their
accustomed share; and therefore first stop, and then invert their motions.
Owing to deficiency of sensorial power, as explained more at large in Sect.
XXXV. 1. 3.

2. _Nausea a conceptu._ The nausea, which pregnant women are so subject to
during the first part of gestation, is owing to the reverse sympathy
between the uterus and stomach, so that the increased action of the former,
excited by the stimulus of the growing embryon, which I believe is
sometimes attended with sensation, produces decreased actions of the latter
with the disagreeable sensation of sickness with indigestion and consequent
acidity. When the fetus acquires so much muscular power as to move its
limbs, or to turn itself, which is called quickening, this sickness of
pregnancy generally ceases.

M. M. Calcined magnesia. Rhubarb. Half a grain of opium twice a day.
Recumbent posture on a sofa.

3. _Vomitio vertiginosa._ Sea-sickness, the irritative motions of vision,
by which we balance ourselves, and preserve our perpendicularity, are
disturbed by the indistinctness of their objects; which is either owing to
the similarity of them, or to their distance, or to their apparent or
unusual motions. Hence these irritative motions of vision are exerted with
greater energy, and are in consequence attended with sensation; which, at
first is agreeable, as when children swing on a rope; afterwards the
irritative motions of the stomach, and of the absorbent vessels, which open
their mouths into it, become inverted by their associations with them by
reverse sympathy.

For the action of vomiting, as well as the disagreeable sensation of
sickness, are shewn to be occasioned by defect of the sensorial power;
which in this case is owing to the greater expenditure of it by the sense
of vision. On the same account the vomiting, which attends the passage of a
stone through the ureter, or from an inflammation of the bowels, or in the
commencement of some fevers, is caused by the increased expenditure of the
sensorial power by the too great action of some links of the associations
of irritative motions; and there being in consequence a deficiency of the
quantity required for other links of this great catenation.

It must be observed, that the expenditure of sensorial power by the retinas
of the eyes is very great; which may be estimated by the perpetual use of
those organs during our waking hours, and during most of our sleeping ones;
and by the large diameters o