Leeuwenhoek’s figures.
FIG. 53. Capillary network in web of frog’s foot. A, C and E
are arterioles, B, D and F are venules.
]
A most remarkable contemporary microscopist was the Dutchman, Anthony
van Leeuwenhoek (1632-1723). Without medical or scientific training,
desultory and secretive in his mode of working, he was withal an
observer of genius and a very shrewd investigator. During his long
and industrious life he made a series of disconnected discoveries
which for originality and importance have been surpassed by no other
microscopic observer. He improved and extended the knowledge of the
capillary circulation of which Malpighi was the discoverer (Fig. 53),
he gave figures of the blood corpuscles (Figs. 50-1), of spermatozoa
and of fibres of muscles (Figs. 55-56a), and advanced the knowledge of
embryology. He always worked with a simple microscope, using lenses of
exceedingly short focal length (Fig. 49A). It is astounding that, with
such an instrument, he saw and figured bacteria as early as 1683 (Fig.
54).
[Illustration:
FIG. 54. THE FIRST REPRESENTATION OF BACTERIA. They were figured by
Leeuwenhoek in the _Philosophical Transactions of the Royal Society_ of
London in 1683.
]
[Illustration: FIGS. 55-56A. Drawings of the structure of muscle made
about 1682 by Leeuwenhoek.
FIG. 55 shows a muscle teased up into bundles of fibres, magnified.
FIG. 56 is a more magnified view of a bundle of fibres. The cut fibres
are shown at the end.
FIG. 56A is a very highly magnified view of a single fibre showing very
clearly the striations that are very characteristic of voluntary muscle.
]
The short life of a second Dutch microscopist of the seventeenth
century, Jan Jacobz Swammerdam (1637-80), was abbreviated yet further,
as regards scientific achievement, by his insanity. In his brief
working period he produced his _Bible of Nature_ which, alone of
the scientific writings of his age, is still consulted by modern
naturalists for the unique beauty and accuracy of its figures.
He extended the knowledge of embryology and he made a series of
physiological experiments which involved the very modern physiological
device known as the ‘nerve-muscle preparation’. He is thus the founder
of an important department of Physiology. Swammerdam showed that,
during contraction, a muscle does not increase in bulk, and that
therefore the nerve brings nothing to it in the way of the hypothetical
‘nervous fluid’ in which many then believed (Fig. 63). He applied the
same reasoning to the heart (Figs. 57-60). Swammerdam was perhaps the
first to see the blood corpuscles. Like several of his contemporaries
and followers, he made injection preparations of much beauty and
delicacy. His great work was not published till after his death. The
copper plates that he had prepared for it were found and purchased by
Boerhaave (p. 140), who produced them at his own expense.
These microscopists and several others in the seventeenth century
did much to explore the minute structure of the animal body. Their
revelations showed at once an unexpected complexity of all the parts,
and an unexpected resemblance of some of those parts which appear
diverse to the naked eye. Thus, the structure of the body came to be
subjected to a process that we may call ‘microscopic analysis’. For
long after the time of these classical microscopists no effective
improvements were made in the microscope, and the progress of
microscopic analysis lay almost dormant. With the improvements in the
microscope of the nineteenth century, the method was taken up again
with triumphant results.
[Illustration: FIGS. 57-60. Experiments by Swammerdam to illustrate the
nature of muscular contraction.
FIG. 57 is the simplest form of what physiologists call a ‘nerve-muscle
preparation’. It is merely a living muscle dissected away from the
body, but with its nerve still attached. In the experiment the two
tendons of the muscle are held by the two hands. An assistant pinches
the nerve with forceps. The muscle thereon contracts and draws the two
hands together.
FIG. 58 shows the muscle passed through a glass tube. Its two tendons
are fastened by two pins. When the nerve is pinched the pins are drawn
towards each other, and the muscle, in contracting, fills the middle of
the tube.
FIG. 59 shows the nerve-muscle preparation enclosed within a tube.
