OUR LIFE
Development of Physiology in Antiquity and the Middle Ages:
Galen--Experiment and Vivisection--Discovery of the Circulation
of the Blood by Harvey--Vitalism: Haller--Teleological and
Vitalistic Conception of Life--Mechanical and Monistic View
of the Physiological Processes--Comparative Physiology in the
Nineteenth Century: Johannes Müller--Cellular Physiology: Max
Verworn--Cellular Pathology: Virchow--Mammal Physiology--Similarity
of all Vital Activity in Man and the Ape
It is only in the nineteenth century that our knowledge of human life
has attained the dignity of a genuine, independent science; during the
course of the century it has developed into one of the highest, most
interesting, and most important branches of knowledge. This "science
of the vital functions," physiology, had, it is true, been regarded
at a much earlier date as a desirable, if not a necessary, condition
of success in medical treatment, and had been constantly associated
with anatomy, the science of the structure of the body. But it was
only much later, and much more slowly, than the latter that it could
be thoroughly studied, as it had to contend with much more serious
difficulties.
The idea of life, as the opposite of death, naturally became the
subject of speculation at a very early age. In the living man, just
as in other living animals, there were certain peculiar changes,
especially movements, which were wanting in lifeless nature:
spontaneous locomotion, the beat of the heart, the drawing of the
breath, speech, and so forth. But the discrimination of such "organic
movements" from similar phenomena in inorganic bodies was by no means
easy, and was frequently impossible; the flowing stream, the flickering
flame, the rushing wind, the falling rock, seemed to man to exhibit
the same movements. It was quite natural that primitive man should
attribute an independent life to these "dead" bodies. He knew no more
of the real sources of movement in the one case than in the other.
We find the earliest scientific observations on the nature of man's
vital functions (as well as on his structure) in the Greek natural
philosophers and physicians of the sixth and fifth centuries before
Christ. The best collection of the physiological facts which were known
at that time is to be found in the _Natural History_ of Aristotle; a
great number of his assertions were probably taken from Democritus
and Hippocrates. The school of the latter had already made attempts
to explain the mystery; it postulated as the ultimate source of life
in man and the beasts a volatile "spirit of life" (Pneuma); and
Erasistratus (280 B.C.) already drew a distinction between the lower
and the higher "spirit of life," the _pneuma zoticon_ in the heart and
the _pneuma psychicon_ in the brain.
The credit of gathering these scattered truths into unity, and of
making the first attempt at a systematic physiology, belongs to the
great Greek physician Galen; we have already recognized in him the
first great anatomist of antiquity (cf. p. 23). In his researches
into the organs of the body he never lost sight of the question of
their vital activity, their functions; and even in this direction he
proceeded by the same comparative method, taking for his principal
study the animals which approach nearest to man. Whatever he learned
from these he applied directly to man. He recognized the value of
physiological experiment; in his vivisection of apes, dogs, and
swine he made a number of interesting experiments. Vivisection has
been made the object of a violent attack in recent years, not only
by the ignorant and narrow-minded, but by theological enemies of
knowledge and by perfervid sentimentalists; it is, however, one of the
_indispensable_ methods of research into the nature of life, and has
given us invaluable information on the most important questions. This
was recognized by Galen seventeen hundred years ago.
Galen reduces all the different functions of the body to three
groups, which correspond to the three forms of the _pneuma_, or vital
spirit. The _pneuma psychicon_--the soul--which resides in the brain
and nerves, is the cause of thought, sensation, and will (voluntary
movement); the _pneuma zoticon_--the heart--is responsible for the beat
of the heart, the pulse, and the temperature; the _pneuma physicon_,
seated in the liver, is the source of the so-called vegetative
functions, digestion and assimilation, growth and reproduction.
He especially emphasized the renewal of the blood in the lungs,
and expressed a hope that we should some day succeed in isolating
the permanent element in the atmosphere--the _pneuma_, as he calls
it--which is taken into the blood in respiration. More than fifteen
centuries elapsed before this _pneuma_--oxygen--was discovered by
Lavoisier.
In human physiology, as well as in anatomy, the great system of Galen
was for thirteen centuries the _Codex aureus_, the inviolable source of
all knowledge. The influence of Christianity, so fatal to scientific
culture, raised the same insuperable obstacles in this as in every
other branch of secular knowledge. Not a single scientist appeared
from the third to the sixteenth century who dared to make independent
research into man's vital activity, and transcend the limits of the
Galenic system. It was not until the sixteenth century that experiments
were made in that direction by a number of distinguished physicians
and anatomists (Paracelsus, Servetus, Vesalius, and others). In 1628
Harvey published his great discovery of the circulation of the blood,
and showed that the heart is a pump, which drives the red stream
unceasingly through the connected system of arteries and veins by a
rhythmic, unconscious contraction of its muscles. Not less important
were Harvey's researches into the procreation of animals, as a result
of which he formulated the well-known law: "Every living thing comes
from an egg" (_omne vivum ex ovo_).
