It is unwise for old people to undertake unaccustomed or sudden muscular exertion, since the muscular system including the heart muscle grows weak and is generally unable to endure great strain. The bones, moreover, grow brittle and heal with difficulty if broken, so that persons of advanced years no matter how active should avoid walking on icy pavements, climbing on chairs to reach high shelves, and placing themselves in other insecure positions. Assistance must be tactfully given, however, as active old people are inclined to resent it. On the other hand, old people should be encouraged to continue moderate and safe activities, and to take regular exercises suited to their strength. Although increasing muscular weakness tends to make most old people indolent, it is far better for them both in mind and in body to remain as active as they can without danger of too great fatigue. At all events, they should be prevented if possible from becoming bedridden. Since in old age sight, hearing, and other special senses become less acute, one should remember that an old person may not notice the odor of escaping gas, the light of a smouldering match, or the sound of an approaching motor car, and that he must be specially guarded from such dangers of every day life. On account of their dulled perceptions old people are sometimes unjustly considered to be less intelligent than they really are. Young people moreover should be told, if an aged person is untidy and careless in personal habits, that the apparent negligence is caused by dulled perceptions and diminished muscular control for which old people are no more responsible than they are for failing eyesight or for inability to hear. Families should also realize that changes in mind and character are beyond an aged person's control and that they should not be made the cause for remonstrance or arguing. Just as the arteries harden with advancing years, as the bones become brittle and as other tissues become less flexible, so changes are likely to occur in the nervous system. It is not surprising when the brain substance like other tissues is becoming less flexible, that the powers of attention should weaken, that memory for recent events should diminish, or that other mental powers should fail. Changes in disposition are not uncommon: previously controlled persons sometimes become querulous and exacting, while excitable and irritable persons become more placid. With most old people emotions become less intense; feeble old people hardly realize great joy or great sorrow, and seldom look forward to death with apprehension.