introduced a method for determining the amount of urea in a solution.
This substance is found in human blood and urine, and was the first
organic compound to be ‘synthetized’, that is to say, built up from
inorganic materials. It is of very great physiological importance.
This is due to the fact that it is regularly formed in the body in the
process of breaking down the characteristic nitrogenous substances
known as proteins. Along with his colleague, Friedrich Wöhler
(1800-82), who had already synthetized urea, Liebig wrote a famous
paper (1832) in which he showed, for the first time, that a complex
organic group of atoms--or ‘radicle’ as it is called--is capable of
forming an unchanging constituent through a long series of compounds,
behaving throughout as though it were an element. This discovery is of
primary importance for our conceptions of the chemical changes in the
living body.
From 1838 onwards, Liebig devoted himself to attempting a chemical
elucidation of living processes. In the course of his investigations
he did pioneer work along many lines that have since become well
recognized. He taught the true doctrine, then little recognized, that
animal heat is the result of combustion, and is not ‘innate’ (compare
p. 156). He classified articles of food with reference to the functions
that he conceived they fulfilled in the animal economy. An outcome of
this was his food for infants and his extract of meat. Very important
was his teaching that plants derive the constituents of their food,
their carbon and nitrogen, from the carbon dioxide and ammonia in the
atmosphere, and that these compounds are returned by the plants to the
atmosphere in the process of putrefaction. This discovery made possible
a philosophical conception of a sort of ‘circulation’ in Nature. That
which is broken down is constantly built up, to be later broken down
again. Thus the wheel of Life goes on, the motor power being energy
from without, derived ultimately from the heat of the sun.
It was very unfortunate that Liebig conceived and adhered to a totally
wrong view of the nature of putrefaction and fermentation, which it
took Pasteur long years to displace.
(c) _Nervous Physiology in the Earlier Nineteenth Century._
From Chemical Physiology we turn to glance at the knowledge of the
Nervous System. Charles Bell and his contemporaries (p. 145) had
attained to a clear distinction of the nature of motor and sensory
nerves and their separate origin from the two spinal roots (Fig.
98). The next fundamental contribution was made by Marshall Hall
(1790-1857), who established the difference between volitional action
and unconscious reflex (1833).
The fundamental ideas in the conception of reflex action had already
been adumbrated by Descartes. In the view of that philosopher, any
stimulus is transmitted along nerve-fibers to the central nervous
system. There, on account of existing nervous connections, it gives
rise to a fresh impulse which passes along outgoing nerve-fibers
to the active organ, muscle, or gland, which is thereby excited to
activity (p. 128). Thus, every action of the organism, and its life
as a whole, conforms to definite laws. These laws must be directed
to its preservation, or organisms would cease to exist. It is thus
possible to look on organisms simply as elaborate mechanisms. Except
that we know that we ourselves think and feel, we might eliminate mind
from our consideration of the action of beings other than ourselves.
Such was the view taken by the mechanists and other members of the
iatro-physical school (pp. 127-131), which followed, to a greater or
less extent, the teaching of Descartes. The course of physiological
advance may be described, briefly, as the expulsion of the mental
element from process after process associated with vital activity. This
avenue leads on to a philosophical discussion whither we shall not now
follow. It will suffice, at the moment, to remind the reader that only
through the channel of _his own_ thinking and feeling is he able to
follow these physiological discussions at all.
The conceptions of Descartes and of his successors were greatly
extended by Marshall Hall. Interest was lent to Hall’s work by the
contemporary discovery by French observers of what was regarded as a
special center governing respiration--a very important reflex--in the
lower part of the brain. Hall’s work gave ‘reflex action’ a permanent
place in Physiology.
[Illustration: FIG. 98. DIAGRAM OF TRANSVERSE SECTION OF THE SPINAL
CORD AND THE MAIN NERVES DERIVED FROM IT.]
[Illustration: FIG. 99. DIAGRAM TO ILLUSTRATE SIMPLEST FORM OF REFLEX
(cf. Fig. 98).
