me time,
and some of them occasionally by them all. Thus respiration is
generally caused by the stimulus of blood in the lungs, or by the
sensation of the want of oxygen; but is also occasionally voluntary.
The actions of the heart also, though generally owing to the stimulus
of the blood, are also inflamed by the association of its motions with
those of the stomach, whence sometimes arises an inequality of the
pulse, and with other parts of the system, as with the capillaries,
whence heat of the skin in fevers with a feeble pulse, see Zoonomia.
They are also occasionally influenced by sensation, as is seen in the
paleness occasioned by fear, or the blush of shame and anger; and
lastly the motions of the heart are sometimes assisted by volition;
thus in those who are much weakened by fevers, the pulse is liable to
stop during their sleep, and to induce great distress; which is owing
at that time to the total suspension of voluntary power; the same
occurs during sleep in some asthmatic patients.

2. The debility of approaching age appears to be induced by the
inactivity of many parts of the system, or their disobedience to their
usual kinds and quantities of stimulus: thus the pallid appearance of
the skin of old age is owing to the inactivity of the heart, which
ceases to obey the irritation caused by the stimulus of the blood, or
its association with other moving organs with its former energy;
whence the capillary arteries are not sufficiently distended in their
diastole, and consequently contract by their elasticity, so as to
close the canal, and their sides gradually coalesce. Of these, those
which are most distant from the heart, and of the smallest diameters,
will soonest close, and become impervious; hence the hard pulse of
aged patients is occasioned by the coalescence of the sides of the
vasa vasorum, or capillary arteries of the coats of the other
arteries.

The veins of elderly people become turgid or distended with blood, and
stand prominent on the skin; for as these do not possess the
elasticity of the arteries, they become distended with accumulation of
blood; when the heart by its lessened excitability does not contract
sufficiently forcibly, or frequently, to receive, as fast as usual,
the returning blood; and their apparent prominence on the skin is
occasioned by the deficient secretion of fat or mucus in the cellular
membrane; and also to the contraction and coalescence and consequent
less bulk of ma