Glossary

Definitions of recurring terms. A term explained once serves every entry that uses it.

A

Aberrant artery: An artery whose origin, course or territory departs from the usual description, although it supplies a normal region. The adjective does not imply disease: it describes an atypical route, of considerable surgical and radiological importance because the vessel may lie where it is not expected. Some authors use 'aberrant' only for an anomalous origin and 'accessory' for a vessel added to the usual ones.
Accessory muscle: A supernumerary muscle present in part of the population and absent from the standard anatomical description. It is often asymptomatic and found incidentally, but becomes clinically relevant when it occupies a confined space and compresses a nerve or tendon, as with the accessory soleus or the anconeus epitrochlearis.
Accessory ossicle: A small supernumerary bony nucleus arising from an independent ossification centre that has not fused with the main bone. It is frequent in the foot and the carpus and must be recognised and distinguished from a fracture fragment, with which it is routinely confused in the emergency setting.
Agenesis: Complete absence of a structure due to failure of the embryonic primordium to develop. It is distinguished from aplasia, in which the primordium forms but does not differentiate, and from hypoplasia, in which the structure is present but reduced in size. In these entries it always denotes congenital absence, never acquired loss.
Anastomosis: A natural connection between two vessels, two nerves or two segments of a duct, placing otherwise separate territories in communication. It is among the most frequent findings in anatomical variation: the presence, calibre and site of an anastomosis govern collateral circulation and the clinical picture of an occlusion or a nerve injury. The term also denotes the surgical joining of two structures.
Anatomical position: The conventional reference posture: the subject standing, gaze directed forward, upper limbs by the sides with the palms facing forward, feet parallel. All terms of position and direction are understood to refer to it, whatever the subject's actual posture.
Anatomical variation: A non-pathological difference in the shape, number, course, position or relations of a structure compared with the commonest description. It is present from birth, does not evolve over time as a lesion does, and requires no treatment in itself. It may nevertheless alter the field of an operation, the success of an anaesthetic block or the reading of a radiological examination.
Anomaly: A departure from the usual configuration that differs from it markedly and may carry functional or clinical significance. The boundary with 'variation' is not sharp: many authors reserve 'variation' for findings without consequence and 'anomaly' for potentially symptomatic ones, but the distinction is not codified and usage varies between sources.
Aplasia: Failure of a structure to develop although its embryonic primordium formed but did not differentiate, so that the definitive organ is absent. In clinical practice and in much of the anatomical literature 'aplasia' and 'agenesis' are used interchangeably; the embryological distinction is kept here, but usage varies.
Aponeurosis: A broad, flattened tendinous sheet by which a muscle attaches to bone or continues into a fascia. Compared with a cord-like tendon it spreads traction over a wide surface. In muscular variants the attachment may shift from a tendon to an aponeurosis or the reverse.
Asymmetry: A difference between the right and left sides of the same individual in the shape, calibre or course of a structure. A degree of asymmetry is physiological. When describing a variant it must always be stated whether the finding is unilateral or bilateral and, if unilateral, on which side.
Atresia: Congenital absence or obliteration of the lumen of a duct or orifice that is normally patent. It differs from stenosis, a narrowing with a preserved lumen, and from agenesis, which involves the whole structure and not only its lumen.
Atrophy: An acquired reduction in the volume of a structure through a decrease in the size or number of its cells. It is a process, not a condition present from birth: it must not be confused with hypoplasia, which is deficient formation from development onwards.
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B

Bifurcation: The point at which a tubular structure divides into two branches. Its level is one of the most variable features of vascular anatomy: the height of a bifurcation, for example the carotid or the aortic one, shifts by several vertebral levels between individuals.
Bilateral: Present on both sides of the body in the same individual. It should always be reported, because how often a variant is bilateral is a descriptive feature in its own right: many muscular variants are bilateral in fewer than half of the subjects who show them.
Branch: A structure arising from a larger trunk, typically a vessel or a nerve, to reach a more circumscribed territory. The origin, number and territory of branches are among the most variable features of human anatomy.

