ANATOMY AND PHYSIOLOGY IN THE EIGHTEENTH CENTURY
ALBRECHT VON HALLER
An epoch in physiology was made in the eighteenth century by the
genius and efforts of Albrecht von Haller (1708-1777), of Berne,
who is perhaps as worthy of the title "The Great" as any
philosopher who has been so christened by his contemporaries
since the time of Hippocrates. Celebrated as a physician, he was
proficient in various fields, being equally famed in his own time
as poet, botanist, and statesman, and dividing his attention
between art and science.
As a child Haller was so sickly that he was unable to amuse
himself with the sports and games common to boys of his age, and
so passed most of his time poring over books. When ten years of
age he began writing poems in Latin and German, and at fifteen
entered the University of Tubingen. At seventeen he wrote
learned articles in opposition to certain accepted doctrines, and
at nineteen he received his degree of doctor. Soon after this he
visited England, where his zeal in dissecting brought him under
suspicion of grave-robbery, which suspicion made it expedient for
him to return to the Continent. After studying botany in Basel
for some time he made an extended botanical journey through
Switzerland, finally settling in his native city, Berne, as a
practising physician. During this time he did not neglect either
poetry or botany, publishing anonymously a collection of poems.
In 1736 he was called to Gottingen as professor of anatomy,
surgery, chemistry, and botany. During his labors in the
university he never neglected his literary work, sometimes living
and sleeping for days and nights together in his library, eating
his meals while delving in his books, and sleeping only when
actually compelled to do so by fatigue. During all this time he
was in correspondence with savants from all over the world, and
it is said of him that he never left a letter of any kind
unanswered.
Haller's greatest contribution to medical science was his famous
doctrine of irritability, which has given him the name of "father
of modern nervous physiology," just as Harvey is called "the
father of the modern physiology of the blood." It has been said
of this famous doctrine of irritability that "it moved all the
minds of the century--and not in the departments of medicine
alone--in a way of which we of the present day have no
satisfactory conception, unless we compare it with our modern
Darwinism."[1]
The principle of general irritability had been laid down by
Francis Glisson (1597-1677) from deductive studies, but Haller
proved by experiments along the line of inductive methods that
this irritability was not common to all "fibre as well as to the
fluids of the body," but something entirely special, and peculiar
only to muscular substance. He distinguished between irritability
of muscles and sensibility of nerves. In 1747 he gave as the
three forces that produce muscular movements: elasticity, or
"dead nervous force"; irritability, or "innate nervous force";
and nervous force in itself. And in 1752 he described one
hundred and ninety experiments for determining what parts of the
body possess "irritability"--that is, the property of contracting
when stimulated. His conclusion that this irritability exists in
muscular substance alone and is quite independent of the nerves
proceeding to it aroused a controversy that was never definitely
settled until late in the nineteenth century, when Haller's
theory was found to be entirely correct.
It was in pursuit of experiments to establish his theory of
irritability that Haller made his chief discoveries in embryology
and development. He proved that in the process of incubation of
the egg the first trace of the heart of the chick shows itself in
the thirty-eighth hour, and that the first trace of red blood
showed in the forty-first hour. By his investigations upon the
lower animals he attempted to confirm the theory that since the
creation of genus every individual is derived from a preceding
individual--the existing theory of preformation, in which he
believed, and which taught that "every individual is fully and
completely preformed in the germ, simply growing from microscopic
to visible proportions, without developing any new parts."
In physiology, besides his studies of the nervous system, Haller
studied the mechanism of respiration, refuting the teachings of
Hamberger (1697-1755), who maintained that the lungs contract
independently. Haller, however, in common with his
contemporaries, failed utterly to understand the true function of
the lungs. The great physiologist's influence upon practical
medicine, while most profound, was largely indirect. He was a
theoretical rather than a practical physician, yet he is credited
with being the first physician to use the watch in counting the
pulse.
BATTISTA MORGAGNI AND MORBID ANATOMY
A great contemporary of Haller was Giovanni Battista Morgagni
(1682-1771), who pursued what Sydenham had neglected, the
investigation in anatomy, thus supplying a necessary counterpart
to the great Englishman's work. Morgagni's investigations were
directed chiefly to the study of morbid anatomy--the study of the
structure of diseased tissue, both during life and post mortem,
in contrast to the normal anatomical structures. This work cannot
be said to have originated with him; for as early as 1679 Bonnet
had made similar, although less extensive, studies; and later
many investigators, such as Lancisi and Haller, had made
post-mortem studies. But Morgagni's De sedibus et causis
morborum per anatomen indagatis was the largest, most accurate,
and best-illustrated collection of cases that had ever been
brought together, and marks an epoch in medical science. From the
time of the publication of Morgagni's researches, morbid anatomy
became a recognized branch of the medical science, and the effect
of the impetus thus given it has been steadily increasing since
that time.
