ESTHESIOLOGY 305 



fowl is round, but in the goose it is elongate transversely; and in 

 the owl, a vertical oval. 



The optic nerve approaching the sclerotic coat becomes altereH 

 into a conical extremity, which enters a sheath and is directed down- 

 ward and obliquely forward. The extremity of the optic nerve in 

 the interior of the eye presents a white narrow streak. Branches of 

 the opthalmic artery enter the eye between the lamina of the retina, 

 along the whole extent of the oblique slit, and penetrate the fold of 

 the pecten upon which they form a delicate ramification. 



The crystalline lens is of soft texture. It is inclosed in a capsule 

 and is nearly round. It adheres very firmly in the depression in the 

 anterior part of the vitreous humor. The capsule is lodged between 

 two layers of hyaloidea, which as they recede from each other, leave 

 around its circumference the sacculated canal of Petit. 



The cornea is of horny consistency and is transparent. Light 

 thus rapidly passes through it to the posterior part of the eye. 



The vitreous chamber, lying back of the crystalline lens, contains 

 a clear jelly-like substance. 



THE ORGAN OF HEARING 



The ear, the organ of hearing, has in the fowl no conchal cartilage. 

 The external auditory meatus, or canal opening, is found on each 

 side of the head, and is usually guarded by a few stiff, short feathers. 

 In some kinds of birds these feathers are capable of being erected 

 so as to direct the waves of sound into the inner ear. The outer 

 entrance of the ear contains glands. This canal is short. It leads 

 to the drum, which is somewhat convex from the outside, and which 

 has a membrane forming a complete curtain stretched over the 

 outer part. The irregularly formed drum has connection with the 

 air-containing cavity of the skull, and by a thin cartilaginous canal, 

 the Eustachian tube, with the pharynx. The auditory ossicles are 

 represented by only a single bone, called the columella, which 

 most closely represents the stapes of mammals. This is attached 

 by processes of cartilage to the tympanic membrane. Owen con- 

 siders these cartilages as representing the malleus of mammals. 

 Huxley on the other hand considers them to represent the incus. 

 Originating from the processes is a small muscle which is attached 

 to the drum. This is by Shufeldt considered the Tensor tympani. 

 The drum cavity through the fenestra vestibularis and cochlearis 

 is connected with the labyrinth. 

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