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Illustrated Encyclopedia of Human Anatomic Variation

Introduction to the Series

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD

Peer Review Status: Internally Peer Reviewed

We have seen, over many years, thousands of anatomic variants whose frequency can be predicted, having been observed on many occasions, for hundreds of years, by many investigators, in many countries, and in many laboratories. These variations arise primarily from our genetic composition, an inheritance carried over from our ancient origins.It is important to understand that no two living organisms are structurally or functionally identical - animals or plants!

We believe that the use of the terms anomaly and abnormality convey an idea of pathology, something strange, probably never to be seen again, a true "lusus naturae" (sport of nature). Many or most variations are totally benign; some are errors of embryologic developmental timing or persistence of an embryologic condition. Some of these variations may seriously compromise parts of the muscular, vascular, nervous, skeletal and/or other organ systems; they are all fascinating because they teach us about our development, and something about our genetic heritage.

In the middle 1800s, comparative anatomy flourished as a science and it was used to explain the appearance of human anatomic variants but this time is now in the past! However, the lessons learned suggest that we, as humans, carry a significant, not always desirable, amount of excess developmental baggage in our DNA from some prehistoric period.

Every aspect of life depends on muscular activity, whether it be speech, eating and digestion, respiration, all expressions of brain function. Muscle responds to sensory stimulation, provides for defensive action, and mobility. Muscular activity is the visible manifestation of being alive. Death exists when muscular activity ceases.

What we are trying to convey to interested readers is that the things we describe here are "normal " even though they may differ from the mean or usual. They are found in "normal" long-lived individuals, and they are statistically (for the most part) predictable. Man is not machine made but rather more subjectively fashioned with many developmental and environmental factors intervening in the process.

Image 48


The problem of anatomic variability has a long history and many students, anatomists, and physicians appear to subscribe to the notion of Galen that the human body has been created in the best possible way, reflecting the perfection of the Creator, and that variations are the result of imperfect or unnatural development. That any variance in the structural arrangement can only be for the worst, it follows logically that these are outside the anatomic standard representing the Creator's perfect human, i.e., man being created in God's image. It is thought that even Vesalius believed in a "canon" of the human body and this is revealed by a passage of the Fabrica where he details some of his pedagogic principles. After discussing some rather rare variations of the azygos vein, he says "that they should be judged like a sixth finger or any other monstrosity."

Image 216

"A Sixth-Finger Nipper."
from Ambroise Paré.

It has been noted that Vesalius passed over variations in public dissections lest students believe that they occur in all bodies. It is a disadvantage for students to attend such a dissection "which varies very much from the canon of man, unless they have frequently witnessed dissections of perfect and not monstrous cadavers."

Image 3

Woodcut portrait of Andreas Vesalius

Vesalius described several skulls that varied from each other and concluded that "all but one were to be considered unnatural." Vesalius also described an extremely rare anomaly, in fact , "one of the most exotic of all skeletal aberrations, os Vesalianum carpi" (Straus and Temkin). According to these same authors, typical is considered normal. This definition is probably a too limited view of the problem of variability. From the work of Galen , the idea of the normal or perfect man came down to Vesalius and, faced with the problem of human variation,Vesalius had to decide what was perfect, natural or normal. We are not entirely sure what is normal. It is still not clear exactly what the word normal means in terms of human anatomy.

Vesalius used the terms always, most frequently, frequently, usually, sometimes, rarely, very rarely, etc. and describes variation without speculation as to significance. Vesalius said that the human sacrum normally has five vertebrae but one of six is by no means rare. In a study of 631Caucasian bodies, Schultz found sacra with four vertebrae in 1%, five in 77%, six in 21.7%, and seven vertebrae in 0.2%.

The thorny issues of variability were handled gingerly by Vesalius because the Inquisition and Holy Office were still terrorizing anyone thought to be a heretic and this may have had a significant role to play in Vesalius' later scientific life and, ultimately, his untimely death.

