1 86 THE HUMAN BODY. 



between the osseous and membranous labyrinths. We shall 

 first consider the osseous labyrinth. 



The vestibule is an ovoid cavity placed in the centre of the 

 internal ear, between the semicircular canals and the cochlea. 

 It communicates with the drum by the fenestra ovalis, which 

 is closed by the base of the stapes. In it are seen the open- 

 ings of the five semicircular canals, of the vestibular stair of 

 the cochlea, and of the vestibular canal. This latter is the 

 opening of a vascular canal which traverses the petrous bone. 



Semicircular canals. This is the name given to three 

 curved tubes forming axes of circles, one which is horizontal 

 and is placed between the two others, which are vertical. 

 They are each enlarged into a bulbous cavity (ampulla) at 

 one extremity, and communicate with the vestibule by five 

 orifices. 



Cochlea (or snail-shell). This is the name given to a 

 conoid cavity which is separated from the semicircular 

 canals by the vestibule, with which it communicates, and 

 terminates at the fenestra rotunda. The cavity of the cochlea 

 is a spiral, describing two turns and a half round its columella 

 or axis; it is divided transversely into two portions by a 

 partition -the lamina spiralis, throughout its entire length. 

 That portion opening into the vestibule is called the scala 

 vestibuli; and that opening into the fenestra rotunda the 

 scala tympani by which it would, if it were not for the 

 membrane which closes it, communicate with the cavity of 

 the tympanum. 



The lamina spiralis is divided lengthwise into a bony por- 

 tion, which corresponds at its internal border to the axis; 

 and a membranous portion, which attaches the osseous por- 

 tion to the external wall of the cochlea. This wall is formed 

 by the spiral plate. The cochlea is lined by a fibro-mucous 

 membrane, which appears to be a continuation of the perios- 

 teum of the other two cavities of the labyrinth; the mem- 

 branous portion of the spiral plate may be considered as a 

 prolongation of the membranous labyrinth. Lastly, the 

 vascular canal, called the canal of the cochlea, analogous to 

 that of the vestibule, communicates also with the cavity of 

 the skull. The base of the cochlea rests on the bottom of 



