736 



AVES. 



Fig. 366. 



ratus is given, by which its movements are effected. This is placed at 

 the back of the eyeball, and may easily be displayed by turning aside 

 the recti and obliqui muscles, as in fig. 366, B. Two muscles are then 

 perceived arising from the globe of the eye, taking their origin from the 

 outside of the sclerotic coat : one of these (c), named the quadratics 

 membrance nictltantis, arising from near the upper aspect of the eye, de- 

 scends towards the optic nerve ; but instead of being inserted into any- 

 thing, as muscles usually are, it terminates in a most remarkable man- 

 ner, ending in a tendinous sheath or pully, through which the tendon 

 of the next muscle passes 

 as it winds around the optic 

 nerve. The second muscle 

 (d), called the pyramidalis 

 memb. nictitantis, arises from 

 the inner aspect of the eye- 

 ball ; and its fibres are col- 

 lected into a long slender 

 tendon, which, as it turns 

 round the optic nerve, 

 passes through the tendinous 

 sheath formed by the qua- 

 dratus, as a rope through a 

 pully, and then is continued 

 in a cellular sheath formed 

 by the sclerotic, underneath 

 the eye, to the lower angle 

 of the third eyelid, into 

 which it is inserted. The 

 reader will at once perceive 

 how beautifully these two 

 muscles, acting simulta- 

 neously, cause the nicti- 

 tating membrane to sweep 

 over the cornea, which returns again into the inner canthus of the eye 

 by its own elasticity. 



(2085.) Being thus provided with moveable eyelids, a lacrymal appa- 

 ratus is, of course, indispensable ; and accordingly, birds are supplied 

 with two distinct glands, one being appropriated to the secretion of 

 tears, while the other furnishes a lubricating fluid, apparently destined 

 to facilitate the movements of the membrana nictitans. 



(2086.) The lacrymal gland is situated, as in Man, at the outer angle 

 of the eye, and its duct pours the lacrymal secretion upon the eyeball 

 near the external canthus. The lacrymal canal, whereby the tears, 

 after moistening the cornea, are discharged into the nose, commences 

 by two orifices (fig. 366, A, e) situated just behind the internal commis- 



Muscles of the nictitating membrane. 



