SENSE OF HEARING. 



877 



internal ear is formed of the vestibule, semicircular canals, and cochlea ; 

 the latter, being but little developed, is not convoluted in a spiral form, 

 but, resembling that of lizards and serpents, consists of an almost straight 

 osseous tube canal terminating in a cul-de-sac. 



In mammals the ear, for the convenience of study, may be divided 

 into three parts — the external, the middle, and the internal ear. Of these 

 three the internal ear is the essential, and the others are simply for the 

 purpose of receiving or modifying impressions from the sounding bod}'. 



In the external ear we include the auricle, or pinna, and the external 

 auditory meatus, bounded internally by the membrane of the tympanum ; 

 in the middle ear, the tympanum, or drum of the ear, -with its contained 

 ossicles ; and in the internal ear, that portion situated in the petrous por- 

 tion of the temporal bone, consisting of the semicircular canals, the 

 vestibule, and the cochlea (Fig. 398). 



Fig. 398.— Scheme of the Organ of Hearing. (Landois.) 



AG, external auditory meatus ; T, tympanic membrane ; K, malleus with its head (h), short process 

 (Kf), and handle (m) : a, incus with its short process (x) and long process: the latter is united to the 

 stapes (s) by means of the Sylvian ossicle (z) ; P. middle ear ; o, fenestra ovalis ; r, fenestra rotunda ; x, 

 beginning of the lamina spiralis of the cochlea ; pt, its scala tympani ; vt, its scala vestibuli ; V, vestibule ; 

 S, saccule; U, utricle; H, semicircular canals; TE. Eustachian tube. The long arrow indicates the 

 line of traction of the tensor tympani; the short, curved one, that of the stapedius. 



In different groups of mammals a marked difference is found in the 

 form and size of the auricle, or pinna. This, in the majority of cases, is 

 a trumpet-shaped dilatation of the external auditory canal formed for the 

 purpose of receiving the undulations communicated to the atmosphere, 

 collecting them, and transmitting them inward to the middle ear. This 

 portion of the external auditory canal owes its shape to the cartilages 

 present in it, which in some instances, as in the horse, the ass, the goat, 

 and the rabbit, are erect and straight ; while in other cases the cartilages 

 are more delicate and soft, folding on themselves so that the auricles lie 



