ORGANS OF SENSE. 159 



the lamina spiralis are called scalce. The superior is called vestibular scala, 

 from communicating directly with the vestibule ; the other is the scala tym- 

 panic and communicates with the tympanum through the fenestra rotunda. 

 Although elsewhere separated, the two scalae communicate near the summit 

 of the cochlea b}' a common opening called helicotrema. Near the termina- 

 tion of the scala tympani at the fenestra, is the opening of the aqueduct of 

 the cochlea^ a canal for a small vein. 



Membrane lining the labyrinthic cavity. This is an extremely delicate mem- 

 brane, of a libro-serous character. One surface is closely adherent to the 

 bone, like a periosteum ; the other is smooth, and secretes the perilymph or 

 aqua Cotunnii., a semi-fluid substance. 



5. The Membranous Labyrinth. This is not so extensive as the osseous 

 labyrinth, as it does not enter the cochlea. It is separated from the walls 

 of the osseous portion in which it occurs by the perilymph. The mem- 

 branous labyrinth is itself filled by a liquid called endolymph or liquor of 

 Scarpa. The labyrinth consists of the common sinus, the sacculus, and the 

 membranous semicircular canals. 



The common sinus or vestibular ventricle is an elongated, laterally com- 

 pressed pouch, which occupies the posterior part of the vestibule ; the semi- 

 circular canals are continuous with it, and open into it by fine orifices ; it 

 floats in the perilymph, and is distended by the endolymph. 



The sacculus vestibuli is much smaller than the sinus; it is round and 

 situated inferior and anterior to the sinus, to which it adheres ; anatomists 

 are not agreed as to whether the two communicate or not. 



The membranous semicircular canals have precisely the same form as the 

 osseous canals, presenting the same number of ampullae or ovoid vesicles. 

 They are surrounded by perilymph and distended by endolymph. The 

 otolithes of certain fishes are represented in man by a powder composed of 

 carbonate and phosphate of lime, and known as otoconia. The grains com- 

 posing this powder are supposed to play an important part in the physiology 

 of hearing, by communicating to the nervous expansion a more vivid and 

 energetic impression than a single liquid could efiect. 



6. Nerves of the Ear. The essential nerve of sense is the portio mollis 

 of the seventh pair, or the auditory nerve proper. This enters the meatus 

 internus, and at the bony crest at the bottom of this canal divides into two 

 branches, an anterior, larger, for the cochlea, and a posterior for the vestibule 

 and semicircular canal. The vestibular nerve immediately separates into three 

 sets of fasciculi, superior, middle, and inferior. The superior pass into the 

 superior ventricle of the vestibule, and are expanded on the sacculus com- 

 munis and on the ampullae of the semicircular canals. The middle set pass 

 through the macula cribrosa into the anterior ventricle, and expand on the 

 sacculus proprius ; while the posterior set pass through the posterior wall of 

 the vestibule, and are lost on the ampullaiy dilatation of the obhque semi- 

 circular membranous canal. 



The cochlear nerve resembles a flat tape rolled on itself lengthwise. It 

 passes downwards to the depression at the bottom of the internal meatus, 

 where it divides into a number of fine filaments, which enter the small 



ICONOGRAPHIC ENCYCLOPAEDIA. — VOL. II. 55 866 



