All sound does not come through the canal of the ear. The bones of the head vibrate and carry sound. So there are instruments for the deaf which are put in the ear and others which are placed between the teeth. The _semicircular canals_ are not essential to hearing but have something to do with a person’s power of maintaining his equilibrium. Injury to them may cause dizziness and loss of equilibrium. =The Eye.=--One more feature, perhaps the most expressive, remains to be described, the eye. The senses are all modifications of the original cutaneous sensibility and the nerve of sight is no more sensitive to light than any other nerve. It therefore needs an end organ that is sensitive to the motions of the ether in order to give impressions of light. This organ is provided in the eye, which is not only itself capable of being moved in every direction, but is placed in the most movable part of the body, the head, which can be turned in almost a complete circle. The _eyeball_ is spherical and lies in the cavity of the orbit upon a cushion of fat, where it has a large range of sight but is securely protected from injury by its bony surroundings. The sunken eyes following protracted illness are due to the using by the system of the fat on which the eyeball ordinarily rests. Each _orbital cavity_ is formed by the juncture of some seven bones and communicates with the cavity of the brain through the _optic foramen_ and through the sphenoidal fissure. Above the orbits are arched eminences of skin, the _eye-brows_, from which several rows of short hairs grow longitudinally and which serve to protect the eyes and to limit the amount of light to a certain extent, as in frowning. Still further protection is afforded by the _eyelids_, longitudinal folds of skin, the one above, the other below, which close like curtains over the eye. Beneath the external layer of skin in the lids is fatty tissue and then the orbicularis palpebrarum muscle by means of which they are closed. They are kept in shape by the tarsal plates or cartilages, in whose ocular surface are embedded the _Meibomian glands_, whose secretion prevents the free edges of the lids from sticking together. Along these edges grows a double or triple row of stiff hairs, the _eye-lashes_, which curve outward so as not to interfere with each other and also to prevent the entrance into the eye of foreign bodies. Lining the inner surface of the lids and reflected thence over the anterior surface of the sclerotic coat of the eye is a mucous membrane, the _conjunctiva_, which is thick, opaque, and vascular on the lids but thin and transparent on the eyeball. The angles between the lids are known as the _internal_ and the _external canthus_. [Illustration: FIG. 23.--The external ocular muscles. (Pyle.)]