426 ADAPTATIONS TO MEDIA AND SUBSTRATES 



The Meibomian glands, embedded in the palpebral tarsi, appear in 

 the mammals for the first time. They could not well have evolved sooner, 

 since they represent glorifications of the oil-glands associated with hairs. 

 They are lacking in some species; and true eyelashes are absent in the 

 elephants and whales, nearly so in the sea-cows and hippopotami. 



The lacrimal gland lies near the temporal canthus. Associated as 

 always with the more mobile of the lids, it may lie wholly above this 

 level and has most of its 1-15 ducts opening under the upper lid. It is 

 usually lobed or divided, as it is in man. In murid rodents it is tiny or 

 lacking, though Harder's glands (and other glands, peculiar to rodents) 

 are present, Harder's gland sometimes being very large and forming a 

 cushion over the whole back half of the eyeball (mice; also, shrews). 

 The lacrimal is said to be lacking in the pronghorn, Antilocapra ameri- 

 cana. In the pig its secretion is not watery as usual, but is rich in mucus. 

 The drainage canal is vertical and opens by two punctae, one on or near 

 each lid-margin at the nasal canthus, with a caruncle (Fig. 16) usually 

 lying between them. 



The nictitans has its ups and downs in the mammals. Where it is well 

 developed, it usually has a cartilaginous tarsus, but it never has a special 

 musculature behind the globe as in lower vertebrates. Hence, it slips over 

 the eye only passively when the globe is slightly or markedly retracted. 

 Contrary to logical expectations, it is most rudimented in the lower mam- 

 mals and has come back to greatest usefulness in some of the higher ones. 

 This probably explains the absence of its characteristic muscles, these 

 having been discarded in early mammals to whom the nictitans was un- 

 important. It is present in the duck-bill but lacking in the echidnas, and 

 it is vestigial in rodents and others of the lower orders such as the insec- 

 tivores, primates, and 'edentates'. One of the latter group however, the 

 aard-vark (Orycteropus) , has a nictitans which is on a par with that of 

 the horse — ^probably as a protection against the termites on which the 

 beast feeds. The scaly ant-eaters (Manis) also have it decently developed. 



All carnivores except the skunk, whose eye protrudes greatly like that 

 of a mouse, have a nictitans. All can move it, though not all ever do so. 

 In only a few could it possibly be drawn all the way over the cornea, the 

 'haw' of the domestic cat being a familiar example of this rare degree of 

 development. In bears, the nictitans is not ordinarily moved, but it drifts 

 partway over the cornea when the animal becomes sleepy. The same 

 reaction is seen in the rhinoceroses. The white bear, however, has an 

 excellent nictitans, and uses it as a defense against snow-blindness (as 