This tube has a narrow neck in which, at _e_, is a drop of water. The
other end of the tube is closed by a cork. The nerve may be squeezed by
pulling the thread _c c_, which passes through the cork and drags the
nerve into a narrow wire loop.
FIG. 60 is a similar experiment with the heart, which contracts and
expands spontaneously and needs no irritation.
The experiments 59 and 60 show that during ‘contraction’ no new
substance passes into the muscle, since it does not then increase in
size. This gives the death-blow to the conception of a ‘nervous fluid’
passing into the muscle to cause contraction by distending it.
]
§ 7. _From Alchemy to Chemistry._
During the sixteenth and the first part of the seventeenth century
the basic science of Mechanics had been placed on a firm footing
by Galileo. Astronomy, with Galileo and Kepler, had made the great
break with the past. Anatomy and Physiology had put on their modern
dress. Chemical knowledge, however, remained peculiarly backward.
Many advances, it is true, had been made in technical processes,
but investigations designed to throw light on theory were mostly
prosecuted by the band of dupes and charlatans who, since the Middle
Ages, had been seeking the Philosopher’s Stone. The old theory of the
four elements, earth, air, fire, and water (p. 34), formed an ill basis
for experiment. Some philosophers, it is true, had put forward crude
atomic theories, but they had little experimental evidence to adduce.
Nevertheless even the alchemists had made some advance and had, for
instance, perfected a system of weighing.
The great defect of the ancient view of matter was that it contained
no definite conception of the nature of a _pure_ substance. Metals,
for instance, were regarded, like other substances, as a mixture
in certain proportions of the four elements of Aristotle (p. 34).
Thus, the transmutation of one metal or one substance into another
by distillation did not seem an absurdity, or even a task of special
theoretical difficulty.
The main agent in changing the chemical outlook was Robert Boyle
(1627-91). He was a member of a small association of scientific men,
the _Invisible College_, which met first in London, then in Oxford,
and finally in 1663 was incorporated by Royal Charter as the _Royal
Society_. These men were satisfied that the only way to learn anything
effective about Nature was by observation and experiment. From their
discussions all purely speculative views were excluded. They agreed
to meet together solely to compare experiences, to demonstrate
experiments, and to draw immediate deductions. None of them was more
active in these matters than Boyle.
The actual chemical and physical discoveries of Boyle were very
numerous, but his great achievement, the real service he rendered to
learning in general and to medicine in particular, was his introduction
of a new spirit into Chemistry. Under him that study was no longer
prosecuted for purely practical ends; it was set free from the mystic
factor in Alchemy and it was loosed from the chains which bound it
to Medicine, to the disadvantage of both. Chemistry thus became an
independent science, the principles of which were to be ascertained by
experiment, and its truths pursued for their own sake.
Boyle demonstrated that the air is a material substance and has weight.
By means of his air-pump, he proved clearly that this substance is
necessary for the support of respiration (Fig. 63). The law of the
compressibility of gases is still known by his name. Most important of
all Boyle’s contributions to chemical theory was his adumbration of the
conception of a chemical element in our modern sense, and his view,
which he borrowed from another philosopher, of the atomic structure of
matter.
Under the inspiration of Boyle and his colleagues, chemical works
of the second half of the seventeenth century exhibit in general a
positive, cautious, experimental spirit, and show a great contrast to
the mystical and obscure writings of the first half of the century,
which have much affinity with Alchemy. A fine exponent of this new
spirit was John Mayow (1645-79), who was prevented by an early death
from fulfilling all his promise. He was the first to recognize clearly
that there is a substance or principle in air which is concerned at
once with combustion, respiration, and the conversion of venous into
arterial blood. In this sense he was the discoverer of Oxygen (Figs. 74
and 75).
[Illustration: FIG. 61. ONE OF ROBERT BOYLE’S AIR-PUMPS. A cat has been
placed in the receiver. It shows signs of asphyxia as soon as the air
is exhausted by the pump.]
