of action, as shewn in Section X. on
Associate Motions. Some constitutions more easily establish these
associations, whether by voluntary, sensitive, or irritative repetitions,
and some more easily lose them again, as shewn in Section XXXI. on
Temperaments.

When the beginning of such a train of actions becomes by any means
disordered, the succeeding part is liable to become disturbed in
consequence, and this is commonly termed sympathy or consent of parts by
the writers of medicine. For the more clear understanding of these
sympathies we must consider a tribe or train of actions as divided into two
parts, and call one of them the primary or original motions, and the other
the secondary or sympathetic ones.

The primary and secondary parts of a train of irritative actions may
reciprocally affect each other in four different manners. 1. They may both
be exerted with greater energy than natural. 2. The former may act with
greater, and the latter with less energy. 3. The former may act with less,
and the latter with greater energy. 4. They may both act with less energy
than natural. I shall now give an example of each kind of these modes of
action, and endeavour to shew, that though the primary and secondary parts
of these trains or tribes of motion are connected by irritative
association, or their previous habits of acting together, as described in
Sect. XX. on Vertigo. Yet that their acting with similar or dissimilar
degrees of energy, depends on the greater or less quantity of sensorial
power, which the primary part of the train expends in its exertions.

The actions of the stomach constitute so important a part of the
associations of both irritative and sensitive motions, that it is said to
sympathize with almost every part of the body; the first example, which I
shall adduce to shew that both the primary and secondary parts of a train
of irritative associations of motion act with increased energy, is taken
from the consent of the skin with this organ. When the action of the fibres
of the stomach is increased, as by the stimulus of a full meal, the
exertions of the cutaneous arteries of the face become increased by their
irritative associations with those of the stomach, and a glow or flushing
of the face succeeds. For the small vessels of the skin of the face having
been more accustomed to the varieties of action, from their frequent
exposure to various degrees of cold and heat become more easily excited
into increa