Behind each eye in a vertebrate there is a cavity in a part of the skull called the auditory capsule, which contains a fluid called peri- Anterior lymph in which a complex structure, the labyrinth, floats.
This contains endolymph.
This labyrinth or inner ear is hollow, consisting of the utriculus, on the upper side of which are three tubes, semi- circular canals, one vertical and directed forwards, one vertical but directed at right angles to the first, and the third lying horizontally, I.e. each lies in a plane at right angles to the other two. Each canal has a bulge, the ampulla, at its base. Inside the ampulla is a cushion containing sense hairs embedded in a gelatinous material in which there are chalky particles. On the under side of the utriculus is a large bulge, the sacculus, and other structures, known as the lagena in the frog and the cochlea in the rabbit. The labyrinth functions as a sense organ of balance. The tilting of the head in any direction will cause the endolymph in the semicircular canals to flow in at least one of them. A jerk of the head will cause the liquid to swirl in the canals, just as water in a glass will slop out of it if it is suddenly moved sideways. This flow of endolymph will cause the chalky particles to knock against the sense hairs of the ampulla? causing them to set up impulses which pass along fibres of a branch of the auditory nerve. These impulses are relayed from the cerebellum to limb, trunk and tail muscles for maintaining balance. Some impulses may reach the scat of consciousness in the brain so that the animal knows its, but human beings are not normally conscious of any sensation as a result of impulses from the ampulla?. It is when the canals are functioning in an unusual fashion that we become conscious of their activity, for then we experience a sensation of giddiness and the muscular response causes us to lose our balance. We are able to tell our position by other ways : for example, by judging the pressure upon each part of the sole of each foot. Animals when flying or swimming have no contact with a solid surface, so that in such animals a sense of position is of the greatest importance, as it is also in the case of animals which run swiftly.
The Ear as a Sense Organ of Hearing
The cochlea in mammals and birds and the lagena in other vertebrates contains sense cells which are affected by vibrations set up in air or water by sound. In forms higher than fish there are acoessory parts which are concerned in receiving these vibrations. Vibrations of high frequency due to high notes are readily damped and absorbed by solids and water, so that they do not travel far compared with low vibrations due to low sounds—a fact that every tenderfoot scout knows once he has put his ear to the ground. Low-frequency vibrations are set up in water, for example, when it swirls around rocks and uneven surfaces. It is this type of vibration to which the ears of fish are sensitive—or rather that extension of the auditory system known as the lateral line.
Ear of Frog
The external ear-drum of the frog forms the outer wall of a cavity, the middle ear. The opposite wall of this cavity has a membrane, the fenestra ovalis, on the other side of which lies the inner ear. Opening into the floor is a tube, the Eustachian tube. from the side of the back of the mouth. Slung across the cavity is a bone, the columella, one end being in contact with the inner side of the ear-drum and the other touching a small mass of cartilage lying against the fenestra. Any sounds in air cause the ear-drum to vibrate. The vibrations are transmitted in sucoession through the columella, fenestra, perilymph and labyrinth walls to the endolymph, which in turn affects the sense cells in the lagena so that impulses pass along the auditory branch of nerve VIII to the auditory centre in the brain, where a sensation of sound is experienced. Thus the parts of the middle ear are the acoessory parts concerned with picking up sounds from outside the animal and transmitting the resultant vibrations to a delicate sense organ situated right inside the head. As the skin of the animal is continued over the ear-drum it is not very likely that the apparatus is very sensitive in air at any rate. The Eustachian tube enables pressure to be equalized on each side of the drum, otherwise when the animal goes down into deep water the pressure on the ear-drum would severely strain it.
Ear of Rabbit
The cochlea in the inner ear of a rabbit is a delicate spiral structure which lies floating in a bony case, just as the soft parts of a snail or winkle fit into the shell. In internal structure it is extremely complex, but all that matters to us is that it contains sense cells which are affected by vibrations due to the different sounds in the world, those affected by the vibrations due to higher notes being situated near the base of the spiral. Thus every combination of sounds will affect a corresponding set of sense cells.
In addition to the inner ear, the rabbit, like nearly all mammals, possesses a middle and outer ear. The middle ear is a cavity containing air, the inner wall having a tiny membrane, the fenestra ovalis, on the other side of which lies the inner ear, and the outer wall has another membrane, the car-drum, stretched across it. The cavity communicates with the pharynx by the Eustachian tube. Slung by ligaments across the middle ear are three bones, a long malleus attached to the car-drum and articulating with incus, which in turn articulates with a stapes attached to the fenestra. The outer ear consists of a pinna, which leads by a tube, the meatus, to the ear-drum.
Sound waves are picked up by the pinna and pass along the meatus to strike against the ear-drum, causing it to vibrate. In the rabbit they are also reflected by the bulla on to the ear-drum. The vibrations are transmitted by the chain of ear bones to the fenestra, so that they are concentrated on to the fenestra which transmits them through perilymph, labyrinth and endolymph to the sense cells in the cochlea. Those affected by the particular vibrations thereupon set up impulses which pass along the fibres of the acoustic branch A of nerve VIII to that part of the brain where the sensation of a particular type of sound is experienced. Vibrations can also be conducted by the bones of the skull to the perilymph, etc., e.g. by placing a ticking watch against the forehead and stopping up the ears.