by experiment that they contain blood. “Ourselves, having
tied the exposed arteries above and below, opened them, and
showed that they were indeed full of blood.” Realdus Columbus
(1559), though he did not discover the general or “systematic”
circulation of the blood, yet seems to have discovered, by experiment,
the pulmonary circulation. “The blood is carried through
the pulmonary artery to the lung, and there is attenuated; thence,
mixed with air, it is carried through the pulmonary vein to the
left side of the heart. Which thing no man hitherto has noted or
left on record, though it is most worthy of the observance of all
men. . . . And this is as true as truth itself; for if you will look
not only in the dead body but also in the living animal, you will
always find this pulmonary vein full of blood, which assuredly it
would not be if it were designed only for air and vapours. . . .
Verily I pray you, O candid reader, studious of authority, but more
studious of truth, to make experiment on animals. You will find
the pulmonary vein full of blood, not air or fuligo, as these men call
it, God help them.” Harvey’s treatise De Motu Cordis et Sanguinis
in Animalibus was published at Frankfort in 1621. It begins thus:
“When by many dissections of living animals, as they came to
hand,—Cum multis vivorum dissectionibus, uti ad manum dabantur,—I
first gave myself to observing how I might discover, with my
own eyes, and not from books and the writings of other men, the
use and purpose of the movement of the heart in animals, forthwith
I found the matter hard indeed and full of difficulty; so that I
began to think, with Frascatorius, that the movement of the heart
was known to God alone. . . . At last, having daily used greater
disquisition and diligence, by frequent examination of many and
various living animals—multa frequenter et varia animalia viva
introspiciendo—I came to believe that I had succeeded, and had
escaped and got out of this labyrinth, and therewith had
discovered what I desired, the movement and use of the heart and the
arteries. And from that time, not only to my friends but also in
public in my anatomical lectures, after the manner of the Academy,
I did not fear to set forth my opinion in this matter.” Here, and
again at the end of the Preface, and again in the eighth chapter
of the De Motu, he puts his experiments in the very foreground
of the argument. Take the headings of his first four chapters:
1. Causae, quibus ad scribendum auctor permotus fuerit. 2. Ex
vivorum dissectione, qualis fit cordis motus. 3. Arteriarum motus
qualis, ex vivorum dissectione. 4. Motus cordis et auricularum
qualis, ex vivorum dissectione. He had, of course, help from other
sources—from anatomy and from physics; but it is certain, from
his own words, that he attributed his discovery, in a very great
measure, to experiments on animals. Malpighi (1661), professor of
medicine at Bologna, by examining with a microscope the lung and
the mesentery of the live frog, made out the capillary vessels.
He
writes to Borelli, professor of mathematics at Pisa, that he has
failed in every attempt to discover them by injecting fluids into
the larger vessels, but has succeeded by examining the tissues with
the microscope: “Such is the divarication of these little vessels
coming off from the vein and the artery, that the order in which a
vessel ramifies is no longer presented, but it looks like a network
woven from the offshoots of both vessels” (De Pulmonibus, 1661).
Stephen Hales (1733), rector of Farringdon and minister of Teddington,
and a Fellow of the Royal Society, made the first exact estimates
of the blood pressure, the real force of the blood, by inserting
one end of a vertical glass tube into the crural artery of a mare,
and noting the rise of the blood in the tube (Statical Essays,
containing Haemostaticks, &c., 1733). John Hunter, born 1738, made
many observations on the nature and processes of the blood; and,
above all, he discovered the facts of collateral circulation.
These
facts were fresh in his mind when he first ventured, in December
1785, to tie the femoral artery in “Hunter’s canal” for the cure
of aneurism in the popliteal space. The experiment that gave
him his knowledge of the collateral circulation was made on one of
the deer in Richmond Park: he tied its external carotid artery,
to see what effect would be produced on the shedding of the antler.
Some days later he found that the circulation had returned in
the antler. He had the buck killed, and found that the artery
had been completely closed by the ligature, but the small branches
coming from it, between the heart and the ligature, were enlarged
and were in communication with others of its branches beyond
the ligature; and by this collateral circulation the flow of blood
to the antler had been restored. Among later observations on
the circulation must be mentioned the use of the mercurial manometer
by Poiseuille (1828) and Ludwig (1849), the study of the
blood pressure within the heart by Hering (1849) and the
permanent tracing of the pressure curves by Chauveau and Marey
(1863). Finally came the study of those more abstruse problems
of the circulation that the older physiologists had left alone—the
influences of the central nervous system, the relations between
blood pressure and secretion, the automatism of the heart-beat, and
the influence of gravitation. Professor Starling, in 1906, writes
as follows of this part of physiology: “Among the researches of
the last thirty years, those bearing on the circulation of the blood
must take an important place, both for their physiological interest
and for the weighty influence they have exerted on our knowledge
and treatment of disorders of the vascular system, such as heart
disease. We have learned to measure accurately the work done
by the great heart-pump; and by studying the manner in which
this work is affected by different conditions, we are enabled to
increase or diminish it, according to the needs of the organ.
