_Of the Joint Connection of the Brain, Heart, Lungs, &c._
These are the principal actions in the grand movement of the human
automaton.
The nerves are the subtile springs of the sensation, by which the whole
becomes sensible of irritation, and agitated to its functions. Their
ramifications are extended _ad infinitum_, and so curiously distributed,
that though there is not a part in the whole system exempt from their
spreading, yet they are distributed with such regularity, that they
don’t interfere with each other. A regularity absolutely requisite to
the order in the animal œconomy: For whereever the vibration of the
nerves is obstructed, life or motion must in consequence cease.
The lungs are the ventilators in the system: they receive and let in the
air; they sift it from all its gross particles, and extract the useful
parts from this universal element of life; thus they give from every
little vesicle, a due proportion of extracted air to the blood by the
adjacent veins; and by the neighbouring arteries, again extract the foul
air from the circulated blood, which by the mouth and nostrils is
expelled again; at that instant a fresh quantity of air is drawn in
again for a new supply.
Thus by intervals, the lungs, like a perfect ventilatory machine,
repeatedly, and without intermission, keep time with _systolic_ and
_diastolic_ motions of the heart, in exchanging fresh and foul air for
the support of this wonderful animal structure. This is the function of
breathing _respiration_, namely _inspiration_, or the drawing in the
fresh air; and _expiration_, the expelling the foul air.
The heart may be considered as the grand perpetual pumping engine,
constructed on principles, so as to have the least friction possible.
This curious pump, _the heart_, sends the fluid of life, _the blood_, to
all the parts, even the remotest in the whole machine. The _arteries_
are the canals of the conveyance; they are strong elastic tubes, whose
fibres are of spiral direction, and receive from the heart a power of
contraction; which is continued from the heart in the moment of the
_systole_, even to the remotest and most minute ramifications; by which
the blood is pushed along to the minutest extremities. This is
immediately succeeded with a _diastolic_ reception of a fresh quantity
of blood; so that on succession, the _diastole_ and _systole_ is
perpetuated not only in the heart, but throughout all the arteries, in a
friendly correspondence over the whole system: And this constitutes the
_pulse_.
The veins, however, are somewhat of a different texture from the
arteries; whilst the arteries transport the blood by an elastic and
pulse-like contraction, the veins on the contrary, receive it from them
at the extremities, in an easy and imperceptible gradation; for whereas
the fibres in the arteries are in a spiral direction, elastic and
strong, the coat of the veins are tender, and the fibres more
longitudinal.
Hence, the wounds in the arteries and veins differ greatly; for whilst
the arteries are for ever in a strong agitation, the veins perform their
functions with less exercise; and therefore the wounds in arteries are
always attended with danger, whilst the veins heal easy. From the same
reason also, the arterial blood differs from the venal; for as the
arterial blood is sent to all the Parts, as the grand nutriment, from
whence the glands secrete their peculiar humours; the veins only
transport it back again, to prepare it for a new rotation. Whence the
blood in the arteries is of a high colour, and of a more alkalascent
nature, than that in the veins.
Collateral with the minute ramifications of the veins and arteries, over
the whole system, are the _lymphatic vessels_, which separate from the
blood as it passes along, a transparent lymphatic humour, adapted for
the different parts, which they are connected with.[6] These vessels are
materially different, from either arteries and veins in their structure,
and justly to be compared to a continuation of little cells; so
constructed, as to transport the humours, without admitting a return.
Footnote 6:
An opinion has lately been broached, that the lymphatics are all
absorbing vessels, of the same nature with the lacteals, and that they
all join to empty themselves in the subclavian vein to the common mass
of the blood; but this to me, seems too general, and contradicts
experience.
From hence we see plainly how these three organs, the _brains_, the
_lungs_, and the _heart_, are concerned, and mutually assist each other
in the actions, and circulation of the fluid of life; nothing could move
without the sensation of the nerves; these could not exist without the
blood, from which they receive their support; the blood could not
circulate without the heart, arteries, and veins; and in them it could
not move with a necessary freedom, except its being supplied with a
sufficiency of air from the lungs: So that evidently all three are so
interested in this grand movement, that the stopping of the one, must
needs be the stopping of the whole.
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