The powerful impetus which Harvey gave to physiological observation and
experiment led to a great number of discoveries in the sixteenth and
seventeenth centuries. These were co-ordinated for the first time by
the learned Albrecht Haller about the middle of the last century; in
his great work, _Elementa Physiologiae_, he established the inherent
importance of the science, independently of its relation to practical
medicine. In postulating, however, a special "sensitive force or
sensibility" for neural action, and a special "irritability" for
muscular movement, Haller gave strong support to the erroneous idea of
a specific "vital force" (_vis vitalis_).
For more than a century afterwards, from the middle of the eighteenth
until the middle of the nineteenth century, medicine and (especially)
physiology were dominated by the old idea that a certain number of the
vital processes may be traced to physical and chemical causes, but that
others are the outcome of a special vital force which is independent
of physical agencies. However much scientists differed in their
conceptions of its nature and its relation to the "soul," they were all
agreed as to its independence of, and essential distinction from, the
chemico-physical forces of ordinary "matter"; it was a self-contained
force (_archaeus_), unknown in inorganic nature, which compelled
ordinary forces into its service. Not only the distinctly psychical
activity, the sensibility of the nerves and the irritability of the
muscles, but even the phenomena of sense activity, of reproduction,
and of development seemed so wonderful and so mysterious in their
sources that it was impossible to attribute them to simple physical
and chemical processes. As the free activity of the vital force
was purposive and conscious, it led, in philosophy, to a complete
_teleology_; especially did this seem indisputable when even the
"critical" philosopher Kant had acknowledged, in his famous critique
of the teleological position, that, though the mind's authority to
give a mechanical interpretation of all phenomena is theoretically
unlimited, yet its actual capacity for such interpretation does not
extend to the phenomena of organic life; here we are compelled to have
recourse to a _purposive_--therefore _supernatural_--principle. This
divergence of the _vital_ phenomena from the _mechanical_ processes of
life became, naturally, more conspicuous as science advanced in the
chemical and physical explanation of the latter. The circulation of the
blood and a number of other phenomena could be traced to mechanical
agencies; respiration and digestion were attributable to chemical
processes like those we find in inorganic nature. On the other hand,
it seemed impossible to do this with the wonderful performances of the
nerves and muscles, and with the characteristic life of the mind; the
co-ordination of all the different forces in the life of the individual
seemed also beyond such a mechanical interpretation. Hence there arose
a complete physiological dualism--an essential distinction was drawn
between inorganic and organic nature, between mechanical and vital
processes, between material force and life force, between the body and
the soul. At the beginning of the nineteenth century this vitalism was
firmly established in France by Louis Dumas, and in Germany by Reil.
Alexander Humboldt had already published a poetical presentation of it
in 1795, in his narrative of the _Legend of Rhodes_; it is repeated,
with critical notes, in his _Views of Nature_.
In the first half of the seventeenth century the famous philosopher
Descartes, starting from Harvey's discovery of the circulation of the
blood, put forward the idea that the body of man, like that of other
animals, is merely an intricate machine, and that its movements take
place under the same mechanical laws as the movements of an automaton
of human construction. It is true that Descartes, at the same time,
claimed for man the exclusive possession of a perfectly independent,
immaterial soul, and held that its subjective experience, thought,
was the only thing in the world of which we have direct and certain
cognizance ("_Cogito, ergo sum_"). Yet this dualism did not prevent
him from doing much to advance our knowledge of the mechanical life
processes in detail. Borelli followed (1660) with a reduction of the
movements of the animal body to purely physical laws, and Sylvius
endeavored, about the same time, to give a purely chemical explanation
of the phenomena of digestion and respiration; the former founded the
_iatromechanical_, the latter the _iatrochemical_, school of medicine.
However, these rational tendencies towards a natural, mechanical
explanation of the phenomena of life did not attain to a universal
acceptance and application; in the course of the eighteenth century
they fell entirely away before the advance of teleological vitalism.
The final disproof of the latter and a return to mechanism only became
possible with the happy growth of the new science of comparative
physiology in the forties of the present century.
