ORGANS OF SENSE. 159 
the lamina spiralis are called scale. The superior is called vestibular 
scala, from communicating directly with the vestibule; the other is the 
scala tympani, and communicates with the tympanum through the fenestra 
rotunda. Although elsewhere separated, the two scale communicate near 
the summit of the cochlea by a common opening called helicotrema. Near 
the termination 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 
membrane, of a fibro-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. Toe Memsranous Lasyrinta. 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 
membranous 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 
semicircular 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 ampulle 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 
composing this powder are supposed to play an important part in the phy- 
slology of hearing, by ‘communicating to the nervous expansion a more 
vivid and energetic impression than a single liquid could effect. 
6. Nerves or 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 communis and on the ampulle of the semicircular canals. 
The middle set pass through the macula cribrosa into the anterior ven- 
tricle, and expand on the sacculus proprius; while the posterior set pass 
through the posterior wall of the vestibule, and are lost on the ampullary 
dilatation of the oblique semicircular 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 ENCYCLOP£DIA.—VOL. II. 55 865 