WILLIAM HUNTER
William Hunter (1718-1783) must always be remembered as one of
the greatest physicians and anatomists of the eighteenth century,
and particularly as the first great teacher of anatomy in
England; but his fame has been somewhat overshadowed by that of
his younger brother John.
Hunter had been intended and educated for the Church, but on the
advice of the surgeon William Cullen he turned his attention to
the study of medicine. His first attempt at teaching was in 1746,
when he delivered a series of lectures on surgery for the Society
of Naval Practitioners. These lectures proved so interesting and
instructive that he was at once invited to give others, and his
reputation as a lecturer was soon established. He was a natural
orator and story-teller, and he combined with these attractive
qualities that of thoroughness and clearness in demonstrations,
and although his lectures were two hours long he made them so
full of interest that his pupils seldom tired of listening. He
believed that he could do greater good to the world by "publicly
teaching his art than by practising it," and even during the last
few days of his life, when he was so weak that his friends
remonstrated against it, he continued his teaching, fainting from
exhaustion at the end of his last lecture, which preceded his
death by only a few days.
For many years it was Hunter's ambition to establish a museum
where the study of anatomy, surgery, and medicine might be
advanced, and in 1765 he asked for a grant of a plot of ground
for this purpose, offering to spend seven thousand pounds on its,
erection besides endowing it with a professorship of anatomy. Not
being able to obtain this grant, however, he built a house, in
which were lecture and dissecting rooms, and his museum. In this
museum were anatomical preparations, coins, minerals, and
natural-history specimens.
Hunter's weakness was his love of controversy and his resentment
of contradiction. This brought him into strained relations with
many of the leading physicians of his time, notably his own
brother John, who himself was probably not entirely free from
blame in the matter. Hunter is said to have excused his own
irritability on the grounds that being an anatomist, and
accustomed to "the passive submission of dead bodies,"
contradictions became the more unbearable. Many of the
physiological researches begun by him were carried on and
perfected by his more famous brother, particularly his
investigations of the capillaries, but he added much to the
anatomical knowledge of several structures of the body, notably
as to the structure of cartilages and joints.
JOHN HUNTER
In Abbot Islip's chapel in Westminster Abbey, close to the
resting-place of Ben Jonson, rest the remains of John Hunter
(1728-1793), famous in the annals of medicine as among the
greatest physiologists and surgeons that the world has ever
produced: a man whose discoveries and inventions are counted by
scores, and whose field of research was only limited by the
outermost boundaries of eighteenth-century science, although his
efforts were directed chiefly along the lines of his profession.
Until about twenty years of age young Hunter had shown little
aptitude for study, being unusually fond of out-door sports and
amusements; but about that time, realizing that some occupation
must be selected, he asked permission of his brother William to
attempt some dissections in his anatomical school in London. To
the surprise of his brother he made this dissection unusually
well; and being given a second, he acquitted himself with such
skill that his brother at once predicted that he would become a
great anatomist. Up to this time he had had no training of any
kind to prepare him for his professional career, and knew little
of Greek or Latin--languages entirely unnecessary for him, as he
proved in all of his life work. Ottley tells the story that,
when twitted with this lack of knowledge of the "dead languages"
in after life, he said of his opponent, "I could teach him that
on the dead body which he never knew in any language, dead or
living."
By his second year in dissection he had become so skilful that he
was given charge of some of the classes in his brother's school;
in 1754 he became a surgeon's pupil in St. George's Hospital, and
two years later house-surgeon. Having by overwork brought on
symptoms that seemed to threaten consumption, he accepted the
position of staff-surgeon to an expedition to Belleisle in 1760,
and two years later was serving with the English army at
Portugal. During all this time he was constantly engaged in
scientific researches, many of which, such as his observations of
gun-shot wounds, he put to excellent use in later life. On
returning to England much improved in health in 1763, he entered
at once upon his career as a London surgeon, and from that time
forward his progress was a practically uninterrupted series of
successes in his profession.
Hunter's work on the study of the lymphatics was of great service
to the medical profession. This important net-work of minute
vessels distributed throughout the body had recently been made
the object of much study, and various students, including Haller,
had made extensive investigations since their discovery by
Asellius. But Hunter, in 1758, was the first to discover the
lymphatics in the neck of birds, although it was his brother
William who advanced the theory that the function of these
vessels was that of absorbents. One of John Hunter's pupils,
William Hewson (1739-1774), first gave an account, in 1768, of
the lymphatics in reptiles and fishes, and added to his teacher's
investigations of the lymphatics in birds. These studies of the
lymphatics have been regarded, perhaps with justice, as Hunter's
most valuable contributions to practical medicine.
