Physiology
Physiology is study of the physical
and chemical processes that take place in living organisms during the
performance of life functions. It is concerned with such basic activities as
reproduction, growth, metabolism, respiration, excitation, and contraction as
they are carried out within the fine structure, the cells, tissues, organs, and
organ systems of the body.
Physiology is closely linked with
anatomy and was historically considered a part of medicine. Its emphasis on
investigating biological mechanisms with the tools of physics and chemistry
made physiology a distinct discipline in the 19th century; the
tendency today, however, is towards a fragmentation and merging with the many
specialized branches of the life sciences. Three broad divisions are
recognized: general physiology, concerned with basic processes common to all
life forms; the physiology and functional anatomy of humans and other animals,
including pathology and comparative studies; and plant physiology, which
includes photosynthesis and other processes of plant life.
History of Physiological Investigation
The first studies in animal
physiology were probably undertaken about 300 BC by the Alexandrian doctor Herophilus, who
reportedly vivisected the bodies of criminals. For about 1,900 years
thereafter, few physiological studies were performed.
Beginnings of Modern Physiology
Modern animal physiology dates from
the discovery of the circulation of the blood by 
the English doctor William Harvey in 1616. Shortly
thereafter, the Flemish chemist Jan
Baptista van Helmont developed the concept of gases and suggested the use
of alkalis in treating digestive disturbances; the Italian biophysicist Giovanni
Alfonso Borelli published studies of animal motion, suggesting that the basis
of muscle contraction lay in the muscle fibers; the Dutch microscopist Antoni van Leeuwenhoek gave the first descriptions of red
blood cells and spermatozoa; and the Italian histologist Marcello Malpighi demonstrated the
existence of capillaries and studied the physiology of the kidney, liver, and
spleen. During the second half of the century the study of glands was initiated
by the English doctor Thomas Wharton, who demonstrated salivary secretion, and
by the Danish anatomist Nicolaus Steno, who
demonstrated the secretions of the tear glands and salivary glands. The Dutch
doctor Regnier de Graaf
furthered glandular study by his discovery of the follicles in the ovary; he
also performed studies on pancreatic juices and bile. The English doctor
Richard Lower was the first to transfuse blood from one animal to another, and
the French doctor Jean-Baptiste Denis first gave a
human being a successful blood transfusion.
In the 17th century
advances were made in the study of respiration. The English physiologist John Mayow showed that air was not a single substance but a
mixture of several, not all of which were necessary for life. In the 18th
century, the British chemist Joseph Priestley showed that the proportion of
oxygen essential for animal life is identical with the proportion of oxygen
needed to support combustion. Antoine Laurent Lavoisier,
the French chemist, isolated and named oxygen shortly thereafter and showed
that the by-product of respiration is carbon dioxide.
18th and 19th Century Physiology
Modern physiology owes much to the
work performed during the 18th century by the Dutch doctor
Hermann Boerhaave and his pupil, the
Swiss scientist Albrecht von Haller.
Through their criticism of the iatrochemists
(scientists who believed physiology involved only chemical reactions) and the iatrophysicists (scientists who believed physiology
involved only physical reactions), they laid the foundation for an integrated
study of physiology. Haller was the first scientist to state that all living
matter possesses irritability.
During the second half of the 18th
century, the Italian doctor Luigi Galvani showed that
the muscles of a frog's leg could be made to contract by stimulation with an
electric current, and the Italian physiologist Lazzaro
Spallanzani investigated the activity of gastric
juice in digestion. Spallanzani also studied
fertilization and artificial insemination in lower animals.
The principal figure in animal
physiology in the 19th century was the French physiologist Claude Bernard, 
who investigated carbohydrate
metabolism in humans; he also studied 
the autonomic nervous system and
described many of its functions. His greatest contribution was his statement of
the principle that living organisms are never at rest but constantly undergoing
dynamic changes to maintain internal equilibrium. The basis of health,
according to Bernard, is the organism's success in maintaining this balance.
Bernard's principles were amplified during the first half of the 20th
century by the American physiologist Walter Bradford Cannon, who named the
dynamic state homeostasis and showed that the body could adjust to meet serious
external danger. Cannon demonstrated such processes of the human body as
internal regulation of body heat, alkalinity of the blood, and preparation of
the body for defense by the secretion of adrenaline in the adrenal gland.
