ORGANS OF HEARING. 93 



levator palpebrac snperioris arises from the roof of the orbit, and is 

 inserted near the external canthus of the lid. Both the upper and 

 lower eyelids are provided with short feather-like bristles, as eye- 

 lashes. The nictitating membrane can be swept by a very peculiar 

 mechanism over the whole anterior surface of the eye. The muscu- 

 lar structures by which this is effected may be best studied in the 

 eye of the Owl. A very broad, flat, thin, and quadrate muscle (m. 

 quadratus mcmb. nictitantis) is situated upon the back part of the 

 globe of the eye, and arises from the superior and posterior margins 

 of the sclerotica. Its fibres descend toward the entrance of the optic 

 nerve, and terminate in a free concave tendinous canal. A second 

 muscle (m. pyramidalis memb. nictitantis") has a short narrow belly 

 which arises from the inner or nasal aspect of the eyeball, and gives 

 off a long slender tendon, which, as it turns round the optic nerve, 

 passes through the canal formed by the tendon of the quadratus, and 

 is inserted into the lower margin of the third eyelid, which by the 

 contraction of this muscle is drawn like a curtain in front of the 

 globe of the eye. In the Owls a small ossicle (ossiculum tuberculare) 

 is situated upon the inferior surface of the bony ring of the sclerotica, 

 to support the long tendon of the pyramidalis muscle. 



Birds have very generally a lacrymal gland situated in the posterior 

 angle of the eye, two excretory ducts, and a membranous lacrymal 

 canal leading into the nasal passages. The Harderian gland, which 

 has been already alluded to (p. 26) as occurring in many Mammalia, 

 is much more largely developed in Birds than the lacrymal gland, 

 and especially in the order Palmipedes, as the Goose and Pelican. 

 It is situated upon the anterior or internal angle of the eye, and 

 through an excretory duct which opens upon the inner surface of the 

 nictitating membrane, pours out a viscid secretion to moisten and 

 lubricate the latter. 



Organs of Hearing. 



The Organs of Hearing in Birds admit of being easily demonstra- 

 ted, from the very superficial position which the labyrinth occupies 

 in relation to the bones of the cranium. 



An external ear or auricle is wanting, and it is only a few spe- 

 cies, such as the Owls, which have a large membranous crescentic 

 fold or auditory conch, provided with tufts of short feathers, and 

 which can be used as a valve. The largest ear-conch is met with 

 among our indigenous Owls in the long-tufted Hibou (strix otus). 