In 1767 he met with an accident by which he suffered a rupture of
the tendo Achillis--the large tendon that forms the attachment of
the muscles of the calf to the heel. From observations of this
accident, and subsequent experiments upon dogs, he laid the
foundation for the now simple and effective operation for the
cure of club feet and other deformities involving the tendons.
In 1772 he moved into his residence at Earlscourt, Brompton,
where he gathered about him a great menagerie of animals, birds,
reptiles, insects, and fishes, which he used in his physiological
and surgical experiments. Here he performed a countless number of
experiments--more, probably, than "any man engaged in
professional practice has ever conducted." These experiments
varied in nature from observations of the habits of bees and
wasps to major surgical operations performed upon hedgehogs,
dogs, leopards, etc. It is said that for fifteen years he kept a
flock of geese for the sole purpose of studying the process of
development in eggs.
Hunter began his first course of lectures in 1772, being forced
to do this because he had been so repeatedly misquoted, and
because he felt that he could better gauge his own knowledge in
this way. Lecturing was a sore trial to him, as he was extremely
diffident, and without writing out his lectures in advance he was
scarcely able to speak at all. In this he presented a marked
contrast to his brother William, who was a fluent and brilliant
speaker. Hunter's lectures were at best simple readings of the
facts as he had written them, the diffident teacher seldom
raising his eyes from his manuscript and rarely stopping until
his complete lecture had been read through. His lectures were,
therefore, instructive rather than interesting, as he used
infinite care in preparing them; but appearing before his classes
was so dreaded by him that he is said to have been in the habit
of taking a half-drachm of laudanum before each lecture to nerve
him for the ordeal. One is led to wonder by what name he shall
designate that quality of mind that renders a bold and fearless
surgeon like Hunter, who is undaunted in the face of hazardous
and dangerous operations, a stumbling, halting, and "frightened"
speaker before a little band of, at most, thirty young medical
students. And yet this same thing is not unfrequently seen among
the boldest surgeons.
Hunter's Operation for the Cure of Aneurisms
It should be an object-lesson to those who, ignorantly or
otherwise, preach against the painless vivisection as practised
to-day, that by the sacrifice of a single deer in the cause of
science Hunter discovered a fact in physiology that has been the
means of saving thousands of human lives and thousands of human
bodies from needless mutilation. We refer to the discovery of the
"collateral circulation" of the blood, which led, among other
things, to Hunter's successful operation upon aneurisms.
Simply stated, every organ or muscle of the body is supplied by
one large artery, whose main trunk distributes the blood into its
lesser branches, and thence through the capillaries. Cutting off
this main artery, it would seem, should cut off entirely the
blood-supply to the particular organ which is supplied by this
vessel; and until the time of Hunter's demonstration this belief
was held by most physiologists. But nature has made a provision
for this possible stoppage of blood-supply from a single source,
and has so arranged that some of the small arterial branches
coming from the main supply-trunk are connected with other
arterial branches coming from some other supply-trunk. Under
normal conditions the main arterial trunks supply their
respective organs, the little connecting arterioles playing an
insignificant part. But let the main supply-trunk be cut off or
stopped for whatever reason, and a remarkable thing takes place.
The little connecting branches begin at once to enlarge and draw
blood from the neighboring uninjured supply-trunk, This
enlargement continues until at last a new route for the
circulation has been established, the organ no longer depending
on the now defunct original arterial trunk, but getting on as
well as before by this "collateral" circulation that has been
established.
The thorough understanding of this collateral circulation is one
of the most important steps in surgery, for until it was
discovered amputations were thought necessary in such cases as
those involving the artery supplying a leg or arm, since it was
supposed that, the artery being stopped, death of the limb and
the subsequent necessity for amputation were sure to follow.
Hunter solved this problem by a single operation upon a deer, and
his practicality as a surgeon led him soon after to apply this
knowledge to a certain class of surgical cases in a most
revolutionary and satisfactory manner.
What led to Hunter's far-reaching discovery was his investigation
as to the cause of the growth of the antlers of the deer. Wishing
to ascertain just what part the blood-supply on the opposite
sides of the neck played in the process of development, or,
perhaps more correctly, to see what effect cutting off the main
blood-supply would have, Hunter had one of the deer of Richmond
Park caught and tied, while he placed a ligature around one of
the carotid arteries--one of the two principal arteries that
supply the head with blood. He observed that shortly after this
the antler (which was only half grown and consequently very
vascular) on the side of the obliterated artery became cold to
the touch--from the lack of warmth-giving blood. There was
nothing unexpected in this, and Hunter thought nothing of it
until a few days later, when he found, to his surprise, that the
antler had become as warm as its fellow, and was apparently
increasing in size. Puzzled as to how this could be, and
suspecting that in some way his ligature around the artery had
not been effective, he ordered the deer killed, and on
examination was astonished to find that while his ligature had
completely shut off the blood-supply from the source of that
carotid artery, the smaller arteries had become enlarged so as to
supply the antler with blood as well as ever, only by a different
route.