C

Cadaver: A human body donated or assigned to anatomical study, the historical and still principal basis of variation series. Cadaveric material allows the finding to be verified directly, but it usually comes from elderly and selected populations, which limits how far the results generalise.
Canal: A tubular passage hollowed out in bone or bounded by fibrous structures, transmitting vessels, nerves or tendons. The number, calibre and contents of canals are subject to variation, and an accessory canal may carry a supernumerary branch.
Case material: The set of cases observed and described by a study or a centre, together with its denominator. It is the basis of prevalence estimates in anatomy and must always be read alongside the method by which the cases were collected.
Case report: A detailed description of a single subject, often the only way a rare variant is documented. It records that the finding exists and illustrates its practical relevance, but it supports no estimate of frequency.
Case series: A description of a group of subjects showing the same finding, without a comparison group and often without a known denominator. It documents that a variant recurs and shows its spectrum of presentation, but it does not measure its frequency in the population.
Caudal: Referring to the direction towards the lower end of the body, that is towards the feet in the anatomical position. It is the opposite of cranial and, for the trunk, is in practice equivalent to 'inferior'.
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Coalition: A congenital osseous, cartilaginous or fibrous fusion between two normally separate bones. The best known example is tarsal coalition, which may remain asymptomatic or present in young patients with restricted movement and pain.
Collateral circulation: A network of vessels that allows blood to reach a territory when the main route is occluded or absent. Its effectiveness depends largely on individual variants: the presence and calibre of pre-existing anastomoses explain why the same occlusion is silent in one subject and causes infarction in another.
Confidence interval: The range of values compatible with the observed data, within which the true population value is estimated to lie, at a preset level of confidence, usually 95%. A prevalence reported without a confidence interval, as happens in many small anatomical series, should be treated as a rough estimate.
Contralateral: Situated on, or referring to, the opposite side of the body from a given reference. In describing a unilateral variant, the contralateral side is the natural term of comparison.
Course: The route an elongated structure, typically a vessel, nerve or tendon, follows between its origin and its termination, together with the relations it forms along the way. It is one of the most frequently variable features and one of the most relevant surgically, because an atypical course exposes the structure to injury.
Cranial: Referring to the direction towards the cephalic end of the body, that is towards the head. It is the opposite of caudal and, for the trunk, is in practice equivalent to 'superior' in the anatomical position.

D

Deep: Farther from the surface of the body than another point. It is a relative term, the opposite of superficial; in describing a variant, its relation to the superficial and deep planes is often what determines its surgical relevance.
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Dissection: Anatomical examination carried out by separating the planes and structures of a body by hand and with instruments. It remains the reference method for documenting small-calibre muscular and nervous variants that imaging cannot resolve.
Distal: Farther from the point of origin or attachment of a limb or structure, along its axis. It is a relative term: it is used in comparison with another point of the same structure, never in absolute terms.
Diverticulum: A sac-like outpouching of the wall of a hollow organ, called true or congenital when it involves all layers of the wall. Meckel's diverticulum, a remnant of the omphalomesenteric duct, is the commonest congenital diverticulum of the digestive tract.
DOI: Digital Object Identifier: a permanent alphanumeric code that uniquely identifies a document and keeps it findable over time, independently of the web address hosting it. It should be given for every reference that has one.
Dominance: The criterion by which one of two paired or alternative structures is judged to predominate in supplying a territory. The classic example is coronary dominance, defined by the artery giving rise to the posterior interventricular branch: it is a variant of distribution, not a pathological condition.
Dorsal: Referring to the back, that is the posterior aspect of the trunk; for the hand and the foot it denotes the surface opposite the palm and the sole respectively. In human anatomy it is in practice synonymous with posterior for the trunk, whereas in comparative anatomy it keeps the sense of 'towards the back' regardless of posture.
Duplication: The presence of two structures, complete or partial, where only one is usually described. It is among the commonest numerical variants for ureters, veins and tendons, and is distinguished from fenestration, in which the element splits and then reunites.

