An anatomical variation, anatomical variant, or anatomical variability is a presentation of body structure with morphological features different from those that are typically described in the majority of individuals. Anatomical variations are categorized into three types including morphometric (size or shape), consistency (present or absent), and spatial (proximal/distal or right/left). Variations are seen as normal in the sense that they are found consistently among different individuals, are mostly without symptoms, and are termed anatomical variations rather than abnormalities. Anatomical variations are mainly caused by

genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wor ...

and may vary considerably between different

population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...

s. The rate of variation considerably differs between single organs, particularly in muscles. Knowledge of anatomical variations is important in order to distinguish them from pathological conditions. A very early paper published in 1898, presented anatomic variations to have a wide range and significance, and before the use of

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

technology, anatomic variations were mostly only found on cadaver studies. The use of imaging techniques have defined many such variations. Some variations are found in different species such as polydactyly, having more than the usual number of digits.

Variants of structures

Muscles

Kopsch gave a detailed listing of muscle variations. These included the absence of muscles; muscles that were doubled; muscles that were divided into two or more parts; an increase or decrease in the origin or insertion of the muscle; and the joining to adjacent organs. The

palmaris longus muscle The palmaris longus is a muscle visible as a small tendon located between the flexor carpi radialis and the flexor carpi ulnaris, although it is not always present. It is absent in about 14 percent of the population; this number can vary in Africa ...

in the forearm is sometimes absent, as is the

plantaris muscle The plantaris is one of the superficial muscles of the superficial posterior compartment of the leg, one of the fascial compartments of the leg. It is composed of a thin muscle belly and a long thin tendon. While not as thick as the achilles te ...

in the leg. The

sternalis muscle The sternalis muscle is an anatomical variation that lies in front of the sternal end of the pectoralis major parallel to the margin of the sternum. The sternalis muscle may be a variation of the pectoralis major or of the rectus abdominis. Struc ...

is a variant that lies in front of the pectoralis major and may show up on a

mammogram Mammography (also called mastography) is the process of using low-energy X-rays (usually around 30 Peak kilovoltage, kVp) to examine the human breast for diagnosis and screening. The goal of mammography is the early detection of breast cancer, ty ...

Bones

Usually there are five lumbar vertebrae but sometimes there are six, and sometimes there are four.

Joints

A discoid meniscus is a rare thickened lateral meniscus in the knee joint that can sometimes be swollen and painful.

Organs

The lungs are subject to anatomical variations.

Clinical significance

Accessory small bones called ''ossicles'' may be mistaken for avulsion fractures.

References

{{Reflist

External links

Atlas of human anatomical variations
Anatomy