An afferent impression from a sense organ to the spinal cord may give
rise to an efferent impulse by a purely intra-spinal process. This
impulse may be of the nature of a complex and balanced muscular act
involving a whole system of muscles, some of which may be antagonistic
to each other. All this may take place not only unconsciously, without
any intervention from the higher nerve-centers in the brain, but even
in an animal from which the brain has been removed. On the other hand,
channels exist (and are indicated in the diagram) for passage of
impressions to and impulses from higher centers. These higher centers
in many cases control or modify the resulting muscular or other action
to a greater or less degree.
]
Since Hall’s time there has been a great extension of the conception of
reflexes. It has been shown that, besides the simple nervous arc (Fig.
99), there are more complex nervous arcs which depend for their action
on the integrity of an elaborate mechanism. The nervous system is
‘integrated’ under higher and higher centers, till at last the highest
centers of the brain are reached (pp. 308-11). Many of the ordinary
acts of life, sneezing, coughing, standing, walking, even breathing,
are expressible as reflexes. The attempt has also been made to press
the ‘instincts’ into the same category. But it is difficult to separate
the instinctive from the volitional elements or to define either. Vast,
therefore, as is the development of this department of physiology, it
is a very delicate task for the historian to pass any verdict upon it.
The ultimate value of all this work must depend upon the conception
that the next generation attaches to the mental element in vital
phenomena. There is evidence of reaction at the present time from the
extreme mechanist physiological position.
Lastly, in the discussion of work on the nervous system comes the
question of the localization of functions of the brain. The possibility
of such localization is a very ancient speculation. The idea was
developed along rational lines, in the first third of the nineteenth
century, by certain Viennese workers who, having made important
contributions to science, unfortunately afterwards degenerated into
phrenological quacks. Later several German observers began the study of
the electrical excitation of those parts of the cortex of the brain
which specially control movement (Fig. 100). The work was continued and
developed by a number of distinguished French and English observers,
among whom Paul Broca (1824-80), Hughlings Jackson (1834-1911),
and Sir David Ferrier (1843-) should be commemorated. Under their
influence many operations usually regarded as involving complex mental
processes, such as vision, speech, reading, writing, drawing, have been
represented as depending on simple nervous relationships. Centers for
the initiation of these operations have been described. Of late years,
there has been reaction from this mechanical conception of the brain as
an organ of the mind. The older school has, however, achieved clinical
success especially at the hands of the great French physician Jean
Marie Charcot (1825-93) and his pupils.
[Illustration: FIG. 100. DIAGRAM TO ILLUSTRATE SOME OF THE MAIN FACTS
OF CEREBRAL LOCALIZATION.]
§ 4. _The Experimental Foundations of Modern Medicine._
We may turn back to consider those who have been the immediate
progenitors of modern Physiology. Among these, three men stand out
beyond all others. In order of seniority, and perhaps of genius, they
are Johannes Müller, Claude Bernard, and Karl Ludwig.
(a) _The Work of Johannes Müller._
Johannes Müller (1801-58) was the greatest physiologist Germany has
produced, and perhaps the greatest physiologist of all time. His
genius was of the universal type and, despite his early death, he
attained equal distinction in every department which he touched. Among
these were Comparative Anatomy, Embryology, Physiological Chemistry,
Psychology and Pathology. He was a careful scholar, well versed in the
history of the subjects which he taught, and as great a teacher as he
was an investigator. A very large number of the best-known men who have
advanced Medicine during the nineteenth century were his pupils while
he was a professor at Berlin. His lovable character was pervaded by a
mystical tendency.
Müller’s text-book of Physiology began to appear in 1834. It introduced
into the subject the comparative and psychological points of view,
which were not fully appreciated until the generation that followed.
The most remarkable generalization associated with his name--and
one further developed by Ewald Hering (1834-1918)--is the ‘Law of
Specific Energies’. According to this law each sensory nerve, however
stimulated, gives rise to its own specific sensation and to no other.