Our knowledge of the vital functions, like our knowledge of the
structure of the human body, was originally obtained, for the most
part, not by direct observation of the human organism itself, but by
a study of the more closely related animals among the vertebrates,
especially the mammals. In this sense the very earliest beginning
of human anatomy and physiology was "comparative." But the distinct
science of "comparative physiology," which embraces the whole sphere
of life phenomena, from the lowest animal up to man, is a triumph of
the nineteenth century. Its famous creator was Johannes Müller, of
Berlin (born, the son of a shoemaker, at Coblentz, in 1801). For fully
twenty-five years--from 1833 to 1858--this most versatile and most
comprehensive biologist of our age evinced an activity at the Berlin
University, as professor and investigator, which is only comparable
with the associated work of Haller and Cuvier. Nearly every one of the
great biologists who have taught and worked in Germany for the last
sixty years was, directly or indirectly, a pupil of Johannes Müller.
Starting from the anatomy and physiology of man, he soon gathered all
the chief groups of the higher and lower animals within his sphere
of comparison. As, moreover, he compared the structure of extinct
animals with the living, and the healthy organism with the diseased,
endeavoring to bring together all the phenomena of life in a truly
philosophic fashion, he attained a biological knowledge far in advance
of his predecessors.
The most valuable fruit of these comprehensive studies of Johannes
Müller was his _Manual of Human Physiology_. This classical work
contains much more than the title indicates; it is the sketch of
a comprehensive "comparative biology." It is still unsurpassed in
respect of its contents and range of investigation. In particular,
we find the methods of observation and experiment applied in it as
masterfully as the philosophic processes of induction and deduction.
Müller was originally a vitalist, like all the physiologists of his
time. Nevertheless, the current idea of a vital force took a novel
form in his speculations, and gradually transformed itself into the
very opposite. For he attempted to explain the phenomena of life
mechanically in every department of physiology. His "transfigured"
vital force was not _above_ the physical and chemical laws of the rest
of nature but entirely bound up with them. It was, in a word, nothing
more than life itself--that is, the sum of all the movements which we
perceive in the living organism. He sought especially to give them
the same mechanical interpretation in the life of the senses and of
the mind as in the working of the muscles; the same in the phenomena
of circulation, respiration, and digestion as in generation and
development. Müller's success was chiefly due to the fact that he
always began with the simplest life phenomena of the lowest animals,
and followed them step by step in their gradual development up to the
very highest, to man. In this his method of _critical comparison_
proved its value both from the physiological and from the anatomical
point of view. Johannes Müller is, moreover, the only great scientist
who has equally cultivated these two branches of research, and combined
them with equal brilliancy. Immediately after his death his vast
scientific kingdom fell into four distinct provinces, which are now
nearly always represented by four or more chairs--human and comparative
anatomy, pathological anatomy, physiology, and the history of
evolution. This sudden division of Müller's immense realm of learning
in 1858 has been compared to the dissolution of the empire which
Alexander the Great had consolidated and ruled.
Among the many pupils of Johannes Müller who, either during his
lifetime or after his death, labored hard for the advancement of the
various branches of biology, one of the most fortunate--if not the
most important--was Theodor Schwann. When the able botanist Schleiden,
in 1838, indicated the cell as the common elementary organ of all
plants, and proved that all the different tissues of the plant are
merely combinations of cells, Johannes Müller recognized at once the
extraordinary possibilities of this important discovery. He himself
sought to point out the same composition in various tissues of the
animal body--for instance, in the spinal cord of vertebrates--and
thus led his pupil, Schwann, to extend the discovery to all the
animal tissues. This difficult task was accomplished by Schwann in
his _Microscopic Researches into the Accordance in the Structure and
Growth of Plants and Animals_ (1839). Thus was the foundation laid
of the "cellular theory," the profound importance of which, both in
physiology and anatomy, has become clearer and more widely recognized
in each subsequent year. Moreover, it was shown by two other pupils
of Johannes Müller that the activity of all organisms is, in the
ultimate analysis, the activity of the components of their tissues, the
microscopic cells--these were the able physiologist Ernst Brücke, of
Vienna, and the distinguished histologist Albert Kölliker, of Würzburg.
Brücke correctly denominated the cells the "elementary organisms," and
showed that, in the body of man and of all other animals, they are the
only actual, independent factors of the life process. Kölliker earned
special distinction, not only in the construction of the whole science
of histology, but particularly by showing that the animal ovum and its
products are simple cells.
Still, however widely the immense importance of the cellular theory for
all biological research was acknowledged, the "cellular physiology"
which is based on it only began an independent development very
recently. In this Max Verworn (of Jena) earned a twofold distinction.
In his _Psycho-physiological Studies of the Protistae_ (1889) he
showed, as a result of an ingenious series of experimental researches,
that the "theory of a cell-soul" which I put forward in 1866[11]
is completely established by an accurate study of the unicellular
protozoa, and that "the psychic phenomena of the protistæ form the
bridge which unites the chemical processes of inorganic nature with
the mental life of the highest animals." Verworn has further developed
these views, and based them on the modern theory of evolution, in
his _General Physiology_. This distinguished work returns to the
comprehensive point of view of Johannes Müller, in opposition to the
one-sided and narrow methods of those modern physiologists who think
to discover the nature of the vital phenomena by the exclusive aid of
chemical and physical experiments. Verworn showed that it is only by
Müller's comparative method and by a profound study of the physiology
of the cell that we can reach the higher stand-point which will give us
a comprehensive survey of the wonderful realm of the phenomena of life.
