he system, act upon it perhaps rather as
a causa sine qua non of its movements, than directly as a stimulus; except
when they are accumulated in unusual quantity. We have a sense adapted to
the perception of the excess or defect of one of these fluids; I mean that
of elementary heat; in which all things are immersed. See Class IV. 1. 4.
7. But there are others of them, which as we have no power to evade their
influence, so we have no sense to perceive it; these are the solar, and
lunar, and terrestrial gravitation, in which also all things are immersed;
the electric aura, which pervades us, and is perpetually varying, See Class
IV. 1. 4. 5; the magnetic fluid, Class IV. 1. 4. 5; and lastly, the great
life-preserver oxygen gas, and the aqueous vapour of the atmosphere, see
Class IV. 1. 4. 6. and 7. and 2.

Of these external influences those of heat, and of gravity, have diurnal
periods of increase and decrease; besides their greater periods of monthly
or annual variation. The manner in which they act by periodical increments
on the system, till some effect is produced, is spoken of in Sect. XXXII.
3. and 6.

D. _Associations affected by other Sensorial Motions._

Circles and trains of associate motions are also liable to be affected by
their catenations with other sensorial powers, as of irritation, or
sensation, or volition; which other sensorial powers either thus simply
form some of the links of the catenation, or add to the energy of the
associated motions. Thus when vomiting is caused by the stimulus of a stone
in the ureter, the sensation of pain seems to be a link of the catenation
rather than an efficient cause of the vomiting. But when the capillary
vessels of the skin increase their action from the influence of external
heat, they are excited both by the stimulus of unusual heat, as well as by
the stimulus of the blood, and by their accustomed association with the
actions of the heart and arteries. And lastly, in the blush of anger the
sensorial power of volition is added to that of association, and
irritation, to excite the capillaries of the face with increased action.
See Class IV. 2. 3. 5.

E. _Associations catenated with Sensation._

Pain frequently accompanies associate trains or circles of motion without
its being a cause, or a link, of them, but simply an attendant symptom;
though it frequently gives name to the disease, as head-ach. Thus in the
cramp of the calves of the legs in diarrhoea, the incr