Hunter soon had a chance to make a practical application of the
knowledge thus acquired. This was a case of popliteal aneurism,
operations for which had heretofore proved pretty uniformly
fatal. An aneurism, as is generally understood, is an enlargement
of a certain part of an artery, this enlargement sometimes
becoming of enormous size, full of palpitating blood, and likely
to rupture with fatal results at any time. If by any means the
blood can be allowed to remain quiet for even a few hours in this
aneurism it will form a clot, contract, and finally be absorbed
and disappear without any evil results. The problem of keeping
the blood quiet, with the heart continually driving it through
the vessel, is not a simple one, and in Hunter's time was
considered so insurmountable that some surgeons advocated
amputation of any member having an aneurism, while others cut
down upon the tumor itself and attempted to tie off the artery
above and below. The first of these operations maimed the patient
for life, while the second was likely to prove fatal.
In pondering over what he had learned about collateral
circulation and the time required for it to become fully
established, Hunter conceived the idea that if the blood-supply
was cut off from above the aneurism, thus temporarily preventing
the ceaseless pulsations from the heart, this blood would
coagulate and form a clot before the collateral circulation could
become established or could affect it. The patient upon whom he
performed his now celebrated operation was afflicted with a
popliteal aneurism--that is, the aneurism was located on the
large popliteal artery just behind the knee-joint. Hunter,
therefore, tied off the femoral, or main supplying artery in the
thigh, a little distance above the aneurism. The operation was
entirely successful, and in six weeks' time the patient was able
to leave the hospital, and with two sound limbs. Naturally the
simplicity and success of this operation aroused the attention of
Europe, and, alone, would have made the name of Hunter immortal
in the annals of surgery. The operation has ever since been
called the "Hunterian" operation for aneurism, but there is
reason to believe that Dominique Anel (born about 1679) performed
a somewhat similar operation several years earlier. It is
probable, however, that Hunter had never heard of this work of
Anel, and that his operation was the outcome of his own
independent reasoning from the facts he had learned about
collateral circulation. Furthermore, Hunter's mode of operation
was a much better one than Anel's, and, while Anel's must claim
priority, the credit of making it widely known will always be
Hunter's.
The great services of Hunter were recognized both at home and
abroad, and honors and positions of honor and responsibility were
given him. In 1776 he was appointed surgeon-extraordinary to the
king; in 1783 he was elected a member of the Royal Society of
Medicine and of the Royal Academy of Surgery at Paris; in 1786 he
became deputy surgeon-general of the army; and in 1790 he was
appointed surgeon-general and inspector-general of hospitals. All
these positions he filled with credit, and he was actively
engaged in his tireless pursuit of knowledge and in discharging
his many duties when in October, 1793, he was stricken while
addressing some colleagues, and fell dead in the arms of a
fellow-physician.
LAZZARO SPALLANZANI
Hunter's great rival among contemporary physiologists was the
Italian Lazzaro Spallanzani (1729-1799), one of the most
picturesque figures in the history of science. He was not
educated either as a scientist or physician, devoting, himself at
first to philosophy and the languages, afterwards studying law,
and later taking orders. But he was a keen observer of nature and
of a questioning and investigating mind, so that he is remembered
now chiefly for his discoveries and investigations in the
biological sciences. One important demonstration was his
controversion of the theory of abiogenesis, or "spontaneous
generation," as propounded by Needham and Buffon. At the time of
Needham's experiments it had long been observed that when animal
or vegetable matter had lain in water for a little time--long
enough for it to begin to undergo decomposition--the water became
filled with microscopic creatures, the "infusoria animalculis."
This would tend to show, either that the water or the animal or
vegetable substance contained the "germs" of these minute
organisms, or else that they were generated spontaneously. It was
known that boiling killed these animalcules, and Needham agreed,
therefore, that if he first heated the meat or vegetables, and
also the water containing them, and then placed them in
hermetically scaled jars--if he did this, and still the
animalcules made their appearance, it would be proof-positive
that they had been generated spontaneously. Accordingly be made
numerous experiments, always with the same results--that after a
few days the water was found to swarm with the microscopic
creatures. The thing seemed proven beyond question--providing, of
course, that there had been no slips in the experiments.
