Externus

Externus is a Latin adjective meaning external, outer, or situated on the outside. In scientific and scholarly terminology, externus is used as a descriptive epithet in compound names to indicate a structure’s outward or superficial position relative to another part. The word pairs with internus (internal) to differentiate between opposing locations. In Latin grammar, externus has gendered forms: externus (masculine), externa (feminine), externum (neuter); the plural forms are externi, externa, externa.

In anatomy, externus appears in several well-known muscle names and related terms. A prominent example is the obturator externus (musculus obturator externus), a small hip muscle located more superficially on the lateral side of the pelvis. Another example is the obliquus externus abdominis, the external oblique muscle of the abdomen, which lies on the outer surface of the abdominal wall. The term is often used in contrast to anatomically adjacent structures described as internus (internal) or profundus (deep).

Beyond anatomy, externus appears in botanical and zoological Latin descriptions to denote external characteristics or outward morphology, and in historical or philological texts as part of Latin phrasing. It is not a standalone entity or concept but rather a descriptive descriptor that forms part of longer scientific names and phrases.

In summary, externus is a Latin descriptor used across disciplines to signal external or outward position, most notably in anatomical nomenclature to distinguish external from internal structures.