STRUCTURE OF THE HUMAN BODY 57 



labyrinth, a special conducting apparatus, consisting of middle and 

 external ears, is necessary. The external ear consists of the 

 flap to which the name " ear " is applied in ordinary language, 

 that of concha or pinna scientifically, and of a short auditory tube 

 leading down into the head. Stretched across the inner end of 

 this tube is a firm tympanic membrane. The middle ear consists 

 of a tympanic cavity ("drum" of the ear) internal to the tympanic 

 membrane, and communicating with the pharynx by a tube-like 

 prolongation (Eustachian tube). The internal ear is situated close 

 to the inner side of the tympanic cavity, and at one place the 

 protecting bone is absent, leaving a little oval space, the oval 

 window (fenestra ovalis), filled by membrane only. If this were 

 perforated, the fluid which surrounds the labyrinth would escape 

 into the tympanic cavity. Stretching across the cavity from 

 tympanic membrane to oval window is a chain of minute bones, 

 the auditory ossicles, called, from their shape, hammer, anvil, and 

 stirrup. The handle of the first is fixed to the tympanic mem- 

 brane, while the foot-plate of the last fits into the oval window. 

 Sound-waves enter the auditory tube, throw the tympanic 

 membrane into vibrations, like those of a drum-head, and this 

 moves the chain of ossicles backwards and forwards, thus acting 

 upon the membrane of the oval window, thereby affecting the 

 fluid surrounding the labyrinth. The wall of the labyrinth itself 

 is thus jolted, and the fluid it contains agitated. So far the 

 vibrations (except the sound-waves themselves) are not molecular, 

 but now begins the first part of hearing proper, for the auditory 

 cells of the labyrinth are thrown into molecular vibration, followed 

 by the transmission of impulses along the auditory nerve to 

 the brain. 



The Organs of Sight (fig. 33). — The eyes, which to a very 

 large extent act together, are sheltered in bony depressions of 

 the face known as the orbits, and are provided with muscles for 

 moving them in different directions, which obviously largely adds 

 to their efficiency. There are also other accessory structures, such 

 as eyelids and tear -glands, the former protective, and the latter 

 secreting a fluid which washes the front of the eye, and as it is 

 done with passing down a narrow tube into the nose-cavity. 



The eyeball is covered by three coats, of which the outermost, 

 the sclerotic, is very tough, and forms what is called " the white " 

 of the eye. A circular external area of this is the transparent 



