eer, the organ of hearing. In the lower orders of animals the ear is quite small and capable of perceiving only a few sounds, but it may be more acute than the human ear in recognizing these sounds. In man and all higher orders of animals the ear consists of three divisions, the external ear, the middle ear and the internal ear. THE EXTERNAL EAR, or CONCHA, consists of the auricle, which is the part usually called the ear, and the auditory canal. In many animals the external ear is movable, but in man it is not. The human external ear consists of cartilage, skin and a thin, muscular coat and has a peculiar form. The outer rim is known as the helix, and the structure is so shaped as to attract sound waves into the auditory canal, which is a passage leading to the membrane separating the external from the middle ear. This is known as the membrane of the tympanum or eardrum, and is a circular membrane stretched across the inner end of the auditory canal, its rim being attached to the bone forming the walls of the tympanum. The membrane contains a number of muscles, some of which are joined to the bones of the middle ear, and its inner surface is lined with a mucous membrane. THE MIDDLE EAR or TYMPANUM is a cavity filled with air, which enters it through the Eustachian tube. It contains three small bones known as ossicles. From their peculiar shape these bones are called the hammer, the anvil and the stirrup. The hammer consists of a head, neck and handle, the last being attached to the eardrum. The anvil has a concave surface into which the head of the hammer fits, and the stirrup at one end is joined to the anvil, and at the other to the vestibule of the internal ear. These bones are so placed as to transfer the vibrations of the eardrum to the internal ear. THE INTERNAL EAR, or LABYRINTH, is located in a small cavity in the temporal bone, and is divided into the vestibule, the semicircular canals and the cochlea. The vestibule is a small triangular cavity between the semicircular canals and the cochlea. The base fits into the opening which connects this with the middle ear. The semicircular canals are three in number and occupy vertical, oblique and horizontal positions, as shown in the diagram. Each connects with the vestibule by two openings. The cochlea or snail shell is a gradually tapering canal winding two and one-half times around a central bony axis. The cavity of the cochlea is divided into two passages by a stiff spiral plate and a membrane composed of a large number of strings extending from the outer edge of the bony plate to the walls of the cavity. The internal car is filled with fluid, within which floats a membrane in shape and outline corresponding exactly to those of the bony cavity. Within this membrane is still another fluid, in which are found the sensory fibers of the auditory nerve and the organ of Corti. This consists of two rows of pillars or rods and extends the entire length of the cochlea. The number of rods is estimated at anywhere from 10,500 to 24,000, while the number of strings is 3000. These rods and strings are connected with the filaments of the auditory nerve. They are of different lengths and sizes and it is through them that the sound vibrations which enter the ear are reproduced. HEARING. Hearing involves both physiological and mental action. For the production and transmission of sound, see SOUND. The external ear collects the sound waves and reflects them to the auditory canal, where they strike against the eardrum, which they cause to vibrate. By means of the bones in the middle ear the vibrations of the eardrum are intensified and at the same time transmitted to the internal ear at the vestibule. Here they set up vibrations in the fluid. The strings and rods in the organ of Corti, which can vibrate in harmony with these vibrations, respond just as do the strings of a piano that produce a note sung near the instrument. The vibrations in the organ of Corti stimulate the sensitive fibers of the auditory nerve connected with the vibrating rods, and the nerve impulses thus created are carried to the auditory centers of the brain, by which they are translated into sound. Hence, the final act of hearing is a mental act. The muscles of the middle ear regulate the tension of the eardrum so that it can respond equally well to vibrations that produce high or low tones, and the large number of rods and pillars of the organ of Corti enable us to recognize a multitude of sounds.