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1. Introduction
World of Anatomy and Physiology (WAP) is devoted to the study of the intricate
relationships of form and function within the human body. Although anatomy is
the most ancient of all medical studies, the early years of the twenty-first century
are an exciting time to undertake the study of the structure and function of the
human body. Around the world, thousands of dedicated research and clinical
science specialists provide a constant stream of insights into the most intimate
aspects of our anatomy and physiology.
Recent rapid progress in cell biology allows, for the first time, insight into the
fundamental mechanisms of development. Never before in human history has
information moved so rapidly from the laboratory to the clinical setting. The
development of reproductive medicine and gene therapy promise to propel us
into a new and revolutionary era of biotechnology and biomedical science.
World of Anatomy and Physiology is a collection of 650 entries on topics covering
a range of interests, from biographies of the pioneers of anatomy and physiology
to explanations of the latest developments and advances in embryology and
developmental biology. Despite the complexities of terminology and advanced
knowledge of biochemistry needed to fully explore some of the topics in
physiology, every effort has been made to set forth entries in everyday language

2. and to provide accurate and generous explanations of the most important terms.
The editors intend World of Anatomy and Physiology for a wide range of readers.
Accordingly. Accordingly, WAP articles are designed to instruct, challenge, and
excite less experienced students, while providing a solid foundation and
reference for more advanced students.
The very essence of the attributes that define life is the province of two
complementary branches of science, anatomy and physiology. All of the sub-
disciplines of medical science are built on the fundamental foundations of
anatomy and physiology.
Anatomy studies the structure of body parts and their relationships to one
another, while physiology concerns the function of the body's structural
machinery and how all the body parts work to carry out life sustaining activities.
Questions such as how the living body is able to see, hear, keep warm, or digest
food are at the core of anatomical and physiological research.
As our most accomplished athletes and artists prove, the human body has the
ability to move gracefully, to lift an arm accurately, and perform many various
tasks. The anatomist and physiologist observe and question how these tasks are
performed by body's nervous system, muscles, and joints. To date, the
investigation of such questions has depended largely on anatomical studies,
which involved examination of anatomical structure by dissection. Technological
advances in microcopy and imaging now allow anatomists to view living body
and cellular structure on the molecular scale.
Advances in molecular biology allow researchers to investigate a broad range of
phenomena, including the physio-chemical processes taking place in the cells
and tissues of the body, the electrical events underlying the actions of the
nervous system, and the feedback mechanisms that allow fine control of complex
physiological processes.
With the rapid expansion of scientific knowledge, various disciplines have split off
from the parent disciplines of anatomy and physiology. Biochemistry, the study of

3. chemical processes in cells and tissues was probably the first discipline to
diverge, then also biophysics, which deals with physical processes within cells.
More recently, neurophysiology became established as a separate and
specialized area of study. Quite often, anatomical or physiological research is
carried out in departments of medicine, where basic scientific knowledge is
required for the understanding of disease. The reverse situation is also common:
basic research in anatomy or physiology reveals how a disease occurs. For
example, basic research into the pancreas and the hormone insulin led to the
discovery of successful treatment for the disease diabetes mellitus. Finally,
anatomists and physiologists have long been interested in the science underlying
such things as athletic performance, the function of the respiratory and nervous
systems under anesthesia, the effects of low or high barometric pressures, lack
of oxygen and so on. This all comes under the heading of applied anatomy and
physiology.
In all these aspects of the study of anatomy and physiology, it is important to
appreciate the role of mechanisms that control bodily functions. For example, we
cannot divorce the study of muscular contraction from the feed-back systems that
control the structural elements of muscle. All muscles contain structures that
signal back to the central nervous system the muscle length and the degree of
contractile force. At the spinal level, reflex actions occur to control the
performance as precisely as possible in terms of length and force. Without these
sophisticated control systems, it becomes very difficult to use muscles properly,
precisely, or accurately.
Similar mechanisms exist to control arterial blood pressure. Obviously, if blood
pressure is too high, there will be an increased likelihood of rupture of blood
vessels and consequent hemorrhage as occurs in strokes. If blood pressure falls
too low, the blood supply to the brain will be impaired and consciousness may be
lost. When one considers the fact that the human body contains trillions of cells
in nearly constant activity, and that remarkably little goes wrong with it, one
appreciates what a marvelous system it is. Walter Cannon, an American
physiologist of the early twentieth century, spoke of the "wisdom of the body" and