Conversely, the same stimulus applied to different organs of sense
produces a different sensation in each organ--that sensation, in fact,
that is its specific attribute. Thus electrical, mechanical, thermal
stimulation produce only the sensation of light when applied to the
optic nerve. On the other hand, any particular form of stimulation, for
example electrical, produces sensations of light, smell, hearing or
taste if applied to the appropriate nerves.
A moment’s reflection will enable the reader to realize the very
great philosophical importance of these conclusions. They provide
experimental evidence that the things of the external world are not
in themselves discernible by us. Such external things we know only by
the events to which they give rise acting on our senses, and yet from
one and the same event utterly different sensations arise within us.
To beings with senses different from ours the world also would be
different. The ‘Law of Specific Energies’ is thus fundamental for our
view as to the range of validity of Scientific Method.
Among other important contributions of Johannes Müller to the
physiology of the nervous system were his experimental confirmation
of Bell’s researches on the spinal roots (p. 145) and his experiments
on the production of the voice. He launched important theories in
explanation of color vision, of the mechanism of hearing, and of the
phenomena of fever. He was one of the first to use the microscope in
pathology and he was one of the founders of Physiological Chemistry.
Like every investigator Müller made mistakes. In 1840 he stated that
the velocity of a nervous impulse could never be measured. By 1852 his
gifted pupil, Hermann von Helmholtz (1821-94), had measured it. Much of
Helmholtz’s work hardly comes within our department. He was, however,
inventor of the instrument known as the _Ophthalmoscope_, by means of
which the interior of the eye can be examined. This is the main factor
which has enabled Ophthalmology to develop along true scientific lines
(p. 319).
(b) _The Work of Claude Bernard._
Claude Bernard (1813-78), the great French physiologist, was Müller’s
junior by twelve years and was in almost every respect a contrast to
him. His mind was of that peculiarly French type to which anything
mystical is abhorrent. He had few eminent pupils who owed much to
him directly, but the influence of his ideas, through his writings,
can hardly be exaggerated. Especially Bernard was the founder of
‘Experimental Medicine’, that is of the artificial production of
disease by chemical and physical means. This is one of the most
important scientific movements within our field.
Bernard’s great discovery, which occupied him for over ten years,
was that the liver has the power of building up and storing certain
highly complex substances, derived from the food and brought to it
by the blood. The substances thus stored, and notably that known as
_glycogen_, are distributed to the body according to its needs, in
simplified and modified form. Now Wöhler in 1828 had synthetized urea
(p. 206) and it was well recognized that this substance is a final
degradation product which the body manufactures by breaking down the
substances derived from food. It was also recognized that from this
breaking-down process the bodily energy is obtained. Bernard showed
that the body could build up complex chemical substances as well as
break them down. This destroyed the conception, then still dominant,
that the body could be regarded as a bundle of organs, each with its
appropriate and separate functions. Bernard thus introduced what we may
call a ‘Physiological Synthesis’, a conception of great import for the
development of medical ideas.
No less important, and bearing on the synthetic view of the working of
the animal body, was Bernard’s work on the physiology of digestion.
Up to the time of Bernard, an elementary knowledge of the facts of
digestion in the stomach constituted the whole of digestive physiology.
While Bernard was working on the glycogenic function of the liver,
another worker had suggested that the secretion of the organ known as
the ‘pancreas’, or sweetbread, emulsifies fats. Soon after, a German
researcher showed that pancreatic juice acts on starch. Bernard now
stepped in and cleared up the whole subject. He showed that digestion
in the stomach is, as he described it, ‘only a preparatory act’. He
proceeded to demonstrate that the pancreatic juice, passing into the
intestine, emulsifies the fatty food substances there and splits them
up into fatty acids and glycerin. He further demonstrated the power
of the pancreatic juice to convert starch into sugar, and he showed
that it has a solvent action on such ‘proteids’ or organic nitrogenous
substances as have not been dissolved in the stomach.