E

Ectopia: The location of a structure in a site other than the usual one, through arrested or excessive migration during development. It is distinguished from acquired displacement, which affects a structure formed in its normal site and moved afterwards.
Editorial review: The internal check of an entry before publication: verification of references, terminology, consistency between text and sources, and compliance with licences. It is distinct from peer review, which involves reviewers external to and independent of the editorial team.
Embryonic persistence: The persistence into postnatal life of a structure that normally regresses during development. Many vascular variants, such as the carotid-basilar anastomoses, are explained in this way, and recognising their embryological origin makes the vessel's course predictable.
Entry: The unit of content on this portal devoted to a structure or a variation, with its description, frequency data, clinical relevance and references. It is the format in which this glossary is used: the terms defined here are those that recur in the entries.
Eponym: The naming of a structure or a finding after the person who first described it. Terminologia Anatomica favours descriptive names, but eponyms remain widespread in clinical usage and in the literature on variation; here they are given as synonyms, alongside the descriptive term.

F

Fascia: A sheet of connective tissue that invests, separates or groups muscles and other organs, forming compartments and gliding planes. Fascial planes guide dissection and the spread of collections, and the relations between a fascia and an accessory muscle are often the first sign that a variant is present.
Fenestration: The splitting of a segment of a vessel or nerve into two channels that then reunite, enclosing an opening. It is frequent in the basilar artery and in the cerebral veins and sinuses; it is distinguished from duplication, in which the two elements remain separate.
Foramen: A natural opening in a bone or a membrane, traversed by vessels or nerves. The number, site and size of foramina are highly variable, and accessory foramina are among the commonest findings in cranial variability.
Frontal plane: A vertical plane parallel to the forehead, dividing the body into an anterior and a posterior part. It is also called the coronal plane.

H

Head: Each of the portions by which a muscle arises from separate points before converging into a common belly or tendon. The number of heads is among the best documented muscular variations: supernumerary heads are described for biceps brachii, biceps femoris and many other muscles.
Heterogeneity: The degree of disagreement between the results of the studies included in a review or meta-analysis, beyond what chance alone would produce. In prevalence estimates for variants it is almost always high, because the method of investigation, the definition of the finding and the population differ from study to study.
Hyperplasia: An increase in the volume of a structure due to an increase in the number of its cells. It is distinguished from hypertrophy, in which the size of individual cells increases; in permanent-cell tissues such as skeletal muscle, hypertrophy predominates.
Hypertrophy: An increase in the volume of a structure through an increase in the size of its cells, their number being unchanged. In muscle entries, acquired functional hypertrophy must be kept distinct from the presence of a supernumerary belly or head, which is a developmental variant.
Hypoplasia: Incomplete development of a structure, which is present but appreciably smaller than normal. It is common among vascular variants, for instance in the segments of the circle of Willis, and carries greater risk when that segment is called upon to compensate for an occluded circulation.

I

Incidence: The number of new cases arising in a population over a given period of time. Applied to anatomical variation it is inappropriate, because a variant is present from birth and does not arise: the correct measure is prevalence, and the use of 'incidence' in this sense, though widespread in the literature, should be avoided.
Insertion: The point or area at which a muscle or ligament attaches to bone; for a muscle it conventionally denotes the attachment on the more mobile segment. A shift of insertion is among the commonest forms of muscular variation and may alter the muscle's action.
Interindividual variability: The set of anatomical differences observable between different individuals in the same population. It is the normal condition of the human body: the typical description found in textbooks represents the commonest form, not the only correct one.
Ipsilateral: Situated on, or referring to, the same side of the body as a given reference. It is the opposite of contralateral.