But Abbe Spallanzani thought that he detected such slips in
Needham's experiment. The possibility of such slips might come
in several ways: the contents of the jar might not have been
boiled for a sufficient length of time to kill all the germs, or
the air might not have been excluded completely by the sealing
process. To cover both these contingencies, Spallanzani first
hermetically sealed the glass vessels and then boiled them for
three-quarters of an hour. Under these circumstances no
animalcules ever made their appearance--a conclusive
demonstration that rendered Needham's grounds for his theory at
once untenable.[2]
Allied to these studies of spontaneous generation were
Spallanzani's experiments and observations on the physiological
processes of generation among higher animals. He experimented
with frogs, tortoises, and dogs; and settled beyond question the
function of the ovum and spermatozoon. Unfortunately he
misinterpreted the part played by the spermatozoa in believing
that their surrounding fluid was equally active in the
fertilizing process, and it was not until some forty years later
(1824) that Dumas corrected this error.
THE CHEMICAL THEORY OF DIGESTION
Among the most interesting researches of Spallanzani were his
experiments to prove that digestion, as carried on in the
stomach, is a chemical process. In this he demonstrated, as Rene
Reaumur had attempted to demonstrate, that digestion could be
carried on outside the walls of the stomach as an ordinary
chemical reaction, using the gastric juice as the reagent for
performing the experiment. The question as to whether the stomach
acted as a grinding or triturating organ, rather than as a
receptacle for chemical action, had been settled by Reaumur and
was no longer a question of general dispute. Reaumur had
demonstrated conclusively that digestion would take place in the
stomach in the same manner and the same time if the substance to
be digested was protected from the peristalic movements of the
stomach and subjected to the action of the gastric juice only. He
did this by introducing the substances to be digested into the
stomach in tubes, and thus protected so that while the juices of
the stomach could act upon them freely they would not be affected
by any movements of the organ.
Following up these experiments, he attempted to show that
digestion could take place outside the body as well as in it, as
it certainly should if it were a purely chemical process. He
collected quantities of gastric juice, and placing it in suitable
vessels containing crushed grain or flesh, kept the mixture at
about the temperature of the body for several hours. After
repeated experiments of this kind, apparently conducted with
great care, Reaumur reached the conclusion that "the gastric
juice has no more effect out of the living body in dissolving or
digesting the food than water, mucilage, milk, or any other bland
fluid."[3] Just why all of these experiments failed to
demonstrate a fact so simple does not appear; but to Spallanzani,
at least, they were by no means conclusive, and he proceeded to
elaborate upon the experiments of Reaumur. He made his
experiments in scaled tubes exposed to a certain degree of heat,
and showed conclusively that the chemical process does go on,
even when the food and gastric juice are removed from their
natural environment in the stomach. In this he was opposed by
many physiologists, among them John Hunter, but the truth of his
demonstrations could not be shaken, and in later years we find
Hunter himself completing Spallanzani's experiments by his
studies of the post-mortem action of the gastric juice upon the
stomach walls.
That Spallanzani's and Hunter's theories of the action of the
gastric juice were not at once universally accepted is shown by
an essay written by a learned physician in 1834. In speaking of
some of Spallanzani's demonstrations, he writes: "In some of the
experiments, in order to give the flesh or grains steeped in the
gastric juice the same temperature with the body, the phials were
introduced under the armpits. But this is not a fair mode of
ascertaining the effects of the gastric juice out of the body;
for the influence which life may be supposed to have on the
solution of the food would be secured in this case. The
affinities connected with life would extend to substances in
contact with any part of the system: substances placed under the
armpits are not placed at least in the same circumstances with
those unconnected with a living animal." But just how this writer
reaches the conclusion that "the experiments of Reaumur and
Spallanzani give no evidence that the gastric juice has any
peculiar influence more than water or any other bland fluid in
digesting the food"[4] is difficult to understand.
The concluding touches were given to the new theory of digestion
by John Hunter, who, as we have seen, at first opposed
Spallanzani, but who finally became an ardent champion of the
chemical theory. Hunter now carried Spallanzani's experiments
further and proved the action of the digestive fluids after
death. For many years anatomists had been puzzled by pathological
lesion of the stomach, found post mortem, when no symptoms of any
disorder of the stomach had been evinced during life. Hunter
rightly conceived that these lesions were caused by the action of
the gastric juice, which, while unable to act upon the living
tissue, continued its action chemically after death, thus
digesting the walls of the stomach in which it had been formed.
And, as usual with his observations, be turned this discovery to
practical use in accounting for certain phenomena of digestion.
