MEMBRANOUS LABYRINTH. AUDITORY NERVE. 187 



the internal auditory canal, by which the auditory nerve 

 enters the organ of hearing. 



Mffttbranous labyrinth. -The bony walls of the vestibule 

 and of the semicircular canals inclose and protect a mem- 

 branous apparatus of the same form, which is separated from 

 them by a space filled with a limpid fluid vd\\z& perilymp/i or 

 liquor Cotunnii. The membranous labyrinth is therefore 

 smaller in proportion than the osseous ; the difference in size 

 is about one-half. Its cavities contain a fluid analogous to 

 the perilymph, and which De Blainville has compared to the 

 vitreous humor of the eye; they also contain semi-transparent 

 tubes and membranous sacs, the appearance of which is 

 closely analogous to that of the retina. The membranous 

 vestibule is composed of two distinct parts, the saccule and 

 utricle, in which there exists a calcareous dust, which appears 

 to represent in man and the mammifera the auditory stones 

 or otoliths of fishes. 



Auditory nerve. The auditory or acoustic nerve specially 

 belongs to the organ of hearing, and is remarkable for the 

 softness of its texture: it enters the ear by the internal 

 auditory canal, and divides into two branches, one of which 

 distributes itself to the vestibule and to the ampullar extremi- 

 ties of the semi-circular canals; the other goes to the cochlea, 

 and has been called the cochlear branch. Its ramifications 

 are extremely minute; they line the surface of the modiolus 

 and spread themselves regularly over the spiral plate, dimin- 

 ishing in length from the base to the summit in such a manner 

 that if we suppose the spiral plate to be placed upright, and 

 forming a triangular plane, these filaments would resemble 

 the strings of a harp the longest at the base of the triangle 

 and the shortest at the summit. They are called the fibres of 

 Corti, from the anatomist who first described them. The 

 microscope enables us to count more than three thousand, 

 and we shall see later the part they are supposed to fill in 

 audition. 



But before opening the physiological question, we will 

 notice summarily some of the phenomena, the existence of 

 which is revealed to us by the ear. 



Noises arid sounds. Physicists divide sounds into two 