The third great achievement of Bernard was his exposition of how the
blood-supply to the different parts of the body is regulated. This we
now call the ‘Vaso-Motor Mechanism’. In 1840 the existence of muscle
fibers in the coats of the smaller arteries was discovered. Bernard
showed that the contraction and expansion of the ‘arterioles’ is
associated with a complex nervous apparatus. The reactions of this
apparatus depend upon a variety of circumstances in a variety of other
organs; again an illustration of the close and complex interdependence
of the various functions of the body upon each other.
(c) _The Work of Karl Ludwig._
Karl Ludwig (1816-95) held a series of professorships at Marburg,
Zürich, Vienna and Leipzig. He was, after Müller, the greatest of
German physiological teachers, and he surpassed even Müller in the
number of his pupils. As a physiologist he was chiefly remarkable for
his ingenuity as an inventor, for his wide and deep knowledge of the
physical sciences and for his extreme generosity in handing over his
work to his pupils.
Among the many lines of investigation of fundamental importance which
Ludwig initiated, some of the most remarkable depended on the discovery
of new methods. Just as the microscope had opened to the anatomist
unexplored fields of research by bringing him into closer relation with
objects which were hitherto beyond his scrutiny, so the rapid progress
of physics and chemistry had placed more exact modes of observation and
of measurement within reach of the physiologist. But the application
of these methods was attended with great difficulty; there was no
physiological laboratories, no instruments, no capable mechanicians
to whom the physiologist could apply for assistance. Under such
conditions, ingenuity and resource were indispensable to success, and
in these qualities Ludwig was pre-eminent.
Accordingly, we find that two of the most important of the early
investigations of Ludwig were as much due to his ingenuity as an
inventor as to his clear grasp of the physiological questions which
his inventions were intended to elucidate. The most interesting of
these inventions, or rather adaptations, is the mechanically rotating
drum or _kymograph_, as it is called. The word itself is derived from
two Greek words which mean ‘wave writer’. This instrument is now
widely used, not only in Physiology but in every department of Science
in which permanent records of any continuous movement are desired.
The most familiar instance is the self-recording barometer. The
kymograph--the use of which had been suggested by Thomas Young (p. 319,
and Fig. 101) in 1807--led to much wider applications of the method of
automatic record. Ludwig himself applied it to indicate the movements
of respiration, as well as the variations in arterial pressure.
Subsequently it became further adapted to the ‘graphic method’, and
it serves not only for the investigation of animal movements of every
conceivable kind, but even for the transient and delicate electrical
changes which are associated with vital action.
An instrument invented by Ludwig is the mercurial blood-pump, the
purpose of which is to separate from a known quantity of blood, derived
directly from the circulation, the mixture of gases which it yields to
a vacuum. This is an indispensable apparatus for the investigation of
the physiology of breathing.
Ludwig devoted much attention to the physiology of secretion. Here his
work has been of great importance in connection with the time-honored
discussion between the ‘vitalists’ and the ‘mechanists’. He succeeded
in showing that the process of secretion can be so transformed
experimentally as to do external mechanical work. This was victory
for the mechanist theory. The idea has since been applied to many
structures.
It is impossible to attempt here any general summary of the conclusions
reached by physiological research since Ludwig. Some have affected
the actual practice of Medicine. Others are too recent or too little
certain to have reacted in this manner. It is, however, safe to say
that the more important conclusions of the three modern founders of
the science, Müller, Bernard, and Ludwig, form the main scientific
background of the clinical practice of our time. The results of the
movement that they represent, together with the knowledge of the
cellular structure of the body (§ 5, p. 219) and of the life-histories
of the disease-causing organisms (§ 6, p. 224), are the three main
groups of ideas which separate the physician of our day from Laënnec
(p. 161).
[Illustration: FIG. 101. THOMAS YOUNG’S KYMOGRAPH.
The cylinder H turns with the axis AB on which it is rigidly fixed. It
is rotated by a handle at A which raises the weight C. When the weight
is allowed to fall the cylinder rotates automatically. The rest of the
apparatus is devised to secure constancy in rate of rotation. This was
done by utilizing the effects of centrifugal force.
