ny capillary arteries.

3. Not only the muscular fibres lose their degree of excitability from
age, as in the above examples; and as may be observed in the tremulous
hands and feeble step of elderly persons; but the organs of sense
become less excitable by the stimulus of external objects; whence the
sight and hearing become defective; the stimulus of the sensorial
power of sensation also less affects the aged, who grieve less for the
loss of friends or for other disappointments; it should nevertheless
be observed, that when the sensorial power of irritation is much
exhausted, or its production much diminished; the sensorial power of
sensation appears for a time to be increased; as in intoxication there
exists a kind of delirium and quick flow of ideas, and yet the person
becomes so weak as to totter as he walks; but this delirium is owing
to the defect of voluntary power to correct the streams of ideas by
intuitive analogy, as in dreams: see Zoonomia: and thus also those who
are enfeebled by habits of much vinous potation, or even by age alone,
are liable to weep at shaking hands with a friend, whom they have not
lately seen; which is owing to defect of voluntary power to correct
their trains of ideas caused by sensation, and not to the increased
quantity of sensation, as I formerly supposed.

The same want of voluntary power to keep the trains of sensitive ideas
consistent, and to compare them by intuitive analogy with the order of
nature, is the occasion of the starting at the clapping to of a door,
or the fall of a key, which occasions violent surprise with fear and
sometimes convulsions, in very feeble hysterical patients, and is not
owing I believe (as I formerly supposed) to increased sensation; as
they are less sensible to small stimuli than when in health.

Old people are less able also to perform the voluntary exertions of
exercise or of reasoning, and lastly the association of their ideas
becomes more imperfect, as they are forgetful of the names of persons
and places; the associations of which are less permanent, than those
of the other words of a language, which are more frequently repeated.

4. This disobedience of the fibres of age to their usual stimuli, has
generally been ascribed to repetition or habit, as those who live near
a large clock, or a mill, or a waterfall, soon cease to attend to the
perpetual noise of it in the day, and sleep dining the night
undisturbed. Thus all medicines, if repeated