THE CIRCULATION AND THE FEEDING OF THE TISSUES
That Harvey frequently took refuge in speculation need excite no
surprise. In the seventeenth century, even with his extraordinary
contributions of observed fact to the knowledge of the circulation
of the blood, the paucity of physiological knowledge in general and
of experimental methods was so great that at every turn a thinking
man was tempted to fill in the gaps with that which was beyond his
powers of ocular demonstration. Contemplation of the circulation,
indeed, led Harvey into contemplation of widely diverse problems of
the life process. The feeding of the tissues, the significance of
respiration, the cause of the heart-beat, the relative importance of
the heart and the blood in the bodily hierarchy, the bodily heat and
its source, and the seat of the soul--to these and other topics he
gave much attention, and these we must consider. Let us begin with
the circulation and its relation to the feeding of the tissues.
In the chapter of Harvey's book which follows at once upon the brief
qualitative statement quoted at the end of our last chapter, Harvey
himself brings us face to face with the difficult quantitative
question raised by his triumphant proof of the circulation. He
says:--
"The blood under the influence of the arterial pulse enters and
is impelled in a continuous, equable, and incessant stream into
every part and member of the body, in much larger quantity than
were sufficient for nutrition or than the whole mass of fluids
could supply."[10]
Here we see that the rapid renewal of the blood in "every part and
member of the body" presented itself to Harvey's own mind as calling
for some other explanation than the simple feeding of the tissues.
The question of "_cui bono_" which his discovery raised is still but
incompletely answered; in Harvey's day it was almost unanswerable.
In dealing from time to time with its main features he himself, as
we shall see, could only bring forward inadequate observations and
shift his ground from one erroneous doctrine to another. In justice
to his opponents, who seem to us so unreasonable, let us remember
how prodigious this new question of "_cui bono_" must have seemed
when the circulation itself was a novelty. Let us remember also
that for nearly two thousand years the tissues had been held to
feed themselves tranquilly out of the contents of the vessels in a
way fitly expressed by the old simile of irrigation ditches in a
garden--a simile which Aristotle and Galen had borrowed in turn from
Plato.[11]
But if Harvey saw only too well that the feeding of the tissues
could not explain the circulation, he had at least seen plainly how
the doctrine of the circulation clarified the ancient but current
doctrine as to the absorption of the digested food. The portal
vein had been accepted as the route of this absorption. No doubt
both Aristotle and Galen had seen its ruddy contents; at any rate
both had concluded that the chyle was changed within the portal
vein into a crude approximation to blood.[12] That the same vessel
should carry to the liver altered chyle, and from the liver blood
to nourish the stomach and intestines, had involved a difficulty
which Galen had met with characteristic cleverness. He had cited
in support of such a reversal of flow the flow of the bile into
the gall-bladder and out by the same duct, the movement of food
and vomit into and out of the stomach by the œsophagus, and the
relation of the _os uteri_ to impregnation and parturition.[13]
Harvey says:--
"For the blood entering the mesentery by the cœliac
artery and the superior and inferior mesenteries proceeds
to the intestines, from which along with the chyle that has
been attracted into the veins it returns by their numerous
ramifications into the vena portæ of the liver, and from
this into the vena cava, and this in such wise that the
blood in these veins has the same colour and consistency as
in other veins, in opposition to what many believe to be
the fact. Nor need we hold the improbable belief that two
inconveniently opposed movements take place in the whole
capillary ramification, namely, movement of the chyle upward,
of the blood downward. Is not the thing rather arranged as it
is by the consummate providence of nature? For were the chyle
mingled with the blood, the crude with the concocted in equal
proportions, the result would not be concoction, transmutation,
and sanguification, but rather, because they are reciprocally
active and passive, a mixture, their union with one another
producing something intermediate, precisely as when wine is
mixed with water and [in] vinegar and water [oxicratum]. But
when a minute quantity of chyle is mingled with a large quantity
of blood flowing by, a quantity of chyle that bears no notable
proportion to the blood, the effect is the same, as Aristotle
says, as when a drop of water is added to a cask of wine, or the
contrary; the resulting total is not a mixture, but is either
wine or water. So in the dissected mesenteric veins we do not
find chyme or chyle and blood, separate or mingled, but only
blood, sensibly the same in color and consistency as in the rest
of the veins."[14]
In a second passage of the same chapter,[15] Harvey returns to this
subject; and again, twenty-one years later, in his first exercise to
Riolanus, as follows:--
"Our learned author mentions a certain tract of his on the
Circulation of the Blood: I wish I could obtain a sight of it;
perhaps I might retract. But had the learned writer been so
disposed, I do not see but that, having admitted the circular
motion of the blood (and in the veins, as he says in the
eighth chapter of the third book,[16] the blood incessantly
and naturally ascends, or flows back, to the heart, as in all
the arteries it descends or departs from the heart), all the
difficulties which were formerly felt in connection with the
distribution of chyle and the blood by the same channels are
brought to an equally satisfactory solution; for all the mooted
difficulties vanish when we cease to suppose two contrary
motions at once in the same vessels, and admit but one and
the same continuous motion in the mesenteric vessels from the
intestines to the liver."[17]
From this passage we see, in passing, that Harvey at the age of
seventy made little account of Caspar Aselli's discovery of the
lacteals, published twenty-two years before in 1627,[18] the year
before the announcement of the discovery of the circulation.
Harvey's mind was focused on the blood, its motion and its meaning;
this was to him the subject of prime importance. The ancient
doctrine of the feeding of the tissues provided an insufficient
reason for the existence of what his observations and his
experiments revealed to him.