Image 192

Ecclesiastical Book Censors and Book Burning

Part of a large, panoramic dome painting in the Ecclesiastical Library in Eger, Hungary

If man was made by God, in His image, how can man's anatomy be variable? The variations, according to Galen and Vesalius, must be unnatural, the handiwork of the Devil. Today, we generally adopt a broader view of God's handiwork. Galen, Vesalius, other anatomists, and the Church did not have the powerful perspective of historical data on anatomy, embryology, or genetics. A most interesting paper by Straus and Temkin entitled "Vesalius and the problem of variability" should be read by those interested in this subject. - Bull. History Med. 14:609-633, 1943.

Some definitions, perhaps, may be useful here but, we believe they are not entirely adequate, they lack precision and their use is, at times, counterproductive:

Variation- The capacity to vary. The act of varying or the extent to which something has varied, a measurable change or modification. Within some normal range of variance.

Normal- A statistical measure of usually observed structures, typical, or represenative type.

We believe that the following terms are very difficult to apply at the present time with precision or assurance:

Anomaly- Outside the norm, inconsistency, irregularity, or abnormal. Any structure, function, or state outside the usual range of variation from the norm. In our view a precise norm has not, and probably will not, be determined.

Teratology- The study of abnormal development. Particularly, it is the study of causes, mechanisms, and manifestations of abnormal development, whether genetically, gestationally, or postnatally induced, and whether expressed as lethality, malformation, growth retardation, or functional aberration. The issue of "abnormal development" is the problem here.

Teratism- The process or processes of abnormal embryogenesis by which malformed structures arise. What is "abnormal embryogenesis"?

Aberration- Abnormally deviant, abnormality, abnormal variant.

Abnormal- Any state, structure, or function that differs substantially from the norm of its kind, exhibiting abnormality.

We believe that the number of ways viable (however imperfect or monstrous by Galen's and Vesalius' definition) and functional human beings can be assembled is very circumscribed, hence viable variations are not unlimited, but rather finite in number and, as such, they are "normal." Viability clearly circumscribes or limits variability and permits the term variation to be applied in a useful way! The entire mechanism of human development is not inflexible but rather pliable. Because variations are finite, we believe it is possible to catalog "all" the variants and have a true, meaningful, and much broader concept of human anatomy than commonly found in modern textbooks. The present work is still not finished.

According to Kopsch (see Books, for reference information), muscle variations are found, generally, to be of seven basic types:

  1. A muscle may be absent,
  2. A muscle may be doubled,
  3. A muscle may be divided into two or more parts,
  4. A muscle may have an increase or decrease in its origin or insertion,
  5. A muscle may join neighboring organs,
  6. A muscle or its tendon may have a deviant distribution, and
  7. A completely new muscle may appear (questioned by present authors).

Kopsch suggests that each of these seven basic types of variation may be subdivided or defined with greater precision depending on the circumstances.

Because of the extensive, worldwide, literature available to us, we believe that the relevant data on human variation can be extracted from that vast literature before the paper on which the data are printed becomes dust. Due to an acid residue from paper manufacturing many older books and journals are already brown and crumbling; the books are moribund!

Most modern textbooks of anatomy are no loss; to put it bluntly, they are aberrations of our time, inaccurate, incomplete, and potentially dangerous. These books transmit the disturbing concept that there is a "normal" (canon) or prototypic, standard anatomy and disregard, in most cases, the more important clinically useful anatomy.

Medical students must understand and come to know human anatomic variability if they are to have a successful medical practice and provide maximum benefit to their patients.

Important notice: The authors have not knowingly altered the data of any author, nor have they unnecessarily modified the text of the original authors. The data and most of the words contained herein are those of the original authors.

Goethe (1749-1832), author and philosopher, said "that there would be little left of him if he were to discard what he owed to others." Reported by Carlotte Cushman

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