"7. We distinguish in sounds-1, the pitch; 2, the intensity or loudness; 3, the quality or timbre.

"The pitch of the sound depends on the rapidity with which the vibrations succeed each other; and any two sounds produced by the same number of vibrations or impulses in the same time, are said to be in unison.

"The loudness or intensity depends upon the violence and extent of the primitive impulse.

"The quality is supposed by Herschel to depend on the greater or less abruptness of the impulses, or generally on the law which regulates the excursions of the molecules originally set in motion.

"8. The velocity with which sound travels is, however, quite independent of its intensity or of its tone; sounds of every pitch and of every quality travel with the same speed through the same medium, as is proved by the fact that distance does not destroy the harmony of a rapid piece of music played by a band.

"9. Water propagates sound with much greater velocity than air does. Colladon concludes from numerous observations that the velocity of sound in water, at 40° Fahr., was at the rate of 4,708 feet in a second.

"10. According to Biot, cast-iron propagates sound at the rate of 11,090 feet in a second.

"11. Sonorous undulations, in passing from one medium to another, always experience a partial reflection, and when they encounter a fixed obstacle, they are almost wholly reflected. The reflection of sound occurs according to the same law which regulates the reflection of light-namely, the angle of reflection is equal to the angle of incidence.

"12. The phenomena of echoes result from the reflection of sound from any prominent object."

The vibrations carried to the labyrinth by the chain of bones and the air in the tympanum will more readily affect the vestibule, and those by the bones of the head, the cochlea. Dugès suggested that the special use of

the cochlea was to estimate the pitch of sound and of voice, as in animals it bears ratio to the development of vocal organs. The semicircular canals hold the same relative position in all animals, and this fact suggested to Wheatstone and others that they are the means by which we determine the direction whence sound issues. Flourens asserts that injury to them will cause animals to move in various directions, according to the canal perforated. The calcareous particles in the endolymph must increase the intensity of vibration to the membraneous labyrinth, as when a stone is placed in a bladder of water, any impulse is most distinctly felt by the hand holding it. Dr. Wollaston calculated the range of human hearing as 9 octaves, the lowest notes of the organ and the cry of some insects being the extremes. That sounds leave a sensation after they have ceased, Savart proved by removing one tooth from a toothed wheel, when no interference with the sound could be detected. Tinnitus aurium, the noise of flowing water, &c., are subjective sensations which are often indicative of cerebral disease. The following interesting parallel between the ear and eye is drawn by Wharton Jones :

"The comparison which I should institute between the component parts of the ear and the eye is the following:

"The osseous labyrinth may be compared to the sclerotica; and the fenestra rotunda, or cochlear fenestra, to the cornea.

"To find a part in the eye analogous to the vestibular fenestra, we must first consider that the latter is a yielding part of the otherwise solid wall of the labyrinth; that, through the medium of it, the chain of small bones and their muscles in the tympanum exercise on the soft parts contained in the labyrinthic cavity a certain degree of tension or compression, fitted propably to accommodate in some manner the ear to the perception of different degrees of sound. In the case of the eye, the sclerotica,

which corresponds to the osseous labyrinth, is thinner and more yielding at the middle of its circumference (remarkably so in the Greenland seal). From this it has been supposed that the action of the muscles of the eyeball might, by their compression, produce a change of shape fitted to accommodate the eye to distances. Hence the vestibular fenestra and middle thin part of the sclerotica might be compared to each other, in as far as regards the function which each performs in the economy of its own organ. However this may be, the vestibular fenestra of the ear and the thin part of the sclerotica correspond to each other as far as can be in relative position; and, if we admit the action just mentioned of the muscles upon the eye-ball, we have, as I shall immediately show, their counterparts in the muscles of the small bones of the tympanum.

"The tympanic scala of the cochlea may be compared to the anterior chamber of the aqueous humour, and the vestibular scala to the posterior chamber.