§ 8. _The Medical Theorists._
The great advances in the physical and biological sciences, instituted
during the sixteenth and seventeenth century, left the old medical
theories derelict. We have already traced the wrecking of the
Galenic physiology. With its destruction, the old ideas concerning
the three types of spirit, natural, vital, and animal, went by the
board. The doctrine of the circulation of the blood (p. 113) and the
investigations of the new Chemistry accorded ill with the old humoral
pathology, which ascribed all disease to excess or defect of one of
the four humors, blood, phlegm, bile, and melancholy (p. 34). Numerous
fresh theories arose, of which the more important can be classed under
the three headings _Iatrophysics_, _Iatrochemistry_, and _Vitalism_.
(_a_) _Iatrophysics._
The physical discoveries of Galileo and the demonstrations of
Sanctorius (p. 108) and of Harvey (p. 111) gave a great impetus to the
attempt to explain the workings of the animal body on purely mechanical
grounds. The writers who took this point of view are known as the
_Iatrophysicists_. One of the earliest and most impressive exponents of
physiological theory along these lines was the French philosopher René
Descartes (1596-1650). His work on the subject appeared posthumously in
1662. It is important as the first modern book entirely devoted to the
subject of Physiology.
Descartes had not himself any extensive practical knowledge of the
subject with which he was dealing. On theoretical grounds he set
forth a very complicated apparatus which he believes to be a model
of animal structure. Subsequent investigation failed to confirm his
findings, and his work soon passed into oblivion. For a time, however,
it attracted much attention and many followers. A strong point in his
theory is the great stress laid upon the nervous system, and its power
of co-ordinating the different bodily activities. Thus stated, his view
may seem not far from the modern standpoint, though in fact he was
grotesquely wrong in detail. An important part of his theory is the
complete separation of Man from all the other animals. Man, according
to him, differs from all other animals by his possession of a soul,
which is situated in a structure in the brain known to physiologists
as the ‘pineal body’! Animals, he held, have no soul, and all their
actions and movements, even those which seem to express pain or fear,
are purely automatic. It is the modern theory of ‘behaviorism’ with
man excluded! (Figs. 62 and 63.)
[Illustration:
FIG. 62. DESCARTES’ conception of the relation of a sensory impression
and a motor impulse. The image of the object ABC passes to the eye and
is formed on the retina. Owing to the optical properties of the eye, it
is there inverted. The image is inverted yet again within the brain,
where it passes to the pineal gland H at the point _b_. The position
and character of the image formed on the retina determines the nature
and distribution of its effect on the pineal body. According to the
nature and distribution of that effect is the result on the nerve,
and through it, by the passage of nervous fluid, on the muscles. The
movement in the nerve is initiated at the point _c_. The relation
between _b_ and _c_ is an insoluble mystery in which is wrapped up the
very nature of the soul. (From the posthumous work of Descartes on
Physiology.) ]
[Illustration:
FIG. 63. DIAGRAM OF DESCARTES to illustrate his theory of nervous
action. P R and _q s_ are nerves which supply the muscles of the eye
T and V V. Descartes held that these nerves were hollow and provided
with valves, which can be seen at the point at which the P R and _q s_
first branch. These valves were partly controlled by little fibrils
(which can be seen in the main stems of P R and _q s_ and in certain of
their branches). These valves control the movement of the fluid within
the hollow spaces of the nerves. Additional complication is lent to
the scheme by the fact that P R and _q s_ intercommunicate at certain
points. The view of Descartes, and all such theories of nervous fluid,
were destroyed by the experiment of Swammerdam (Figs. 57-60), which,
however, long remained unpublished.
]
More lasting was the achievement of Giovanni Alfonso Borelli
(1608-79), an eminent mathematician who was professor at several
Italian universities and the friend of Galileo and Malpighi. Stirred,
like Descartes, by the success of Galileo in giving a mathematical
expression to mechanical events, Borelli attempted to do the same with
the animal body. In this undertaking he was, in fact, very successful.
That department of Physiology which treats of muscular movement on
mechanical principles was effectively founded and largely developed by
him. Here his mathematical and physical training was specially useful.