Experiments in what is often regarded as the most transcendental
department of physiology—i.e. that which treats of muscle and
nerve—have thrown light on the wonderful process of
‘compensation’ by which a diseased heart is able to keep up a normal
circulation.” And Dr James Mackenzie, writing in 1910 of certain
irregularities of the circulation during pregnancy (venous pulse in
the neck and irregular beat of the heart), says, very emphatically,
that these conditions in patients have been interpreted by experiments
on animals. “The outcome of these researches [Wenckebach’s
clinical studies], as well as those of a great number of other
observers, has been to elucidate the nature and meaning of a great
number of abnormal conditions of the heart. It might be said
with truth that, whereas a few years ago irregular action of the
heart was one of the most obscure symptoms in clinical medicine,
it is now one of the best understood. It is needless to repeat that
this advance would have been absolutely impossible without the
knowledge gained by experiment” (Research Defence Society, May
1910).
2. The Lacteals.—Asellius (1622) by a single experiment demonstrated the flow of chyle along the lacteals. The existence of these minute vessels had been known even to Galen and Erastistratus, but they had made nothing of their knowledge. Asellius says: “I observed that the nerves of the intestines were quite distinct from these white threads, and ran a different course. Struck with this new fact, I was silent for a time, thinking of the bitter warfare of words among anatomists as to the mesenteric veins and their purposes. When I came to myself, to satisfy myself by an experiment, I pierced one of the largest cords with a scalpel. I hit the right point, and at once observed a white liquid like milk flowing from the divided vessel.” Jehan Pecquet (1647), in the course of an experiment on the heart, observed the flow of chyle into the subclavian vein, and its identity with the chyle in the lacteals; and by further experiment found the thoracic duct, and the chyle flowing up it: “I perceived a white substance, like milk, flowing from the vena cava ascendens into the pericardium, at the place where the right auricle had been. . . . I found these vessels (the thoracic duct) all along the dorsal vertebrae, lying on the spine, beneath the aorta. They swelled below a ligature; and when I relaxed it, I saw the milk carried to the orifices that I had observed in the subclavian vein.” The existence of this duct, which is empty and collapsed after death, had been overlooked by Vesalius and all the great anatomists of his time.
3. The Gastric Juice.—Our knowledge about digestion dates back to the end of the 17th century, when Valisnieri first observed that the stomach of a dead animal contained a fluid which acted on certain bodies immersed in it—“a kind of aqua fortis.” In 1752 Réaumur began his observations on this fluid, making birds swallow fine fenestrated tubes containing grain or meat, or sponges with threads attached; and observed that digestion consists in the dissolution of food, not in any sort of mechanical action or trituration. His observations were extended and perfected by Spallanzani (1777). Then came a period of uncertainty, without further advance; until in 1823 the French Academy offered a prize for the best work on the subject, and Tiedemann and Gmelin submitted their observations to them: “The work of Tiedemann and Gmelin is of especial interest to us on account of the great number of their experiments, from which came not only the absolute proof of the existence of the gastric juice, but also the study of the transformation of starch into glucose. Thus the theory of digestion entered a new phase: it was finally recognized, at least for certain substances, that digestion is not simply dissolution, but a true chemical transformation” (Claude Bernard, Physiologie opératoire, 1879). Beaumont’s experiments on Alexis St Martin (vide supra) were published in 1838. They were, of course, based on the work of the physiologists: “I make no claim to originality in my opinions as respects the existence and operation of the gastric juice. My experiments confirm the doctrines (with some modifications) taught by Spallanzani and many of the most enlightened physiological writers” (Beaumont’s preface to his book). Eberlé, in 1834, showed how this knowledge of the gastric juice might be turned to a practical use, by extracting it from the mucous membrane of the stomachs of animals after death: hence came the invention of the various preparations of pepsin. Later, Blondlot of Nancy, in 1842, studied the gastric juice by the method of a fistula, like that of St Martin. More recent observations have been made on the movements of the stomach during digestion, and on the influences of the nervous system on the process.
The stomach is, of course not the only organ of digestion: the liver, the pancreas and the intestinal glands, all are concerned. The recent work of Pawlow and of Starling has greatly advanced our knowledge of the actions of the secretions from these organs. The whole chain of processes, nervous and chemical, psychical and physical, from the taking of food into the mouth to the expulsion of the waste residue, is now viewed in its entirety; and especial study has been given to the influences, nervous or chemical, which
