4. A certain quantity of stimulus less than natural induces the moving
organ into feebler and more frequent contractions, as mentioned in No. I.
4. of this Section. For each contraction moving through a less space, or
with less force, that is, with less expenditure of the spirit of animation,
is sooner relaxed, and the spirit of animation derived at each interval
into the acting fibres being less, these intervals likewise become shorter.
Hence the tremours of the hands of people accustomed to vinous spirit, till
they take their usual stimulus; hence the quick pulse in fevers attended
with debility, which is greater than in fevers attended with strength; in
the latter the pulse seldom beats above 120 times in a minute, in the
former it frequently exceeds 140.

It must be observed, that in this and the two following articles the
decreased action of the system is probably more frequently occasioned by
deficiency in the quantity of sensorial power, than in the quantity of
stimulus. Thus those feeble constitutions which have large pupils of their
eyes, and all who labour under nervous fevers, seem to owe their want of
natural quantity of activity in the system to the deficiency of sensorial
power; since, as far as can be seen, they frequently possess the natural
quantity of stimulus.

5. A certain quantity of stimulus, less than that above mentioned, inverts
the order of successive fibrous contractions; as in vomiting the vermicular
motions of the stomach and duodenum are inverted, and their contents
ejected, which is probably owing to the exhaustion of the spirit of
animation in the acting muscles by a previous excessive stimulus, as by the
root of ipecacuanha, and the consequent defect of sensorial power. The same
retrograde motions affect the whole intestinal canal in ileus; and the
oesophagus in globus hystericus. See this further explained in Sect. XXIX.
No. 11. on Retrograde Motions.