ADNEXA IN REPTILES 423 



heads are thrust upward into the air, in line with the axis of the body, 

 the palpebral fissure is then actually parallel to the water (see Fig, 160b, 

 p. 547). 



The lacrimal gland shows much variation. It may be compact with 

 one or many ducts, or scattered along the length of the lower lid as in 

 salamanders. The Harderian gland is present, with a single duct, and 

 the nasolacrimal duct is completely absent in all turtles. A real puzzle is 

 the enormous size of the lacrimal gland in the marine turtles which, one 

 might think, should need none at all. It may be needed during visits to 

 land for egg-laying; and, since males of the marine forms are rarely 

 caught, it is not on record whether the gland is much smaller in that sex. 

 Or perhaps the secretion is mucous or oily, and affords an analogy with 

 the marine mammals — no one seems to know. 



Lizards — In the lizards, again only the lower lid has a tarsal plate and 

 moves, as a rule. In one anole, at least (Anolis alligator) , the two lids 

 do move equally. The lower lid is operated by a muscle somewhat like 

 the orbicularis oculi of the mammals (Chapter 2, section C), but of 

 course is not homologous therewith, since the mammalian muscle is a 

 derivative of the facial platysma peculiar to the class. The tendon of the 

 nictitans is enfolded by the peculiar 'musculus bursalis', from which a 

 special retractor muscle runs to the sclera to keep the apparatus from 

 pressing on the optic nerve (Fig. 143c). These new muscles are supplied 

 by the sixth cranial (abducens) nerve. 



The Harderian gland is large, lies nasoventrally alongside the globe, 

 and has a single duct. The lacrimal gland lies at the temporal canthus 

 of the palpebral fissure and has several contractile apertures. It is lacking 

 in some lizards, notably the chameleons. These aberrant forms have no 

 nictitans, and have the palpebral fissure greatly reduced to about the 

 size of the pupil, the lids clinging as a broad circular fold to the surface 

 of the huge eyeball, and turning with the eye. They seldom close except 

 in sleep; but when they do, they meet along a straight line as usual. This 

 situation might have arisen from one similar to that in some other 

 lizards, for instance the family Agamidae, where the upper and lower 

 lids merge into one another at the canthi (as they do also in toads) . In 

 several lizards, and two turtles as well, the lower lid shows a special mod- 

 ification to permit vision with the eye closed, and in some burrowing and 

 nocturnal forms the palpebral complex has been frozen into a permanent 

 spectacle, like that of the snakes. 



