The eardrum does not act directly upon the sensitive hearing apparatus, but its vibrations are transmitted across a space known as the middle ear. The necessity for this space is found in the fact that atmospheric pressure is not constant, but changes frequently from day to day, besides falling off as one ascends higher above sea level. The free action of the eardrum depends on its not being stretched; if there were no means of readjustment it might be properly set for one air pressure, but greater pressures would bulge it inward, putting it on the stretch, and so cutting down its ability to respond to a wide range of tones. The middle ear, which is the space behind the drum, connects with the outside air by a tube leading from it to the back of the throat, which latter communicates freely with the air through the nose, as well as through the mouth whenever it is open. The tube is known as the Eustachian tube. Its walls are ordinarily collapsed, so it is not an open passage, but every time one swallows the tube is pulled open, thus allowing differences in air pressure on the two sides of the eardrum to equalize. Whenever one ascends a high hill quickly, as by train or automobile, or even in going to the top of a high building by elevator, the difference in air pressure behind and in front of the eardrum can be felt. The sensation is disagreeable, and there is definite impairment of hearing. Repeated swallowing gives relief. The vibrations of the eardrum are transmitted across the space of the middle ear by a chain of three tiny bones; these are very irregular in shape, and are attached to one another in such a way that every movement of the drum is followed exactly as to time and direction, but with reduced size and increased power. The hearing apparatus, which is part of the internal ear, but not the same part as makes up the equilibrium organ, contains liquid which is set moving by the last of the chain of bones, and this liquid acts upon the actual sensitive cells which make up the sense organ proper. These latter are arranged in an exceedingly complicated fashion. There are various theories as to the precise manner in which the vibrations of the liquid in the inner ear arouse the sensitive cells. It is thought that different cells respond to tones of different pitch, but exactly how this is accomplished is not known.