in Cyclostomes, Fishes, aquatic Amphibians, 

 Sphenodon, Ophidians, penguins and owls. 

 3 p. 502. 



Fig. 649. — The Migration 



OF THE LaCRIM.\L GlAND 



IN Phylogenetic Deve- 

 lopment. 



^4, position in Amphibians; 

 /?, position in Reptiles and 

 Birds ; .1/, position in Mam- 

 mals (after Wiedershein). 



