Anatomical architecture of the brachial plexus in the common hippopotamus (Hippopotamus amphibius) with special reference to the derivation and course of its unique branches.

Summary The anatomy of the brachial plexus in the common hippopotamus (Hippopotamus amphibius), which has not been previously reported, was first examined bilaterally in a newborn hippopotamus. Our observations clarified the following: (1) the brachial plexus comprises the fifth cervical (C5) to first thoracic (T1) nerves. These formed two trunks, C5–C6 and C7–T1; in addition, the axillary artery passed in between C6 and C7, (2) unique branches to the brachialis muscle and those of the lateral cutaneous antebrachii nerves ramified from the median nerve, (3) nerve fibre analysis revealed that these unique nerve branches from the median nerve were closely related and structurally similar to the musculocutaneous (MC) nerve; however, they had changed course from the MC to the median nerve, and (4) this unique branching pattern is likely to be a common morphological feature of the brachial plexus in amphibians, reptiles and certain mammals.