Deafness may result from failure of the sensitive inner ear to respond, or from poor transmission of vibrations across the middle ear by the chain of bones, or by interference with the freedom of action of the eardrum, or by preventing the air waves from striking upon the drum. Injury to the inner ear is rare, because of its secure position within the bone. A common form of deafness is the result of hardening of the connections between the ear bones, so that the chain no longer follows well the vibrations of the drum. This usually begins to come on during the twenties or thirties, and causes almost complete deafness by the age of fifty. In most if not all cases it is hereditary. Interference with the action of the eardrum may be due to the partial destruction of the drum itself. Scarlet fever and measles are particularly likely to leave the drum in a delicate condition, and any strain upon it then may rupture it beyond repair. Continuous closing of the Eustachian tube, by preventing equalization of air pressure on the two sides of the drum, causes partial deafness. Inflammation accompanying a cold may cause this, or the growth of adenoids in the back of the throat. Adenoids will be described later; here they are mentioned only because they may press upon and close the Eustachian tubes. There is danger from the closure of the tubes by inflammation, because the inflammation may creep up to the middle ear and cause serious trouble, both there and in the mastoid region adjoining. Earache in children should be carefully watched, since it usually means that the middle ear has been invaded by the same inflammatory condition that is present in the throat in colds, and may do serious damage to the delicate structures there. Often in children, and sometimes in adults, the hearing is impaired by the accumulation of wax in the ear canal. This wax is a sticky secretion that serves to catch particles entering the ear canal and to prevent them from striking against the drum. Unless the ear canals are washed out frequently with _hot_ water the wax accumulates and hardens into a plug which closes the ear canal, shutting off faint sounds. The wax dissolves in hot water, but not in cold, so its accumulation is to be prevented by taking pains that hot water actually gets to the bottom of the ear canal once in a while. Digging the wax out with hard instruments should be done only by an expert with greatest caution, lest the drum be injured.