ORGAN OF SIGHT IN BIEDS. 143 



the margin : in the lower eyelid the tarsal cartilage intervenes 

 between the muscle and the ciliary margin. 



The levator jjcilpehrcs superioris arises from the roof of the 

 orbit, and is inserted near the external angle of the lid. There is 

 also an express muscle for depressing the lower eyelid, as in the 

 Crocodile. In the Owls and Nightjar (^Caprimulgus) the eyelids 

 are closed principally by the depression of the upper one. There 

 are but few Birds that possess eyelashes; of these the Ostrich is an 

 example, as also the Hornbills and the Owls, in which they are 

 arranged in a double series ; but here they are rather to be con- 

 sidered as feathers with short barbs, than true eyelashes. 



The third eyelid, or membrana nictitans, is a thin membrane, 

 transparent in some Birds, in others of a pearly white colour. 



Two muscles are especially provided to eifect its movements, 

 but are so placed as to cause no obstruction to the admission of 

 light to the eye during their actions. One of these is called the 

 quadratus nictitantis, fig. 59, g ; it arises from the sclerotica at the 

 upper and back part of the globe of the eye, and its fibres slightly 

 converge as they descend towards the optic nerve, above which 

 they terminate in a tendinous sheath, having no fixed insertion. 

 The second muscle, called pyramidalis nictitantis, ib. h, arises from 

 the lower and nasal side of the eyeball : its fibres converge toward 

 the upper part of the optic nerve, and terminate in a small round 

 tendon which glides through the pulley at the free margin of the 

 quadratus ; thus, A\dnding over the nerve, it passes down to be in- 

 serted into the lower part of the margin of the third eyelid. By 

 the simultaneous action of the two muscles, that nictitatino; lid is 

 drawn outward and obliquely downward over the fore part of the 

 eyeball. The tendon of the pyramidalis gains the due direction 

 for that action by winding round the optic nerve, and it is 

 restrained from pressing upon the nerve by the counteracting 

 force of the quadratus, which thus augments the power of the 

 antagonist muscle, while it obviates any inconvenience from 

 pressure on the optic nerve, which its peculiar disposition in 

 relation to that part would otherwise occasion. The nictitating 

 membrane returns, on the relaxation of its muscles, by virtue of 

 its own elasticity, to the inner corner of the orbit, where it lies 

 folded when not in use. 



The lacrymal glands are two, as in Reptiles ; but the inner one is 

 the largest, especially subserving the more frequent movements 

 of the nictitating membrane : it is called the * harderian gland,' 

 fig. 56, d', is situated at the inner or nasal can thus, has a lobu- 