4. he coined the term homeostasis to describe its ability to maintain relatively stable
internal conditions even though the outside world changes continuously.
Although the literal translation of homeostasis is "unchanging", the term does not
actually mean a static, or unchanging, state in the body. Rather, it indicates a
dynamic state of equilibrium, or a balance, in which the internal conditions vary,
but always within relatively narrow limits. In general, the body is in homeostasis
when its needs are adequately met and it is functioning smoothly.
Maintaining homeostasis is much more complicated than it appears at first
glance. Virtually every organ plays a role in maintaining the constancy of the
internal environment. Adequate blood levels of vital nutrients must be
continuously present, and heart activity and blood pressure must be constantly
monitored and adjusted so that the blood is propelled to all body tissues.
Metabolic wastes must not be allowed to accumulate, and the body temperature
must be precisely controlled. A wide variety of chemical, thermal, and neural
factors act and interact in complex ways, sometimes helping and sometimes
hindering the body as it works to maintain its "steady rudder" to steer carefully
between the dangers of anatomical and physiological extremes.
A system of communication is essential for homeostasis to function.
Communication is accomplished chiefly by the nervous and endocrine systems,
which use electrical and chemical mechanisms to transmit impulses, information,
and instructions. All homeostatic control mechanisms have at least three
interdependent control mechanisms: receptors, control centers and effectors.
Most homeostatic control systems function as negative feedback mechanisms,
where the output of the system shuts off the original stimulus or reduces its
intensity. In positive feedback systems, the response enhances the original
stimulus so that the activity, or output, is accelerated. Blood clotting is an
example of a process controlled by positive feedback mechanisms.
Although World of Anatomy and Physiology concentrates on topics in classical
anatomy and physiology, the editors have tried to provide insight into important
areas of developmental and reproductive biology. Almost daily, new discoveries

5. extend our understanding of reproductive biology and embryology. At an equally
rapid pace, biotechnologies emerge to expand applications of those discoveries.
The pace of change and innovation is challenging to all publications. For
example, during the writing of World of Anatomy and Physiology v, researchers
announced the creation of cloned human embryos that grew to the six cell stage.
Days before going to press, researchers at the Whitehead Institute for
Biomedical Research announced results in the British journal Nature that
indicated that fully differentiated adult cells can be used to form clones.
Accordingly, World of Anatomy and Physiology has attempted to incorporate
references and basic explanations of the latest findings and applications.
Although certainly not a substitute for in-depth study of important topics such as
stem cell research or cloning, the editors hope to provide students and readers
with the basic information and insights that will enable a greater understanding of
the news and stimulate critical thinking regarding current events in biomedicine.
In the Classical world, the Greek philosopher Aristotle (384-322 B.C.) raised
fundamental questions about form, function, reproduction, and development. The
quest for insight and answers into the embryological development of humans
continues to fascinate and challenge modern scientists. We hope that World of
Anatomy and Physiology inspires a new generation of scientists who will join in
the exciting quest to unlock the remaining secrets of life. It is our modest wish
that this book provide valuable information to students and readers regarding
topics that play an increasingly prominent role in our civic debates, and a
fundamentally important and intimate role in our everyday lives.
K. Lee Lerner & Brenda Wilmoth Lerner, editors
New Orleans
February, 2001
Acknowledgments and Dedication

6. The editors are indebted to Robert G. Best, Ph.D., Director, Division of Genetics, Department of
Obstetrics and Gynecology, University of South Carolina School of Medicine, for his skillful
advising throughout the project.
Many of the academic advisors for World of Anatomy & Physiology, along with others, authored
specially commissioned articles within their field of expertise. The editors would like to specifically
acknowledge the following special contributions: Eric v.d. Luft, Ph.D., M.L.S., Curator of Historical
Collections, SUNY Upstate Medical University, for his diligent and extensive research related to
his compilation of the "History of anatomy and physiology" series of articles. Constance K. Stein,
Ph.D., Director of Cytogenetics, Assistant Director of Molecular Diagnostics at SUNY Upstate
Medical University for her article "Stem Cells;" Danila Morano, M.D., Department of Embryology,
Obstetrics, and Gynecology, University of Bologna, Italy, for her article "Pregnancy: maternal
physiological and anatomical changes;" and Antonio Farina, M.D., Ph.D., Visiting Professor,
Department of Pathology and Laboratory Medicine, Brown University School of Medicine and the
Department of Embryology, Obstetrics, and Gynecology, University of Bologna, Italy, for his
article "Ethical issues in embryological research."
The editors gratefully acknowledge the assistance of Ms. Robyn Young and the Gale Imaging
Team for their guidance through the complexities and difficulties related to graphics. Last, but
certainly not least, the editors thank Ms. Kimberley McGrath, whose continued wit and guidance
kept World of Anatomy & Physiology a labor of love.
The editors lovingly dedicate this book to Mary Josephine Wilmoth.
Contributing editors K. Lee Lerner (scholar.harvard.edu/kleelerner) and Brenda Wilmoth Lerner's (scholar.harvard.edu/brendawilmothlerner) combined
portfolio includes respected journalism, book, and media awards, including multiple RUSA Book and Media Awards, as well as books named
Outstanding Academic Titles. They are cofounders of LMG (London, Paris, Cambridge) Additional information may be found at
scholar.harvard.edu/kleelerner and scholar.harvard.edu/brendawilmothlerner
CC BY-NC-ND otherwise ©LMG All Rights Reserved. Permission to republish this excerpt, with appropriate acknowledgments, is granted for
academic use. Commercial use is strictly prohibited. Republished with permission of Cengage Gale.