The following account of the stomach being digested after death
was written by Hunter at the desire of Sir John Pringle, when he
was president of the Royal Society, and the circumstance which
led to this is as follows: "I was opening, in his presence, the
body of a patient of his own, where the stomach was in part
dissolved, which appeared to him very unaccountable, as there had
been no previous symptom that could have led him to suspect any
disease in the stomach. I took that opportunity of giving him my
ideas respecting it, and told him that I had long been making
experiments on digestion, and considered this as one of the facts
which proved a converting power in the gastric juice. . . . There
are a great many powers in nature which the living principle does
not enable the animal matter, with which it is combined, to
resist--viz., the mechanical and most of the strongest chemical
solvents. It renders it, however, capable of resisting the powers
of fermentation, digestion, and perhaps several others, which are
well known to act on the same matter when deprived of the living
principle and entirely to decompose it. "
Hunter concludes his paper with the following paragraph: "These
appearances throw considerable light on the principle of
digestion, and show that it is neither a mechanical power, nor
contractions of the stomach, nor heat, but something secreted in
the coats of the stomach, and thrown into its cavity, which there
animalizes the food or assimilates it to the nature of the blood.
The power of this juice is confined or limited to certain
substances, especially of the vegetable and animal kingdoms; and
although this menstruum is capable of acting independently of the
stomach, yet it is indebted to that viscus for its
continuance.[5]
THE FUNCTION OF RESPIRATION
It is a curious commentary on the crude notions of mechanics of
previous generations that it should have been necessary to prove
by experiment that the thin, almost membranous stomach of a
mammal has not the power to pulverize, by mere attrition, the
foods that are taken into it. However, the proof was now for the
first time forthcoming, and the question of the general character
of the function of digestion was forever set at rest. Almost
simultaneously with this great advance, corresponding progress
was made in an allied field: the mysteries of respiration were
at last cleared up, thanks to the new knowledge of chemistry. The
solution of the problem followed almost as a matter of course
upon the advances of that science in the latter part of the
century. Hitherto no one since Mayow, of the previous century,
whose flash of insight had been strangely overlooked and
forgotten, had even vaguely surmised the true function of the
lungs. The great Boerhaave had supposed that respiration is
chiefly important as an aid to the circulation of the blood; his
great pupil, Haller, had believed to the day of his death in 1777
that the main purpose of the function is to form the voice. No
genius could hope to fathom the mystery of the lungs so long as
air was supposed to be a simple element, serving a mere
mechanical purpose in the economy of the earth.
But the discovery of oxygen gave the clew, and very soon all the
chemists were testing the air that came from the lungs--Dr.
Priestley, as usual, being in the van. His initial experiments
were made in 1777, and from the outset the problem was as good as
solved. Other experimenters confirmed his results in all their
essentials--notably Scheele and Lavoisier and Spallanzani and
Davy. It was clearly established that there is chemical action
in the contact of the air with the tissue of the lungs; that some
of the oxygen of the air disappears, and that carbonic-acid gas
is added to the inspired air. It was shown, too, that the blood,
having come in contact with the air, is changed from black to red
in color. These essentials were not in dispute from the first.
But as to just what chemical changes caused these results was the
subject of controversy. Whether, for example, oxygen is actually
absorbed into the blood, or whether it merely unites with carbon
given off from the blood, was long in dispute.
Each of the main disputants was biased by his own particular
views as to the moot points of chemistry. Lavoisier, for
example, believed oxygen gas to be composed of a metal oxygen
combined with the alleged element heat; Dr. Priestley thought it
a compound of positive electricity and phlogiston; and Humphry
Davy, when he entered the lists a little later, supposed it to be
a compound of oxygen and light. Such mistaken notions naturally
complicated matters and delayed a complete understanding of the
chemical processes of respiration. It was some time, too, before
the idea gained acceptance that the most important chemical
changes do not occur in the lungs themselves, but in the ultimate
tissues. Indeed, the matter was not clearly settled at the close
of the century. Nevertheless, the problem of respiration had
been solved in its essentials. Moreover, the vastly important
fact had been established that a process essentially identical
with respiration is necessary to the existence not only of all
creatures supplied with lungs, but to fishes, insects, and even
vegetables--in short, to every kind of living organism.
ERASMUS DARWIN AND VEGETABLE PHYSIOLOGY
Some interesting experiments regarding vegetable respiration were
made just at the close of the century by Erasmus Darwin, and
recorded in his Botanic Garden as a foot-note to the verse:
"While spread in air the leaves respiring play."
These notes are worth quoting at some length, as they give a
clear idea of the physiological doctrines of the time (1799),
while taking advance ground as to the specific matter in
question:
"There have been various opinions," Darwin says, "concerning the
use of the leaves of plants in the vegetable economy. Some have
contended that they are perspiratory organs. This does not seem
probable from an experiment of Dr. Hales, Vegetable Statics, p.
