Show YOURSELF S and YOUR BODY SENTINEL NUMBER THREE the ear outer ear ear inner ear balancing Is like playing ball whichever cell catches the ball sends a message to the brain t j tiie third great control Is the ear the ear is a double conj arol it first enables the body to balance and second it acts as a very valuable alarm be cause while the eyes guard right ahead only the ears guard in two dl lons horses can move their ears around in any direction and can therefore tell better than we can how far off and from what direction the sound Is because the ear muscles can put the ears at the angle that gets the sound best borses rely much more on their ears than their eyes so do rabbits and deer fortunately as our ears are on opposite sides of our head it takes sound wavea longer to get to one ear than the other and that helps us to tell how far off the sound Is and which direction it comes from our hearing machine has on the outside a halt horn to collect sound waves and send them down the hole into a tube as the receiver of a telephone does for our voice the tube Is located in solid bone and away down it so as to be safe a drum head Is stretched right across the tube in the tube also are hairs and wax to catch and keep out ene and dirt the drum head Is built up of very fine strings stretch ing from the side of the tube and fastened in the middle to a tiny bone rod called the hammer handle bea cause there Is a knob on the top end of it the handle Is not exactly in the middle so that all the strings are of different lengths and each fiber will start to move in answer to a alt ferent note the our ears can tell us of any note made up of waves that measure anywhere between 40 and vibrations to a second now we must have air on both sides of the delicate drum or it would be pressed flat by the weight of the air outside so the other side of the drum head Is a fine little room called the middle ear there Is a door in the back wall of this room leading to a pipe the other end of alch opens into the big air chamber inside the back of the throat it ou shut your mouth and hold your nose and swallow you will hear air crinkle inside your ear against the inside of the drum big tonsils are apt to block up the inner end of this pipe or when you get a bad cold it gets blocked then you can hardly hear anything because the drum gets pressed in and cant vibrate the arrangement to carry the sound vibrations across this necessary room Is really marvelous a beautiful necklace of little bones joined to the head of the hammer bone Is a tiny called the anvil lie cause of its shape and fast to that Is the stirrup the flat part or plunger of which Is fastened square over the second drum a thin mem brane covering in an oval window on the inside wall like this ahen there Is a noise the outside drum moves with the sound and the little chain prevents the waves from pulling the inner drum too hard the other side of the oval window Is the secret chamber called the inner ear ahls Is beau lined and divided into two halla each full of watery lymph the front hall leads to the music room alila is a long tubular corridor which winds round and round an upright hoi low pillar getting smaller as it goes up like the center piece of tills conch shell see picture there s a bone shelf half way up the side of the corridor running round the central rod and EO many thousands of fibers are stretched from it to the outer wall that U makes a fine curtain the strings getting shorter and shorter as you go up exactly as fiddle string or harp strings get shorter to make higher and higher notes so these do to receive and answer to higher and higher notes A veil only a single cell thick covers over the outer part of j this as you see in the picture and j in this queer passage formed beneath Is the real hearing organ this organ Is made of a few special cells and runs the whole way up the corridor from bottom to top inside the central column run thousands of wires to central the whole Is called the hearing nerve exactly like the seeing nerve one fiber runs out over the bony shelf to every single cel on the hearing or gan the oval window covered by the plunger of the stirrup Is in the upper half of the corridor there Is a large round window in the lower half covered by an elastic membrane exactly like the eskimos use elastic seal bowel skin to cover their windows because they have no glass in the hearing organ you see the rows of cells each with fine hairs in it and close over them Is a special roof made up mostly of live wires that Is of nerves to a cell mass or ganglion thousands of which are lodged on the circular shelf and are local centers sending the messages through the hearing wires to central when the outside drum Is moved by a sound wave the plunger of the stirrup presses the water in the top of the corridor down on the tips of the hairs and that pushes out the elastic round window below the elasticity pulls it back and so it Is just as it the hairs ticked the live wires just the right number of times for each note and so wired a hearing picture up to cen aral where it Is exactly as in the eye immediately preserved in the record chamber called memory as well as interpreted and acted upon by you and me there are little specks floating in the corridors also and when stirred up they may help to send up messages also these records are so well kept that recently a man listening in on a radio discovered his lost brother whose voice he had not heard for years now lets go back to the other or inner end pt tho hall here Is an other room with five doors each lend ing into a big bony tube each Is halt a circle so they are billed semi circular canals here in this abam ber which Is also filled with fluid are again many small specks and also a number of cells with hairs the canals lie in the shapes of big bows at right angles to one another so that one Is down when the other Is up whichever way your head moves and into them also the fluid goes there Is a row of wires that goes to tills room also but it does not come from central front office this time but from the baal nr lower brain office and it 18 these canals which enable us to bal ance see picture ahen they na too much in certain ways they get troubled and we get giddy some think the specks lodge now on this side now on that of the hairs as they moe about and the changing 0 the weight makes pictures in the lower brain sea sickness Is a trouble of these balancing canals when alie ship shakes us up too much we don t 3 et exactly understand bow any of these pictures are interpreted by the man inside perhaps we never shall but the workmanship and W design are so wonderful and are so carefully protected that we ought to know nil we can about them by tb 1311 ayoutt ino |