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 extremities 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, diminishing 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.