30. He, found, by cutting off branches of trees with apples on
them and taking off the leaves, that an apple exhaled about as
much as two leaves the surfaces of which were nearly equal to the
apple; whence it would appear that apples have as good a claim to
be termed perspiratory organs as leaves. Others have believed
them excretory organs of excrementitious juices, but as the vapor
exhaled from vegetables has no taste, this idea is no more
probable than the other; add to this that in most weathers they
do not appear to perspire or exhale at all.
"The internal surface of the lungs or air-vessels in men is said
to be equal to the external surface of the whole body, or almost
fifteen square feet; on this surface the blood is exposed to the
influence of the respired air through the medium, however, of a
thin pellicle; by this exposure to the air it has its color
changed from deep red to bright scarlet, and acquires something
so necessary to the existence of life that we can live scarcely a
minute without this wonderful process.
"The analogy between the leaves of plants and the lungs or gills
of animals seems to embrace so many circumstances that we can
scarcely withhold our consent to their performing similar
offices.
"1. The great surface of leaves compared to that of the trunk
and branches of trees is such that it would seem to be an organ
well adapted for the purpose of exposing the vegetable juices to
the influence of the air; this, however, we shall see afterwards
is probably performed only by their upper surfaces, yet even in
this case the surface of the leaves in general bear a greater
proportion to the surface of the tree than the lungs of animals
to their external surfaces.
"2. In the lung of animals the blood, after having been exposed
to the air in the extremities of the pulmonary artery, is changed
in color from deep red to bright scarlet, and certainly in some
of its essential properties it is then collected by the pulmonary
vein and returned to the heart. To show a similarity of
circumstances in the leaves of plants, the following experiment
was made, June 24, 1781. A stalk with leaves and seed-vessels of
large spurge (Euphorbia helioscopia) had been several days placed
in a decoction of madder (Rubia tinctorum) so that the lower part
of the stem and two of the undermost leaves were immersed in it.
After having washed the immersed leaves in clear water I could
readily discover the color of the madder passing along the middle
rib of each leaf. The red artery was beautifully visible on the
under and on the upper surface of the leaf; but on the upper side
many red branches were seen going from it to the extremities of
the leaf, which on the other side were not visible except by
looking through it against the light. On this under side a system
of branching vessels carrying a pale milky fluid were seen coming
from the extremities of the leaf, and covering the whole under
side of it, and joining two large veins, one on each side of the
red artery in the middle rib of the leaf, and along with it
descending to the foot-stalk or petiole. On slitting one of these
leaves with scissors, and having a magnifying-glass ready, the
milky blood was seen oozing out of the returning veins on each
side of the red artery in the middle rib, but none of the red
fluid from the artery.
"All these appearances were more easily seen in a leaf of Picris
treated in the same manner; for in this milky plant the stems and
middle rib of the leaves are sometimes naturally colored reddish,
and hence the color of the madder seemed to pass farther into the
ramifications of their leaf-arteries, and was there beautifully
visible with the returning branches of milky veins on each side."
Darwin now goes on to draw an incorrect inference from his
observations:
"3. From these experiments," he says, "the upper surface of the
leaf appeared to be the immediate organ of respiration, because
the colored fluid was carried to the extremities of the leaf by
vessels most conspicuous on the upper surface, and there changed
into a milky fluid, which is the blood of the plant, and then
returned by concomitant veins on the under surface, which were
seen to ooze when divided with scissors, and which, in Picris,
particularly, render the under surface of the leaves greatly
whiter than the upper one."
But in point of fact, as studies of a later generation were to
show, it is the under surface of the leaf that is most abundantly
provided with stomata, or "breathing-pores." From the stand-point
of this later knowledge, it is of interest to follow our author a
little farther, to illustrate yet more fully the possibility of
combining correct observations with a faulty inference.
"4. As the upper surface of leaves constitutes the organ of
respiration, on which the sap is exposed in the termination of
arteries beneath a thin pellicle to the action of the atmosphere,
these surfaces in many plants strongly repel moisture, as cabbage
leaves, whence the particles of rain lying over their surfaces
without touching them, as observed by Mr. Melville (Essays
Literary and Philosophical: Edinburgh), have the appearance of
globules of quicksilver. And hence leaves with the upper
surfaces on water wither as soon as in the dry air, but continue
green for many days if placed with the under surface on water, as
appears in the experiments of Monsieur Bonnet (Usage des
Feuilles). Hence some aquatic plants, as the water-lily
(Nymphoea), have the lower sides floating on the water, while the
upper surfaces remain dry in the air.
