ADNEXA IN BIRDS AND MAMMALS 425 



retracting; but here it is because the orbit affords no room at all for a 

 retractor bulbi. 



The lacrimal gland is ventro-temporal in location, with a single duct 

 which opens inside the lower lid. From bird to bird it shows what seem 

 to be inconsistent variations. It is minute, as might be expected, in one 

 group of amphibious birds (the penguins) but is particularly large in 

 another (the dippers or water-ouzels). The owls lack it, and moreover 

 have a very small Harderian gland, as do also their remote ancestors the 

 goatsuckers. The very large Harderian gland 

 of the cormorants makes good sense, for these 

 are marine amphibious birds; and the avian 

 Harderian secretion is a thick, oily emulsion 

 which, if abundant, would shield the eye well 

 from the osmotic and chemical effects of sea- 

 water. In birds there are two slit-like punctae at 

 the nasal canthus, the upper one being the 

 larger of the two. The penguins appear to have 

 lost the nasolacrimal duct, for their oily tears Fig. 144 — Ventral view of 

 are described as spilling down their cheeks tt, ZwrnUTci.t'it 



when they are out on land. and its pulley. After Franz. 



Mammals — In the mammals, the upper lid ordinarily comes down much 

 more than the lower comes up. Exceptions are the elephant and hippo- 

 potamus, the camel and reindeer, the great elephant-seal, and a number 

 of very small forms, such as the mouse. Both lids, or only the upper, may 

 have tarsal plates. The monotreme echidnas carry out their generally 

 sauropsidan-like ocular makeup by having a tarsus in the lower lid alone. 

 The two lids in mammals are approximated by the annular 'orbicularis 

 oculi' muscle, which sweeps around through both like a flattened dough- 

 nut (Fig. 17, p. 39). They' are separated largely by the actions of the 

 levator of the upper lid (a derivative of the superior rectus, which it 

 parallels) and of the more intrinsic depressor muscle of the lower lid. 

 In many forms, particularly primitive ones, the lids are thick and their 

 action slow; but ordinarily they are thin and the 'blink' may be lightning- 

 quick like the movement of a bird's nictitans. Except in forms whose 

 binocular fields are very narrow (like the rabbit), the lids of both eyes 

 react when only one pair is stimulated. We have all seen humans who 

 have never learned to wink one eye! 



