SEC. IV ANATOMY OF BIRDS 281 



tympanic cavity ; this is therefore the scnJa tiimpajd. The first 

 marl)le would also eventually reach the tympanum, through the 

 vestibule, and out of the fenestra ovalis, if the foot of the stapes were 

 unstepped (in life, of course, both these " windows " are closed by 

 membranous curtains). Now in birds the cochlear cavity and its 

 bony or cartilaginous contents are only the beginnings of such 

 structure — a strap-shaped or tongue-like protrusion from the vesti- 

 bule, as if a part of the first mammalian Avhorl, and very incom- 

 pletely divided into scala vestibuli and scala tympani by a gristly 

 structure (representing the modiolus and its lamina), which proceeds 

 from the bony bar or bridge between fenestra ovalis and fenestra 

 rotunda. (See Figs. 84, 85.) This structure is the most intimate 

 and essential part of the organ of hearing, for upon it spread the 

 terminal filaments of the auditory nerve. A human or any well- 

 developed mammalian cochlea is a thing of marvellous })eauty, even 

 as to its bony shell — there is nothing to compare with its exquisite 

 symmetry ; while the spiral radiation of the nervous tissue intro- 

 duces yet other and more wondrous " curves of beauty." 



The vestibule hardly requires special description ; it is simply 

 the central chamber common to the cochlear and canalicular cavities ; 

 receiving the mouth of the scala vestibuli of the cochlea ; the several 

 mouths of the separate or uniting semicircular canals ; opening into 

 tympanum by fenestra ovalis ; conducting to meatus auditorius 

 internus by the course of the auditory nerve. In the eagle, if its 

 irregularities of contour were smoothed out, it would about hold a 

 pea. 



In the language of human anatomy, the three semicircular canals 

 are the (a) anterior or superior vertical, the (h) posterior or inferior 

 vertical, and the (c) external or horizontal ; and the planes of their 

 respective loops are approximately mutually perpendicular, in the 

 three planes of any cubical figure. In birds these terms do not apply 

 so well to the situation of the canals with reference to the axis of the 

 body, nor to the direction of the loops ; neither is mutual perpendicu- 

 larity so nearly exhibited. The whole set is tilted over backward to 

 some degree so that the (((.) " anterior " (though still superior) loops 

 back beyond either of the others ; the (b) " posterior " loops behind 

 and below the (c) horizontal, which tilts down backward ; the verti- 

 cality of the planes of (a) and (b) is better kept. The canals may be 

 better known as the (u) superior (vertical), and (b) inferior (vertical), 

 and (c) internal (horizontal). Whatever its inclination backward, 

 there is no mistaking (a), much the longest of the three, looping high 

 up over the rest, exceeding the petrosal and bedded in the occipital, 

 the upper limb and loop of the arch bas-relieved upon the inner 

 surface of the skull (Fig. 70, asc). It makes much more than a 

 semicircle — rather a horse-shoe. The inferior vertical (b) loops 