"The spiral lamina, considering its vascularity and richness in nerves, and its forming a partition between two chambers containing an aqueous humour, may, as I have already said in a former part of this article, be considered the counterpart of the iris, and the helicotrema, that of the pupil.

"The membrane lining the labyrinthic cavity bears the same relation to the latter as the arachnoidea oculi does to the sclerotica. The place filled with perilymph, between the osseous and membraneous labyrinth, may be considered analogous to that between the sclerotica and choroid. It however communicates with the scale of the cochlea, the parts analogous to the chambers of the aqueous humour, because there is nothing in the ear to be compared to the ciliary ligament.

"Forming the membraneous labyrinth we find: 1, a delicate cellular tissue supporting the branches of the blood-vessels, and which is sometimes found containing

black pigment; 2, a firm, transparent, membraneous coat, within which, 3, is a nervous expansion; 4, the endolymph; 5, suspended in the latter the mass of calcareous matter. The cellulo-vascular layer containing pigment, together with the rest of the walls of the membraneous labyrinth, may be compared to the choroid coat of the eye, the nervous expansion to the retina, the endolymph to the vitreous humour, and the calcareous mass to the lens.

"In the lower animals the cochlea is the first part of the ear-bulb to disappear; in regard to the eye-ball, the aqueous chambers, to which I have compared the scalæ of the cochlea, are, in like manner, the first parts which, in the depreciation of the structure of the eye, in the animals series, disappear; e.g., the eye of the Cephalopodous Mollusca.

"Is the cochlear nerve the same in function with the vestibular? The vestibular nerve is the special nerve of hearing; but does not the cochlear nerve perform some function in the economy of the ear analogous to what the ciliary nerves perform in that of the eye?

"If an example is required in which the optic nervous filaments enter the eye separately, as do the nervous filaments of the ear-bulb, it is to be found in the Cephalopodous Mollusca.

"As in front of the eye-ball there is—or rather would be if it was not that the eyelids are constantly in contact with the eye-ball-a space lined by a mucous membrane, the conjunctiva; so, at the peripheral surface of the ear-bulb there is a space-the tympanic cavitylined by a mucous membrane also. Moreover, as there is a passage into the nose from the space bounded by the conjunctiva, so does the tympanic cavity communicate with the throat by the Eustachian tube. In the tympanic cavity there is a chain of small bones, articulated to each other and moved by muscles, which serves to produce some change in the state of tension of the soft parts

of the ear-bulb; in the conjunctival space there is nothing analogous-although, without pushing the point too far, we might compare the muscles of the eye-ball with those of the ossicles of the tympanum, both being equally, in fact, outside their respective mucous membranes. In regard to the ossicles, I would remark that, according to the views of Weber, they must be reckoned among those which do not belong to the skeleton, and which are of very inconstant occurrence. Such are the bone of the penis in many animals, the teeth, the ring of bony plates round the front of the sclerotica of the bird's eye, &c.

"A part in the composition of the appendages of the eye analogous to the membrana tympani is only to be conceived by supposing the existence of a mediate anchyloblepharon-that is, an irregular membrane stretched between the edges of the eyelids, uniting them together and closing in the space lined by the conjunctiva, which space would now communicate with the exterior only by the lachrymal canalicules and nasal duct, in the same way that the tympanic cavity communicates with the exterior only by the Eustachian tube. A congenital fissure, or total absence of the membrana tympani, is an irregularity of structure in the ear which may be compared to what is regular in the eye. A mind accustomed to trace analogies will perceive a resemblance to the external auditory passage in that short space at the opening of the eyelids, extending from the inner edge of the tarsal margin to the outer; to the ceruminous glands in the Meibomian follicles; and to the hairs at the entrance of the auditory passage in the eyelashes. The auricle, if it is necessary to look for a part corresponding to it, may be placed in the same category with the eyebrows."

FACIAL, PNEUMOGASTRIC, AND ACCESSORY NERVES. The Facial nerve is traceable from the olivary column to a space bounded by the pons, olivary body, and pneu