During the 19th century,
the physiology of the nervous system was studied by the Scottish anatomist
Charles Bell, who described the functions of sensory and motor nerves; the
French physiologist François Magendie,
who described the functions of the spinal nerves and investigated the
mechanisms of swallowing and regurgitation; the French physiologist Pierre
Flourens, who investigated the functions of 
the cerebellum and who was a pioneer
in the physiological investigation of animal psychology; and the German
physiologist Johannes Peter Müller, who showed that perceptions were determined
only by the sensory organ that received the sensory impulse. Ernst Heinrich
Weber, a German physiologist, discovered that two types of nerve stimulate the
human heart: those that increase the heartbeat and those that inhibit it. He
was thus one of the first to recognize that the autonomic nervous system is
composed of two nerve systems. Weber also conducted investigations into the
mechanics of perception.
The first laboratory for the
investigation of the physiological basis of psychology was founded by the
German physiologist and psychologist Wilhelm Wundt during the last quarter of
the 19th century.
During the late 19th and
early 20th centuries, the impetus of the new science of bacteriology
led to research into immunity. The most important
figures were the Russian naturalist Élie
Metchnikoff, who developed the theory of phagocytosis—the destruction of
foreign materials in the blood—and the German bacteriologist and chemist Paul
Ehrlich, who evolved a theory of antibody formation.
At about the same time, the
physiology of the ductless glands was investigated by the British physiologist
Sir Edward Albert Sharpey-Schafer, who showed that an extract of the adrenal
glands, later discovered to be adrenaline, raised the blood pressure after
injection. Several years later, the British physiologist Sir William Maddock
Bayliss and Ernest Henry Starling discovered that an intestinal extract, called
secretin, caused the flow of pancreatic juice upon injection. They proposed the
term hormones for secretions that could act upon other organs through the
bloodstream. Later work on hormones yielded important information on the
mechanics of growth and reproduction.
Recent Advances
Among the most important advances of
the 20th century are the discovery of new hormones; recognition of
the role of vitamins; discovery of blood types; development of the
electrocardiograph and electroencephalograph to record the activity of the
heart and brain; discovery of the cause and cure of pernicious anemia by the
American doctors George R. Minot, William P. Murphy, and George H. Whipple; and
greater understanding of metabolism, the role of enzymes, and the immune
system.
The first part of the 20th
century also witnessed great advances in the understanding of reflexes, first
elaborated by the French philosopher René Descartes as a philosophic concept to
distinguish involuntary reflexes of animals from the more rational reactions of
humans. The concept was refined by the work of German zoologists, who described
it in physical terms and divided behavior into its component reflexes. Further
understanding was facilitated by the British neurophysiologist Charles Sherrington, who showed that
reflexes enable the nervous system to function as a unit. The concept of a
conditioned response, first described in the 18th century by the
Scottish physiologist Robert Whytt, a pioneer in the study of reflex action,
culminated in the later work of the Russian physiologist Ivan Pavlov and that of the Russian neuropathologist Vladimir
Bekhterev. Although there was no validity to Pavlov's attempt to extend the
principles of conditioning—the method by which responses may be elicited more
frequently or more predictably by reinforcement—to complex mental processes,
his work had great impact on psychology and learning. It was one of the primary
influences in the founding of behaviorism by the American psychologist John B.
Watson. The work of the American psychologist B. F. Skinner in programmed
instruction, the basis of so-called teaching machines, was also based on the
theory of conditioning and reinforcement.
The 20th century has also
witnessed other fundamental advances in neurology. The British physiologist
Lord Edgar Douglas Adrian measured and recorded electric potentials from sense
organs and motor nerve fibers. Sherrington investigated the integrative action
of the nervous system. Their work was followed by that of the American
physiologists Joseph Erlanger and Herbert Spencer Gasser, who demonstrated
functional differences in nerve fibers and used the oscilloscope to record the
variation of electrical impulses that occurs in these fibers. Later
investigations by the American biochemist Julius Axelrod, the Swedish
physiologist Ulf von Euler, and the British doctor Bernard Katz demonstrated
the role of specific chemicals in the transmission of nerve impulses. These
investigations were vital to such basic processes as the control of blood
pressure and the mobilization of strength to meet an emergency.