"5. As those insects which have many spiracula, or breathing
apertures, as wasps and flies, are immediately suffocated by
pouring oil upon them, I carefully covered with oil the surfaces
of several leaves of phlomis, of Portugal laurel, and balsams,
and though it would not regularly adhere, I found them all die in
a day or two.
"It must be added that many leaves are furnished with muscles
about their foot-stalks, to turn their surfaces to the air or
light, as mimosa or Hedysarum gyrans. From all these analogies I
think there can be no doubt but that leaves of trees are their
lungs, giving out a phlogistic material to the atmosphere, and
absorbing oxygen, or vital air.
"6. The great use of light to vegetation would appear from this
theory to be by disengaging vital air from the water which they
perspire, and thence to facilitate its union with their blood
exposed beneath the thin surface of their leaves; since when pure
air is thus applied it is probable that it can be more readily
absorbed. Hence, in the curious experiments of Dr. Priestley and
Mr. Ingenhouz, some plants purified less air than others--that
is, they perspired less in the sunshine; and Mr. Scheele found
that by putting peas into water which about half covered them
they converted the vital air into fixed air, or carbonic-acid
gas, in the same manner as in animal respiration.
"7. The circulation in the lungs or leaves of plants is very
similar to that of fish. In fish the blood, after having passed
through their gills, does not return to the heart as from the
lungs of air-breathing animals, but the pulmonary vein taking the
structure of an artery after having received the blood from the
gills, which there gains a more florid color, distributes it to
the other parts of their bodies. The same structure occurs in the
livers of fish, whence we see in those animals two circulations
independent of the power of the heart--viz., that beginning at
the termination of the veins of the gills and branching through
the muscles, and that which passes through the liver; both which
are carried on by the action of those respective arteries and
veins."[6]
Darwin is here a trifle fanciful in forcing the analogy between
plants and animals. The circulatory system of plants is really
not quite so elaborately comparable to that of fishes as he
supposed. But the all-important idea of the uniformity underlying
the seeming diversity of Nature is here exemplified, as elsewhere
in the writings of Erasmus Darwin; and, more specifically, a
clear grasp of the essentials of the function of respiration is
fully demonstrated.
ZOOLOGY AT THE CLOSE OF THE EIGHTEENTH CENTURY
Several causes conspired to make exploration all the fashion
during the closing epoch of the eighteenth century. New aid to
the navigator had been furnished by the perfected compass and
quadrant, and by the invention of the chronometer; medical
science had banished scurvy, which hitherto had been a perpetual
menace to the voyager; and, above all, the restless spirit of the
age impelled the venturesome to seek novelty in fields altogether
new. Some started for the pole, others tried for a northeast or
northwest passage to India, yet others sought the great
fictitious antarctic continent told of by tradition. All these of
course failed of their immediate purpose, but they added much to
the world's store of knowledge and its fund of travellers' tales.
Among all these tales none was more remarkable than those which
told of strange living creatures found in antipodal lands. And
here, as did not happen in every field, the narratives were often
substantiated by the exhibition of specimens that admitted no
question. Many a company of explorers returned more or less laden
with such trophies from the animal and vegetable kingdoms, to the
mingled astonishment, delight, and bewilderment of the closet
naturalists. The followers of Linnaeus in the "golden age of
natural history," a few decades before, had increased the number
of known species of fishes to about four hundred, of birds to one
thousand, of insects to three thousand, and of plants to ten
thousand. But now these sudden accessions from new territories
doubled the figure for plants, tripled it for fish and birds, and
brought the number of described insects above twenty thousand.
Naturally enough, this wealth of new material was sorely puzzling
to the classifiers. The more discerning began to see that the
artificial system of Linnaeus, wonderful and useful as it had
been, must be advanced upon before the new material could be
satisfactorily disposed of. The way to a more natural system,
based on less arbitrary signs, had been pointed out by Jussieu in
botany, but the zoologists were not prepared to make headway
towards such a system until they should gain a wider
understanding of the organisms with which they had to deal
through comprehensive studies of anatomy. Such studies of
individual forms in their relations to the entire scale of
organic beings were pursued in these last decades of the century,
but though two or three most important generalizations were
achieved (notably Kaspar Wolff's conception of the cell as the
basis of organic life, and Goethe's all-important doctrine of
metamorphosis of parts), yet, as a whole, the work of the
anatomists of the period was germinative rather than
fruit-bearing. Bichat's volumes, telling of the recognition of
the fundamental tissues of the body, did not begin to appear till
the last year of the century. The announcement by Cuvier of the
doctrine of correlation of parts bears the same date, but in
general the studies of this great naturalist, which in due time
were to stamp him as the successor of Linnaeus, were as yet only
fairly begun.