with its fundamental tone and overtones, would be transmitted along the nerve filaments to the brain.

And it is suggested that the basilar membrane of the cochlea, consisting as it does of thousands of fibres stretching across from the inside to the outside (from left to right in Fig. 68), with its thousands of epithelial cells and rods of Corti lying upon it, represents, as it were, an assemblage of thousands of tuning-forks of various rates of vibration, with a separate nerve filament attached to each. So that, when a number of vibrations of different periods, such as constitutes an ordinary musical sound, are transmitted by the tympanum to the cochlea, these as they sweep along the canalis cochlearis throw into sympathetic movement those parts, and those parts only, of the basilar membrane with their overlying epithelium and rods of Corti, whose periods of vibration correspond to their own vibrations, and thus excite certain nerve filaments, and these only. It is this excitement of a group of nerve filaments, some more intensely than others, which reaching the brain gives rise to the sensation which we associate with the particular musical sound.

As has been already stated, we know very little definitely about the position in the brain of, and still less about the nature of, the auditory sensorium or central end-organ of the auditory nerve; but it may be conceived that each filament of the cochlear nerve is connected with a particular portion of the nervous matter of the central endorgan, in such a way that the molecular movements of one of these particular portions of nervous matter, brought about by a molecular disturbance reaching it through its appropriate filament, produces a psychical effect of one kind only, more or less intense it may be, but still always of one kind. If this be so, each cochlear fibre or filament may be considered as being provided with two end-organs: one, peripheral, in the organ of Corti, capable of being set in motion by vibrations of one quality only; the other, central, in the brain, capable of producing a psychical effect of one quality only. It does not follow, however, that we are distinctly and separately conscious of the nervous disturbance in each central end-organ, it does not follow that we have as many distinct and separate kinds of

conscious sensation as there are peripheral and central end-organs, though how many such distinct kinds of sensation we may have we do not know. Just as the peripheral mechanism sifts out the several vibrations of which a musical sound is composed, and transmits them separately, so, by a reverse operation, the central mechanism probably pieces together the nervous disturbances of a number of central end-organs, and thus produces a sensation whose characters are determined by a combination of the nervous disturbances taking place in each end-organ.

Some such a view is indeed exceedingly probable; but it must be remembered that we do not at present at all understand the exact mechanism by which each particular vibration excites its corresponding nerve filament. The nerve filaments appear to end in the epithelial cells bearing short hairs, which lie on each side of the rods of Corti ; and we may therefore conclude that these "hair-cells have some share in producing the effect. But the whole matter is at present very obscure; the functions of the rods of Corti are particularly difficult to understand; for these do not seem in any way connected with the nerve filaments, and their movements can only affect the latter by influencing in some way the hair-cells.

31. The fibres of the cochlear nerve, or their endings in the brain itself, may be excited by internal causes, such as the varying pressure of the blood and the like: and in some persons such internal influences do give rise to veritable musical spectra, sometimes of a very intense character. But, for the appreciation of music produced external to us, we depend upon the organ of Corti being in some way or other affected by the vibrations of the fluids in the cochlea.

32. It has already been explained that the stapedius and tensor tympani muscles are competent to tighten the membrane of the fenestra ovalis and that of the tympanum, and it is probable that they come into action when the sonorous impulses are too violent, and would produce too extensive vibrations of these membranes. They may therefore be of use in moderating the effect of intense sound, in much the same way that, as we shall find, the

contraction of the circular fibres of the iris tends to moderate the effect of intense light in the eye; they may however, have other purposes.

The function of the Eustachian tube is, probably, to keep the air in the tympanum, or on the inner side of the tympanic membrane, of about the same tension as that on the outer side, which could not always be the case if the tympanum were a closed cavity.

LESSON IX.

THE ORGAN OF SIGHT.

1. IN studying the organ of the sense of sight, the eye, it is needful to become acquainted, firstly, with the structure and properties of the sensory expansion in which the optic nerve, or nerve of sight, terminates; secondly, with the physical agent of the sensation; thirdly, with the intermediate apparatus by which the physical agent is assisted in acting upon the nervous expansion.

The ball, or globe, of the eye is a globular body, moving freely in a chamber, the orbit, which is furnished to it by the skull. The optic nerve, the root of which is in the brain, leaves the skull by a hole at the back of the orbit, and enters the back of the globe of the eye, not in the middle, but on the inner, or nasal, side of the centre. Having pierced the wall of the globe, it spreads out into a very delicate membrane, varying in thickness from

th of an inch to less than half that amount, which lines the hinder two-thirds of the globe, and is termed the retina. This retina is the only organ connected with sensory nervous fibres which can be affected, by any agent, in such a manner as to give rise to the sensation of light.

2. If the globe of the eye be cut in two, transversely, so as to divide it into an anterior and a posterior half, the retina will be seen lining the whole of the concave wall of the posterior half as a membrane of great delicacy, and, for the most part, of even texture and smooth surface. But almost exactly opposite the middle of the posterior

R

FIG. 72.-DIAGRAMMATIC VIEWS OF THE NERVOUS (A) AND THE CONNECTIVE (B) ELEMENTS OF THE RETINA, SUPPOSED TO BE SEPARATED.

FROM ONE ANOTHER.

A, the nervous structures-b, the rods; c, the cones; b'c', the granules or nuclei of the outer layer, with which these are connected; d d', interwoven very delicate nervous fibres, from which fine nervous filaments, bearing the inner granules or nuclei, ff', proceed towards the inner surface; gg', the continuation of these fine nerves, which become convoluted and interwoven with the processes of the nerve cells h h'; ii, the expansion of the fibres of the optic nerve. B, the connective tissue-a a, external limiting membrane; ee, radial fibres passing to the internal limiting membrane; e e, nuclei; dd, the intergranular layer; gg, the nolecular layer; 7, the inner limiting membrane.

(Magnified about 250 diameters.)