The secondary tendinous slip of the extensor hallucis longus (extensor ossis metatarsi hallucis).

In 80% of a random sample the extensor ossis metatarsi hallucis was present. In 90% of these occurrences it arose off the medial aspect of the extensor hallucis longus tendon and coursed distally, medially, and inferiorly an average of 4.0 cm., inserting into the sling portion of the extensor apparatus. Variation of this insertion occurred in 10% of the cases in which the sling portion of the extensor apparatus continued to illustrate fibers of the extensor ossis metatarsi hallucis inserting into it. There is little variation in the occurrence of this tendon between the sexes. The frequency of occurrence of the extensor ossis metatarsi hallucis from this study supports a theory that this tendon should be considered to occur as a rule rather than an exception, and because the previously suggested name of extensor hallucis capsularis is not biomechanically sound, perhaps the tendon merely be termed the secondary extensor hallucis longus.