a space is left; this is filled with a watery fluid, called the perilymph; and the membranous internal ear is filled by a similar liquid, the endolymph.

FIG. 19.-Casts of the bony labyrinth. 4, left labyrinth seen from the outer side; B, right labyrinth from the inner side; C, left labyrinth from above; Co, cochlea; V, vestibule; Fc, round foramen; Fu, oval foramen; h, horizontal semicircular canal; ha, its ampulla; vaa, ampulla of anterior vertical semicircular canal; vpa, ampulla of posterior vertical semicircular canal; vc, conjoined portion of the two vertical canals.

The Bony Labyrinth.-"The bony labyrinth is described in three portions, the vestibule, the semicircular canals, and the cochlea; casts of its interior are represented from different aspects in Fig. 19. The vestibule is the central part and has on its exterior the oval foramen (Fv) into which the base of the stirrup-bone fits. Behind the vestibule are three bony semicircular canals, communicating with the back of the vestibule at each end, and dilated near one end to form an ampulla. The bony cochlea is a tube coiled on itself somewhat like a snail's shell, and lying in front of the vestibule.

The Membranous Labyrinth.-"The membranous vestibule, lying in the bony, consists of two sacs communicating by a narrow aperture. The posterior is called the utriculus, and into it the membranous semicircular canals open. The anterior, called the sacculus, communicates by a tube with the membranous cochlea. The membranous semicircular canals much resemble the bony, and each has

an ampulla; in the ampulla one side of the membranous

A

tube is closely adherent to its bony protector; at this point nerves enter the former. The relations of the membranous to the bony cochlea are more complicated. section through this part of the auditory apparatus (Fig. 20) shows that its osseous portion consists of a tube wound two and a half times round a central bony axis, the modiolus. From the axis a shelf, the lamina spiralis, projects and partially subdivides the tube, extending farthest across in its lower coils. Attached to the outer edge of this bony plate is the membranous cochlea (scala media), a tube triangular in cross-section and attached by its base to the outer side of the bony cochlear spiral. The spiral lamina and the membranous cochlea thus subdivide the cavity of the bony tube (Fig. 21) into an upper portion, the scala

FIG. 20.-A section through the cochlea in the line of its axis.

FIG. 21.-Section of one coil of the cochlea, magnified. SV, scala vestibuli; R, membrane of Reissner; CC, membranous cochlea (scala media); lls, limbus laminæ spiralis; t, tectorial membrane; ST, scala tympani; lso, spiral lamina; Co, rods of Corti; b, basilar membrane.

vestibuli, SV, and a lower, the scala tympani, ST. Between these lie the lamina spiralis (so) and the mem

branous cochlea (CC), the latter being bounded above by the membrane of Reissner (R) and below by the basilar membrane (b).” *

The membranous cochlea does not extend to the tip of the bony cochlea; above its apex the scala vestibuli and scala tympani communicate. Both are filled with perilymph, so that when the stapes is pushed into the oval foramen, o, in Fig. 17, by the impact of an air-wave on the tympanic membrane, a wave of perilymph runs up the scala vestibuli to the top, where it turns into the scala tympani, down whose whorls it runs and pushes out the round foramen r, ruffling probably the membrane of Reissner and the basilar membrane on its way up and down.

The Terminal Organs.-"The membranous cochlea contains certain solid structures seated on the basilar membrane and forming the organ of Corti. end-organs of the cochlear nerves.

A

This contains the Lining the sulcus

FIG. 22. The rods of Corti. A, a pair of rods separated from the rest; B, a bit of the basilar membrane with several rods on it, showing how they cover in the tunnel of Corti; i, inner, and e, outer rods; b, basilar membrane; r, reticular membrane.

spiralis, a groove in the edge of the bony lamina spiralis, are cuboidal cells; on the inner margin of the basilar membrane they become columnar, and then are succeeded by a row which bear on their upper ends a set of short stiff hairs, and constitute the inner hair-cells, which are fixed below by a narrow apex to the basilar membrane; nervefibres enter them. To the inner hair-cells succeed the

*Martin: op. cit.

rods of Corti (Co, Fig. 21), which are represented highly magnified in Fig. 22. These rods are stiff and arranged side by side in two rows, leaned against one another by their upper ends so as to cover in a tunnel; they are known respectively as the inner and outer rods, the former being nearer the lamina spiralis. The inner rods are more numerous than the outer, the numbers being about 6000 and 4500 respectively. Attached to the external sides of the heads of the outer rods is the reticular membrane (r, Fig. 22), which is stiff and perforated by holes. External to the outer rods come four rows of outer hair-cells, connected like the inner row with nerve-fibres; their bristles project into the holes of the reticular membrane. Beyond the outer hair-cells is ordinary columnar epithelium, which passes gradually into cuboidal cells lining most of the membranous cochlea. From the upper lip of the sulcus spiralis projects the tectorial membrane (t, Fig. 21) which extends over the rods of Corti and the hair-cells."*

23.-Sensory epithelium

from ampulla or semicircular canal, and saccule. At n a nerve-fibre pierces the wall, and after branching enters the two hair-cells, c. At ha 'columnar cell' with a long

hair is shown, the nerve-fibre being broken away from its base. The slender cells at

The hair-cells would thus seem to be the terminal organs for 'picking FIG. up' the vibrations which the airwaves communicate through all the intervening apparatus, solid and liquid, to the basilar membrane. Analogous hair-cells receive the terminal nerve-filaments in the walls of the saccule, utricle, and ampullæ (see Fig. 23). The Various Qualities of Sound.-Physically, sounds consist of vibrations, and these are, generally speaking, aërial waves. When the waves are non-periodic the result is a

f seem unconnected with

rerves.

noise; when periodic it is what is nowadays called a tone, or note. The loudness of a sound depends on the force of the waves. When they recur periodically a peculiar quality called pitch is the effect of their frequency. In addition to loudness and pitch tones have each their voice or timbre, which may differ widely in different instruments giving equally loud tones of the same pitch. This voice depends

on the form of the aërial wave.

Pitch. A single puff of air, set in motion by no matter what cause, will give a sensation of sound, but it takes at least four or five puffs, or more, to convey a sensation of pitch. The pitch of the note c, for instance, is due to 132 vibrations a second, that of its octave c' is produced by twice as many, or 264 vibrations; but in neither case is it necessary for the vibrations to go on during a full second for the pitch to be discerned. "Sound vibrations may be too rapid or too slow in succession to produce sonorous sensations, just as the ultra-violet and ultra-red rays of the solar spectrum fail to excite the retina. The highestpitched audible note answers to about 38,016 vibrations in a second, but it differs in individuals; many persons cannot hear the cry of a bat nor the chirp of a cricket, which lie near this upper audible limit. On the other hand, sounds of vibrational rate about 40 per second are not well heard, and a little below this they produce rather a 'hum' than a true tone-sensation, and are only used along with notes of higher octaves to which they give a character of greater depth."

The entire system of pitches forms a continuum of one dimension; that is to say, you can pass from one pitch to another only by one set of intermediaries, instead of by more than one, as in the case of colors. (See p. 41.) The whole series of pitches is embraced in and between the terms of what is called the musical scale. The adoption of certain arbitrary points in this scale as 'notes' has an ex

*Martin: op. cit.