Hearing, and the Ear. 457 



and suspended in a fluid called the perilymph, while the fluid contained 

 in them is called endolymph. Each of the bags is held in a peculiar 

 shaped bony cavity, and corresponds to its cavity in shape, but does not 

 fill it, the spare space being occupied by the perilymph. The membran- 

 ous labyrinth is above and toward the rear from the scala media, and 

 consists of three hoop-like cylinders called the semicircular canals, 

 which communicate with a common chamber below by five openings, 

 each end of each canal opening into the chamber, except in one case, in 

 which two come together before entering the common chamber. This 

 chamber is called the vestibular sac. At their lower ends the canals 

 are swelled or expanded, such expansions being called ampullae or ' ' bot- 

 tles," fig. 192, e. The scala media is contained in a bony cavity called 

 the cochlea. This is situated a little below and forward of the semicir- 

 cular canals. In shape the cochlea is like a snail shell, and in the hu- 

 man ear it is coiled two and a half times around the central, bony stem, 

 which is called the modiolus. The cavity which thus runs in a spiral di- 

 rection up the cochlea, is divided by two longitudinal partitions, so as to 

 make three long spiral tubes winding from bottom to top. The lower 

 and under one is the scala tympani, the upper one the scala vestibuli, 

 and between these two is the scala media. This, as before stated, is a 

 membranous sac containing the endolymph. Around its upper end the 

 other two scalae connect with each other. They are filled with per- 

 ilymph. The two membranes spoken. of above as separating the ear 

 drum or tympanum from the cavity of the inner ear, are named respect- 

 ively the Fenestra Ovalis and the Fenestra Rotunda ( the oval window 

 and the round window ). The former of these is opposite the vestibular 

 sac before mentioned, the space between it and the sac being named the 

 vestibule. The latter is lower and further forward, and forms one end 

 of the scala tympani. The scala vestibuli at its lower end merges 

 into the vestibule by a narrow opening called " canalis reuniens. " 

 (Fig. 209, cr.) 



The vibrations by which the fluids of the inner ear are agitated are 

 communicated to it by mechanical vibrations upon the membrane of the 

 fenestra ovalis. When that membrane is compressed or pushed in, the 

 jar is communicated to the perilymph in the vestibule, and from thence 

 to that which surrounds the membranous labyrinth in the semicircular 

 canals, and also from the vestibule up the scala vestibuli around the head 

 of the scala media, down the scala tympani to the fenestra rotunda, which 

 is pushed out towards the tympanum, thus acting as a safety valve. Of 

 course, whenever the perilymph is shaken up it communicates the agita- 

 tion to the endolymph which is contained in the two sacs above described. 

 From this it is communicated to the ends of the auditory nerve, and by 

 it conveyed to the balancing organ in the brain. To go back to the 