L

Lateral: Farther from the median sagittal plane than another point. It is a relative term and should not be confused with 'external', which describes a relation to a surface or to the wall of a cavity.
Level of evidence: A classification of the strength of a study based on its design. For anatomical variation the hierarchies devised for therapeutic studies fit poorly, since there is no intervention to compare: the benchmark remains a systematic series with a known denominator.
Ligament: A band of dense connective tissue joining two bones or fixing an organ, limiting movement beyond a certain range. Accessory and ossified ligaments are frequent variants and may compress the structures passing beside them.

M

Malformation: A congenital structural alteration due to an intrinsic defect of the developmental process, with functional or clinical consequences. In the variation literature it lies at the far end of the spectrum that begins with the variant of no significance; the boundary with 'anomaly' is not uniformly defined.
Medial: Closer to the median sagittal plane than another point. It is a relative term and should not be confused with 'internal', which describes a relation to a surface or to the wall of a cavity.
Meta-analysis: A statistical synthesis of the results of several independent studies, combined into a single overall estimate. Applied to variant prevalence it calls for caution, because it pools studies with differing definitions and methods: without an assessment of heterogeneity the combined estimate can be misleading.
Muscle belly: The fleshy, contractile portion of a muscle, lying between its tendinous ends. The number of bellies, their separation by tendinous intersections and the presence of supernumerary bellies are variations described for many muscles.

N

Narrative review: A reasoned synthesis of the literature on a topic, carried out without an explicit protocol for searching and selecting studies. It is useful for general orientation but is exposed to subjective selection of sources and is not suited to grounding a frequency estimate.
Normal variant: A finding that departs from the commonest description without entailing functional or clinical consequences. The adjective 'normal' denotes the absence of pathological significance, not frequency: some normal variants are rare, and others are common yet decisive in surgery.

O

Observational study: A study in which what is observed is recorded without intervening on the subjects or assigning them to groups. Almost the whole literature on anatomical variation, cadaveric or radiological, belongs to this type.
Open access: A form of publication that makes a work freely readable online without subscription, often under a licence that also allows reuse. Free access and freedom to reuse are not the same thing: only the stated licence determines what may be done with the text and the figures.
Origin: The point at which a muscle attaches to the segment regarded as more fixed; for a vessel, the point at which it arises from its parent trunk. In vascular variation the origin is the feature most often atypical, and its site determines the course of the entire branch.

P

Peer review: Assessment of a work by independent experts before publication, to check its method, accuracy and originality. It does not guarantee that the result is correct: it removes obvious errors, but many anatomical series pass review despite small samples and heterogeneous methods.
Plexus: A network of nerves, vessels or lymphatics in which the elements exchange fibres or branches before being distributed peripherally. The composition of plexuses is variable: the prefixed and postfixed forms of the brachial plexus are the best known example and account for many discrepancies between the expected and the observed territory.
PMID: PubMed Identifier: a number that uniquely identifies a record in the PubMed database. It allows a reference to be retrieved quickly; unlike a DOI it identifies the bibliographic record rather than the document itself.
Prevalence: The proportion of individuals showing a given characteristic in a defined population at a given time. It is the correct measure for anatomical variation, which is present from birth and does not arise over time. It should always be reported with the denominator, the method of detection and the unit of analysis, because the same variant may show very different prevalence figures in cadaveric and in radiological studies.
Primary source: A publication that reports for the first time the original data of a study, a dissection or a case. Frequency estimates in these entries must be traced back to the primary source, not to the review that cites them.
Provenance: A documented statement of where a content item, a datum or an image comes from and of the path by which it reached the entry. It makes every statement verifiable and establishes which reuse rights accompany it.
Proximal: Closer to the point of origin or attachment of a limb or structure, along its axis. It is a relative term, the opposite of distal.
Public domain: The status of a work whose copyright has expired or has been waived by the author, and which may therefore be reused without seeking permission. It covers many historical anatomical plates, but recent photographic reproductions may carry rights of their own, so each case must be checked individually.

