1G4 ON THE SENSES. 



name of aqueductus vestibuli ; the remaining five openings lead into tlie three 

 semicircular canals, while each of these canals has its opening orifice in the ves- 

 tibule and runs back with a terminal orifice into the same ; but as one of these 

 apertxn-es is common to two of the canals, there are, in effect, only five of them 

 instead of six. The vestibule itself shelters in its water-filled cavity two little 

 membranaceous sacs, also filled with fluid and imbedded in separate depressions 

 ffovece) of its wall, being channels for the nerves, of Avhich we shall hereafter 

 speak. The semicircular canals are narrow bony tubes, bent somewhat into the 

 shape of a horseshoe, enlarged at their commencement and termination, and 

 from their position and direction are distinguished as the superior, posterior and 

 horizontal canal. As nothing results from their special position, we will not 

 detain the reader with the pedantry of a scrupulous anatomical description of 

 them ; the above outline conveys an adequate idea of their form and direction. 

 In each of these canals is fixed a membranous tube, which has the same form 

 as the canal, as if it were a reduced cast of it, and dilating therefore at the be- 

 ginning and end of the canal into a small roundish cavity called the ampulla. 

 The interspace between the membranous tube and the osseous wall of the canal, 

 as well as the tube itself, is filled with a watery secretion, which is in immediate 

 communication with that of the vestibule. The sack-shaped origin of each 

 membranous tube is a conduit for the nerves. The cochlea well deserves its 

 name ; the best idea of its form may be acquired by observing the shell of the 

 common garden snail. As in this shell a spiral and constantly diminishing 

 channel winds, from the base to the apex, around a central spindle, so the inte- 

 rior chamber of the cochlea consists of a similar screw-shaped canal, which de- 

 scribes, with a like progressive diminution, two and a half turns around the 

 modiolus or central axis, terminating, not in a point, but in a funnel-shaped 

 dilatation. A more striking difference between the shell of a snail and the 

 cochlea consists in this : that the canal of the latter is not single, but is divided, 

 throughout iis length, by a transverse partition into two passages, one of which 

 runs above, the other belov/ the dividing wall, but which communicate with one 

 another in the funnel-shaped expansion ; resembling in some sort a winding stair- 

 case by which one might ascend to the top, and, stepping over again, arrive at the 

 bottom. An idea of this arrangement may be conveyed by the annexed figure, 



which represents a central longitudinal section 

 •^^^" -"^v of the cochlea from its base to its apex. A 



denotes the central axis, S the partition wall 

 running around it, V and P the winding pas- 

 sages separated from one another by this wall. 

 These passages bear the name of scalce (stairs) ; 

 that above the partition, marked V, being 

 called the scala vestibuli, that below, marked 

 P, the scala tympani. They owe these names 

 respectively to the circumstance that the former 

 communicates, by an oval aperture, with the 

 vestibule, while the latter is closed below by 

 the membrane of the fenestra rotunda, which faces the cavity of the tympanum. 

 Both passages are filled with fluid which, at the entrance of the scala vestibuli, 

 is in immediate communication with the fluid of the vestibule. The partition 

 S, which divides the canal into two passages, is constituted, as the figure repre- 

 sents, by a thin lamina winding spirally around the axis, to which it is attached 

 by its inner edge, while its outer edge adheres to the outer wall of the cochlea. 

 It consists of tv/o zones, as they are called, united with one another, the one 

 osseous, the other membranous, which together extend quite across the canal ; 

 that part of it which lies contiguous to the axis being composed of two thin 

 lamellaj of bone and reaching as far as the middle, while the outer part, by 

 which it is continued to the opposite wall, consists of a soft membranous sub- 

 stance, serving, as we shall see, to support the tender apparatus of the nerves. 



