it is known to be
effected by an admixture of oxygene, or vital air; which, according to a
discovery of Dr. Priestley, passes through the moist membranes, which
constitute the sides of these vessels. As the blood passes through the
capillary vessels, and glands, which connect the aorta and its various
branches with their correspondent veins in the extremities of the body, it
again loses the bright red colour, and undergoes some new combinations in
the glands or capillaries, in which the matter of heat is given out from
the secreted fluids. This process therefore, as well as the process of
respiration, has some analogy to combustion, as the vital air or oxygene
seems to become united to some inflammable base, and the matter of heat
escapes from the new acid, which is thus produced.

V. After the blood has passed these glands and capillaries, and parted with
whatever they chose to take from it, the remainder is received by the
veins, which are a set of blood-absorbing vessels in general corresponding
with the ramifications of the arterial system. At the extremity of the fine
convolutions of the glands the arterial force ceases; this in respect to
the capillary vessels, which unite the extremities of the arteries with the
commencement of the veins, is evident to the eye, on viewing the tail of a
tadpole by means of a solar, or even by a common microscope, for globules
of blood are seen to endeavour to pass, and to return again and again,
before they become absorbed by the mouths of the veins; which returning of
these globules evinces, that the arterial force behind them has ceased. The
veins are furnished with valves like the lymphatic absorbents; and the
great trunks of the veins, and of the lacteals and lymphatics, join
together before the ingress of their fluids into the left chamber of the
heart; both which evince, that the blood in the veins, and the lymph and
chyle in the lacteals and lymphatics, are carried on by a similar force;
otherwise the stream, which was propelled with a less power, could not
enter the vessels, which contained the stream propelled with a greater
power. From whence it appears, that the veins are a system of vessels
absorbing blood, as the lacteals and lymphatics are a system of vessels
absorbing chyle and lymph. See Sect. XXVII. 1.

VI. The movements of their adapted fluids in the various vessels of the
body are carried forwards by the actions of those vessels in consequence of
two kinds of st