The eyelids are lined by a mucous membrane which we call the conjunctiva, which secretes fluid which continually moistens the front part of the eye. That mucous membrane is not continued over the transparent cornea, but only its epithelial lining. The eye is-kept still moister by means of the secretion of a gland on the upper side of the eyeball, called the lachrymal gland or tear gland, which secretes watery fluid continually, keeping the front part of the eyeball moist. The excess of fluid is conveyed by some small ducts into a tube which leads into the nose, and which is a kind of drainage tube for the eye-socket. When that secretion is excessive, that drainage tube is not able to carry it all off, and it rolls out on to the surface of the face in the form of tears.

The colour of the eye, as we call it, is due-to the colour of the iris. That colour is caused by the pigment in the iris and by the blood in the blood-vessels of the iris, and in that way we get all the varieties of black, grey, blue, brown, etc. that we see in people's eyes. Persons who have no pigment in their eyes and no pigment in their hair are called albinos, and they have pink eyes, the pink colour being due to the colour of the blood in the iris.

Such persons always have weak eyes, and can only bear a very small amount of light, and that shows you that one of the principal objects of the pigment in the iris and in the choroid membrane, but especially of the pigment in the iris, is to absorb light which is not required for the purposes of clear vision.

The organ of hearing consists of three parts - the external ear, the middle ear, and the internal ear, the two latter being contained in cavities in the hard part of the temporal bone. The external ear is more or less trumpet-shaped, and has a passage leading through a hole in the temporal bone to the middle ear. Across the end of this passage, separating the external ear from the middle ear, is stretched a membrane called the tympanic membrane.

The cavity of the middle ear, on the inner side of this membrane, is called the tympanum, or drum of the • ear, and communicates with the pharynx by a tube called the Eustachian tube. On the inner side of this cavity, opposite to the tympanic membrane, are two openings in the bone closed by membranes which form separations between the middle and the internal ear. A chain of three small bones jointed together stretches between the tympanic membrane, to which the first of them is attached, and the membrane closing one of these two apertures on the other side of the tympanum, to which the third bone is attached. The internal ear or labyrinth is contained in complicated cavities in the bone, called the vestibule, the semicircular canals, and the cochlea (or shell, from its shape). Inside these cavities is a closed sac, prolongations from which extend through the winding passages of the bone. Outside of this sac, between it and the walls of the cavities, is a watery liquid called perilymph, and inside of the sac is a liquid called endolymph. On the inner wall of the sac, and, therefore, in the endolymph, there are in some parts fine hair-like bodies, and in others small calcareous particles, looking something like grains of sand, or little rod-like bodies placed side by side like the keys of a pianoforte (called fibres of Corti). The fibres of the seventh pair of cranial nerves - the auditory nerves - are distributed to the walls of the membranous labyrinth, as this complicated sac is called, and their extremities are in connection with the little bodies just mentioned.

When the waves of sound, caused by vibrations of the particles of the ear, being collected by the external air and directed along the passage leading towards the middle ear strike the tympanic membrane or drum, it vibrates, and its vibrations shake the little chain of bones that cross the cavity of the middle ear, and so shake the membrane closing the aperture on the other side of the tympanum to which the last bone of the chain is attached. At the same time the vibrations of the air in the tympanum cause the membrane covering the other aperture leading into the internal ear to vibrate.

The vibration of these two membranes sets the perilymph in motion, and its vibrations are transmitted to the walls of the sac or membranous labyrinth in the internal ear, and so the little hair-like bodies, calcareous particles, and fibres of Corti are shaken, and their movement irritates the extremities of the auditory nerves, by the fibres of which the stimuli produced are transmitted to the brain.

No doubt the different parts of the labyrinth have different duties to perform. One part, for instance, has to do with the quantity or loudness of sounds, another with the quality or tone; but what I want you specially to note is, that the extremities of the optic nerve are not themselves directly affected by the vibrations of the air, but that, as in the case of the other senses, the stimulus which comes from outside the body affects a structure belonging to the epithelial tissues.