accessory foramen

An accessory foramen is an additional opening or hole found on the humerus (upper arm bone) of tetrapodomorphs — early limbed vertebrates — located beyond the entepicondyle and ectepicondyle foramina. These additional foramina are often denoted as 'c' and 'd' following the classification of Jarvik (1996).

Accessory foramina are found on the distal humerus (the lower end near the elbow joint) of certain tetrapodomorphs. They are positioned beyond the entepicondylar and ectepicondylar foramina, the two more commonly described openings in this region.

Like other foramina in long bones, accessory foramina likely transmitted nerves and blood vessels to the overlying soft tissues. Their specific contents and functional role in the extinct animals in which they are found are inferred from comparative anatomy and functional analogy.

As features studied primarily in fossil tetrapodomorphs and comparative anatomy, accessory foramina are not associated with clinical conditions. In the paleontological context, their preservation and variation among specimens are used to study intraspecific variation and phylogenetic relationships.

The entepicondylar foramen is a well-known opening on the medial epicondyle of the humerus through which the median nerve passes in many early tetrapods, while accessory foramina are additional openings further along the bone. The entepicondylar foramen has been lost in most living mammals including humans.