He endeavored, with some success, to extend mechanical principles to
such movements as the flight of birds and the swimming of fish. When he
came to an analysis of some of the other activities of the body, such
as the action of the heart, or the movements of the intestines, he was
less successful, and he naturally failed altogether in his attempt to
introduce mechanical ideas in explanation of what we now know to be
chemical processes, such as digestion in the stomach.
Undeterred by Borelli’s failure, other writers sought to find
mechanical explanations of physical processes. As is usual in such
cases, the amount of theory was inversely proportional to the amount
of knowledge. The views of some of the later ‘Iatrophysicists’ became
very fantastic. Belated representatives of the school are the writers
of the great French _Encyclopédie_ (1751-72), and notably its main
author, the man of letters, Denis Diderot (1713-84).
[Illustration:
FIG. 64. DIAGRAMS FROM BORELLI, showing one of his attempts to analyse
the movements of the muscles, in this case of the arm, according to the
principles of the science of Mechanics as expounded by Galileo. The
figure should be considered in conjunction with Fig. 65 opposite.
]
(_b_) _Iatrochemistry._
Just as there were some who sought to explain all animal activity on
a mechanical basis so others resorted to chemical interpretation.
These may be termed _Iatrochemists_. The most prominent was Franciscus
Sylvius (1614-72), professor of Medicine at Leyden. That university
had become, in the second half of the seventeenth century, the most
progressive scientific center north of the Alps. It was the seat of the
first University laboratory, built at the instigation of Sylvius.
Sylvius devoted much attention to the study of salts. He recognized
that they were the result of the union of acids and bases, and he
attained to the idea of chemical affinity--an important advance. He
looked at the phenomena of life also from the chemical point of view.
Well abreast of the anatomical knowledge of his day, and accepting
the broader lines of mechanistic advance in Biology, such as the
circulation of the blood and the mechanics of muscular motion, Sylvius
sought to interpret other activities in chemical terms. His position
and abilities as a teacher gave his views wide currency and he and his
pupils occupy a large part of the field of medical theory until well
into the eighteenth century.
Under the influence of this school, almost all forms of vital activity
were expressed in terms of ‘acid and alkali’ and of ‘fermentation’.
The latter process was assumed to be of a chemical order, and no
clear distinction was made between changes that are brought about by
‘unorganized’ ferments, such as gastric juice or rennet, and changes
that are brought about by the action of micro-organisms, such as
alcoholic fermentation or leavening by yeast. Nevertheless, the school
of Sylvius and its immediate successors added considerably to our
knowledge of physiological processes, notably by their examination of
the body fluids, especially the digestive fluids such as the saliva,
and the secretions of the stomach and of the pancreas.
[Illustration:
FIG. 65. DIAGRAM OF MUSCULAR ACTION involved in lifting a weight in the
hand. It illustrates how muscular movement may sometimes be resolved
into terms of the lever. In practice, however, it is usually necessary
to involve a whole system of levers, pulleys, resistances, &c., as
Borelli clearly perceived. (Compare Fig. 64.)
]
(_c_) _Vitalism._
Yet another school of medical theorists arose under the leadership of
the German chemist and physician, George Ernest Stahl (1660-1734).
Stahl is best remembered as the author of the famous theory of
_phlogiston_, a hypothetical substance with which bodies were supposed
to part during the process of burning (p. 151). He is important in the
history of science for his success in grouping chemical phenomena and
therefore in systematizing the study of the subject. For our purpose,
however, Stahl stands as the protagonist of that view of the nature of
the organism which now goes under the term _Vitalism_. Though expressed
by him in obscure and mystical language, it is, in effect, a return to
the Aristotelian position and a denial of the view of Descartes. To
Descartes the animal body was a machine. To Stahl the word _machine_
expressed exactly what the animal body was not. The phenomena
characteristic of the living body are, he considered, not governed by
physical and chemical laws, but by laws of a wholly different kind.
These laws are the laws of the _sensitive soul_. The _sensitive soul_
of Stahl is, in its ultimate analysis, not dissimilar to the _psyche_
of Aristotle (p. 32). Stahl held that the immediate instruments, the
natural slaves of this sensitive soul, were chemical processes and his
Physiology develops along lines of which Aristotle could know nothing.
