492 THE EYE IN EVOLUTION 



are large and modified in some Rodents (Loewenthal, 1931). Glands 

 of Moll are present in many Ungulates (ox, pig), Carnivores (dog, cat), 

 and Primates (apes, man) ; but in Rodents they are absent (rabbit, 

 guinea-pig, rat, mouse) (Ikeda, 1953). Most Mammals have cilia 

 (Zietzschmann, 1904), the whale, elephant and hippopotamus being 

 exceptions (Matthiessen, 1893) ; among domestic animals those of 

 the lower lid are rudimentary, while localized absences occur, such as in 

 the mid-region of the upper lid of the horse (F. Smith, 1922). Eyebrows 

 are specialized in many Placentals (particularly the cat) into long 

 tactile vibrissas ; the camel has a somewhat similar formation on its 

 lower lid. 



The movements of the two main lids are elaborately controlled by 

 muscles. In terrestrial Placentals they are closed by the con- 

 traction of the annular orbicularis oculi muscle with a sphincter-like 

 interlacing system of fibres (Zietzschmann, 1904 ; Meinertz, 1932-42 ; 

 Rohen, 1953-54). All are provided with a levator palpebrae superioris, 

 except the aquatic Cetaceans which have a dilatator rimae palpebrarum 

 distributed round the lids (Stannius, 1846, in dolphins ; Virchow, 1910, 

 in whales). The elephant has a depressor palpebrae inferioris similar 

 to the levator of the upper lid (Virchow, 1910), and in Herbivores the 

 external malar muscle serves as a depressor of the former. 



The palpebral muscles of Miiller are more fully developed in lower Mammals 

 than in man : in aquatic Mammals the fibres are striated, in terrestrial Mammals 

 they are plain. According to CJroyer (1903) they are developed in association 

 with the superior and inferior recti : these divide into two parts, one of which 

 is striated and is inserted into the eyeball, the other is inserted into the lids. 

 Owing to the great development of the upper lid, the muscle running to it 

 divides again into two, forming a large levator muscle anteriorly, and a small 

 palpebral mviscle posteriorly. In those cases wherein the palpebral muscles are 

 composed of plain fibres, they are supplied by the sympathetic nerve, but where 

 they are striated they are supplied by the nerves to the recti. 



The third eyelid in Placentals is characteristically rudimentary ; 

 although often reinforced by a plate of hyaline cartilage it lacks a 

 specific musculature as is found in so many lower Vertebrates. 

 Entirely passive in its movements, it is rarely functional, slipping 

 over the eye when the globe is retracted. Occasionally, as in the bear 

 and the rhinoceros, it drifts partly across the cornea when the animal 

 becomes sleepy. The mechanism of its movement is much less specialized 

 than in lower Vertebrates, for any muscular elements it contains are 

 merely vestigial. It seems to be forced out from the canthus across 

 the cornea by the propulsive action of the retractor bulbi muscle as it 

 pulls the eyeball inwards ; while the return of the membrane, although 

 probah\\ largely due to its own elasticity, may be helped by the opposite 

 action the orbital muscle of Miiller. It is most rudimentary in the 