Only thus do we become convinced that the vital processes in man are
subject to the same physical and chemical laws as those of all other
animals.
The fundamental importance of the cellular theory for all branches of
biology was made clear in the second half of the nineteenth century,
not only by the rapid progress of morphology and physiology, but also
by the entire reform of that biological science which has always
been deemed most important on account of its relation to practical
medicine--pathology, or the science of disease. Many even of the
older physicians were convinced that human diseases were natural
phenomena, like all other manifestations of life, and should be studied
scientifically, like other vital functions. Particular schools of
medicine--the Iatrophysical and the Iatrochemical--had already, in
the seventeenth century, attempted to trace the sources of disease to
certain physical and chemical changes. However, the imperfect condition
of science at that period precluded any lasting results of these
efforts. Many of the older theories, which sought the nature of disease
in supernatural and mystical causes, were almost universally accepted
down to the middle of the nineteenth century.
It was then that Rudolf Virchow, another pupil of Müller, conceived
the happy idea of transferring the cellular theory from the healthy to
the diseased organism; he sought in the more minute metamorphoses of
the diseased cells and the tissues they composed the true source of
those larger changes which, in the form of disease, threaten the living
organism with peril and death. Especially during the seven years of
his professorship at Würzburg (1849-56) Virchow pursued his great task
with such brilliant results that his _Cellular Pathology_ (published in
1858) turned, at one stroke, the whole of pathology and the dependent
science of practical medicine into new and eminently fruitful paths.
This reform of medicine is significant for our present purpose in that
it led us to a monistic and purely scientific conception of disease. In
sickness, no less than in health, man is subject to the same eternal
"iron laws" of physics and chemistry as all the rest of the organic
world.
Among the numerous classes of animals which modern zoology
distinguishes the mammals occupy a pre-eminent position, not only on
morphological grounds, but also for physiological reasons. As man
belongs to the class of mammals (see p. 27) by every portion of his
frame, we must expect him to share his characteristic functions with
the rest of the mammals. Such we find to be the case. The circulation
of the blood and respiration are accomplished in man under precisely
the same laws and in the same manner as in all the other mammals--_and
in these alone_; they are determined by the peculiar structure of
their heart and lungs. In mammals only is all the arterial blood
conducted from the left ventricle of the heart to the body by one,
the _left_, branch of the aorta, while in birds it passes along the
_right_ branch, and in reptiles along both branches. The blood of
mammals is distinguished from that of any other vertebrate by the
circumstance that its red cells have lost their nucleus (by reversion).
The respiratory movements are effected largely by the diaphragm in
this class of animals alone, because only in them does it form a
complete partition between the pectoral and abdominal cavities. Special
importance, however, in this highest class of animals, attaches to
the production of milk in the breasts (_mammae_), and to the peculiar
method of the rearing of the young, which entails the supplying of the
offspring with the mother's milk. As this nutritive process reacts most
powerfully on the other vital functions, and the maternal affection of
mammals must have arisen from this intimate form of rearing, the name
of the class justly reminds us of its great importance. In millions of
pictures, most of them produced by painters of the highest rank, the
"madonna with the child" is revered as the purest and noblest type of
maternal love--the instinct which is found in its extreme form in the
exaggerated tenderness of the mother-ape.
As the apes approach nearest to man of all the mammals in point of
structure, we shall expect to hear the same of their vital functions;
and that we find to be the case. Everybody knows how closely the
habits, the movements, the sense activity, the mental life, and the
parental customs of apes resemble those of man. Scientific physiology
proves the same significant resemblance in other less familiar
processes, particularly in the working of the heart, the division
of the breasts, and the sexual life. In the latter connection it is
especially noteworthy that the mature females of many kinds of apes
suffer a periodical discharge of blood from the womb, which corresponds
to the menstruation of the human female. The secretion of the milk in
the glands and the suctorial process also take place in the female ape
in precisely the same fashion as in women.
Finally, it is of especial interest that the speech of apes seems on
physiological comparison to be a stage in the formation of articulate
human speech. Among living apes there is an Indian species which is
musical; the _hylobates syndactylus_ sings a full octave in perfectly
pure, harmonious half-tones. No impartial philologist can hesitate any
longer to admit that our elaborate rational language has been slowly
and gradually developed out of the imperfect speech of our Pliocene
simian ancestors.