This does not, however, alter the fact of his hypothesis being an
essentially vitalistic one of Aristotelian origin.
* * * * *
The language and the theories of the Iatrophysicists, the
Iatrochemists, and the Vitalists of the seventeenth and eighteenth
centuries have long been discarded by men of science in the form in
which they were originally propounded. Nevertheless, they represent
three attitudes to the activities of living things which have present
and current meaning. Each seems to present some aspect of truth.
Whether some physiological thinker will combine all three aspects into
one coherent whole, it is for the future to decide. Yet it is certain
that all three lines of approach remain of value, and the stimulus
provided by each of the three inspires investigation at the present
day. In this sense we enter on the period of modern Medicine in the
seventeenth century. In this sense the foundations of modern rational
Medicine may be said to have been laid by Borelli, Sylvius and Stahl,
with Galileo, Boyle and Harvey standing behind them.
V
THE PERIOD OF CONSOLIDATION
(FROM ABOUT 1700 TO ABOUT 1825)
§ 1. _The Reign of Law._
During the sixteenth and seventeenth centuries the human mind cast
off its medieval vestments and, having refreshed itself at the spring
of Antiquity, turned to array itself in the garments of the New
Philosophy. The advent of new ideas and new knowledge had been very
rapid. The _method_ of Research had been determined by Galileo at the
beginning of the seventeenth century. The _meaning_ of Research was
determined by a second great investigator, Newton, at the end of the
same century.
The change wrought in the thought of their time by Vesalius, Galileo,
Harvey and their like was quantitative rather than qualitative. They
discovered new laws of Nature, but the discovery of such new laws was
hardly unprecedented. Law had been traced in the heavens from of old.
The rules of planetary and stellar motion had been gradually developed
from the simple astronomical theories of the ancients. The great
astronomers of the sixteenth and seventeenth centuries did not hesitate
to appeal to the records and doctrines of medieval writers, for new
mathematical relationships of the heavenly bodies had been elicited
even during the Middle Ages. In the sixteenth century Astronomy under
Tycho (died 1601) put her house in order for the ‘Great Instauration’
of the coming age. And then Galileo startled the world (1604) with that
new star of his (p. 104), among the most remote heavenly bodies in the
very region held by the Aristotelian and Platonic schemes to be utterly
changeless. The Revolution in Thought had begun, though no new order
had been established.
By 1618 Kepler had enunciated his ‘three laws of planetary motion’,
bringing these movements into an intelligible relation with each other.
Then the experimental philosophers set forth to establish terrestrial
mechanics. They determined the mode of action of gravitation, and
Galileo came near to the ‘three laws of motion’ which we call Newton’s.
But it was Newton who first affirmed them clearly and succeeded in
linking them with Kepler’s laws of planetary motion. Before Newton, no
man had shown or perceived that rule by which the natural succession of
earthly phenomena is in relation to that of the heavenly bodies. Nay,
Faith and Reason alike would have been against such a view. To prove
that the relationship amounted to identity, to move men’s minds to see
that the force that causes the stone to fall is that which keeps the
planets in their path, this was Newton’s unique achievement. It was
Newton who first enunciated a law whose writ ran alike in the Heavens
and on the Earth. With Newton the Universe acquired an independent
rationality, and the whole cosmology of Aristotle, of Galen, and of the
Middle Ages lay in the dust.
When Newton had completed his work, the Gravitation of the Earth and of
the Heavens was seen to be one, and all the Mechanism of the Universe
lay spread before him. The vision was set forth in his _Principia_
(1687). It established a view of the structure and working of the
Universe which has survived to our own generation.
And now as to the change wrought in men’s minds. It was something more
than a Revolution. It was the establishment of a New Order. Newton
conceived a working Universe wholly independent of the Spiritual Order.
As to how far his vision is philosophically tenable and as to how far
he realized its nature, these are matters which we need not discuss
here. There can, however, be no doubt that Newton utterly destroyed the
very foundations of medieval thought. With Newton there sets in the
last stage of ‘scientific determinism’.