(As the rate of rotation increases the pendula D and E fly apart, they
separate the weights F and G. These move with friction which increases
as they separate, thus decreasing the rate of rotation.)
The movements of the pen at K are transferred into permanent graphic
form by writing on the rotating cylinder H.
]
§ 5. _The Cell Theory and Cellular Pathology._
During the process of the microscopic analysis of plants that took
place in the seventeenth century a number of observers distinguished
the walls of plant-cells and the word _cell_ was introduced into the
English language. Less clearly, a similar structure was discerned in
animals. No real understanding of the nature of cells was, however,
reached. Little farther progress was made in the eighteenth century,
but just at its close a young French microscopist, Marie François
Xavier Bichat (1771-1802), likened the microscopic structure of the
animal body to the substance of a woven fabric. The word he used was
the old French term _tissu_. He perceived that the different parts
of the body, bones, muscles, nerves, blood-vessels, and the like,
each presented a characteristic microscopic pattern. According to
these appearances he analyzed the parts of the body into twenty-one
‘tissues’. Study of this kind came to be called ‘Histology’ (Greek
_histos_ = web).
During the seventeenth, eighteenth, and early nineteenth centuries,
some advances were made in the knowledge of those organisms whose
bodies are made up of only one cell, but their essential nature was
still unappreciated. In the early nineteenth century a number of
botanists and others were observing cells and cell contents. But no
important advance in the interpretation of the appearances was made
until the matter was taken up by Schleiden.
Matthias Jakob Schleiden (1804-81), professor at Jena in 1838, put the
matter in a new light. He noted, as had certain of his predecessors,
the constant presence in every cell of the structure we now call the
‘nucleus’, and came to the conclusion that it is essential to the
life of every cell. He reached the conception, moreover, that in a
multi-cellular organism, such as a tree, every cell has a double life,
one an essential and independent one, pertaining to its own development
alone, the other an incidental and dependent one, in so far as it is
an integral part of the plant. His work was somewhat vitiated by a
fanciful conception of the origin of new cells.
The work of Schleiden was amplified and corrected in 1839 by Theodor
Schwann (1810-82), a pupil of Müller. He showed that the tissues of
animals, like those of plants, are susceptible of analysis into cells,
and the difficulty of this process arises from the extreme modification
of such cells as have developed for various special purposes. He showed
too that the ovum or egg of animals is, in the first instance, a single
cell, and that the cells of the body are derived and descended from it.
He demonstrated that the entire animal or plant body is composed either
of cells or of substances that are excreted or thrown off by cells.
He gained some insight into the life of animal cells and in doing so
he invented the very useful word _metabolic_. The word means ‘liable
to change’. It was used by Schwann, and is still habitually used in
modern Medicine to indicate chemical changes within the body which are
specially associated with living activity.
Contributions to the cell theory were made by other botanists. Hugo
von Mohl of Tübingen (1805-72) distinguished the contents of the
vegetable cell just under the cell-wall from the watery sap that fills
the interior, introducing for it the term _protoplasm_ (1846). The
Swiss, Karl v. Nägeli (1817-91), by chemical examination proved that
protoplasm is nitrogenous and differs from other cell constituents
(1862).
The cell theory was placed on a firm and clear footing by Max Schultze
(1825-74), successor of Helmholtz (p. 213) as professor of Anatomy at
Bonn. He defined the cell as ‘a lump of nucleated protoplasm’ (1861),
introduced the idea of protoplasm as ‘the physical basis of life’, and
showed that it presented essential similarities, physiological and
structural, whether in plants or animals, and whether in higher or
lower forms.
The study of tissues, Histology, was raised to the status of an
independent science by the Swiss, Albrecht von Kölliker (1817-1905),
pupil of Müller and professor at Würzburg, who wrote the first
text-book on the subject (1850-52). Apart from this achievement,
Kölliker is remarkable for having reached some of the conclusions in
connection with heredity that are associated with the name of Mendel,
of whose work he knew nothing.