R

Radiological study: The detection of a variant on images acquired in vivo, by computed tomography, magnetic resonance imaging, angiography or ultrasound. It allows far larger samples than dissection and less selected populations, but resolution limits the recognition of small-calibre structures: for the same variant, radiological and cadaveric prevalence figures often do not agree.
Retinaculum: A band-like fascial thickening that holds tendons against the skeleton, preventing them from bowstringing during movement. It bounds an already confined space, so a supernumerary tendon or muscle passing beneath it may cause compression.

S

Sagittal plane: A vertical plane dividing the body into a right and a left part; the one passing through the midline is the median sagittal plane. It is the reference against which the terms medial and lateral are defined.
Sample: The set of subjects, sides or specimens actually examined in a study. In anatomical series the unit of analysis must be stated, individual or side, because the same study of fifty cadavers may report one hundred sides and the two prevalence figures do not coincide.
Secondary source: A text that reworks or summarises data published elsewhere: treatises, narrative reviews, textbook chapters. It is useful for general orientation but is not sufficient as the sole support for a numerical figure.
Selection bias: A systematic distortion of results caused by the way subjects entered the study rather than by chance. It is especially treacherous in variation studies: cadaveric series and radiological series examine populations that do not represent the general population, and the prevalence figures derived from them must be read with that caveat.
Sesamoid bone: A bone embedded in the course of a tendon near a joint, altering its lever arm and reducing friction. Beyond the constant sesamoids their presence is variable, and a bipartite sesamoid may mimic a fracture.
Situs inversus: An arrangement of the thoracic and abdominal organs that mirrors the usual one. In the total form the inversion is complete and function is generally preserved; in partial forms, or situs ambiguus, association with cardiac defects and splenic anomalies is frequent.
Superficial: Closer to the surface of the body than another point. It is a relative term, the opposite of deep.
Synonym: An alternative name for the same structure, in Italian, in Latin or as an eponym. Synonyms are recorded so that a term can be found by readers who know it under another name, without implying that they are equivalent to the official term.
Synostosis: Bony fusion between two normally separate skeletal segments, congenital when it results from failure of segmentation during development. Proximal radioulnar synostosis is the classic example; the acquired form, post-traumatic or postsurgical, must be kept distinct.
Synovial sheath: A double-walled, synovium-lined sleeve enveloping a tendon where it runs within an osteofibrous canal, reducing friction. The extent and septa of these sheaths vary between individuals and govern the spread of infection, particularly in the hand.
Systematic review: A review carried out under a protocol defined in advance, stating the databases searched, the inclusion criteria and the appraisal of study quality. It is the type of synthesis on which the soundest prevalence estimates for anatomical variation rest.

T

Tendon: A structure of dense connective tissue that transmits to bone the force generated by the muscle belly. The number, site of insertion and presence of accessory slips are variable, particularly for the extensor tendons of the hand and the flexors of the foot.
Terminologia Anatomica: The international reference nomenclature of anatomy, which fixes the standard Latin terms, and their English equivalents, for the structures of the human body. Many variants do not appear in it, because the nomenclature describes usual anatomy: in these entries the official term, where one exists, is given alongside the eponyms and synonyms in use.
Transverse plane: A horizontal plane, perpendicular to the long axis of the body, dividing it into a superior and an inferior part. It is the plane of the axial sections of computed tomography and magnetic resonance imaging.
Trunk: A vascular or nervous segment from which several branches arise, or in which several elements converge before dividing again. The formation of common trunks, such as the coeliac trunk or the trunks of the brachial plexus, is a typical site of variation.

U

Unilateral: Present on one side of the body only. When describing a variant the side should be stated and, where the information is available, the proportion of cases in which the finding is unilateral rather than bilateral.

V

Ventral: Referring to the anterior aspect of the trunk, on the side of the belly. In human anatomy it is in practice equivalent to anterior, whereas in comparative anatomy and embryology it keeps the sense of 'towards the belly' regardless of posture.