We observed the cervical vertebrae of a Pacific white-sided dolphin, and examined gross anatomically the brachial plexus of a specimen stranded at Toyama bay in 2003. The atlas and the axis were fused into one unit. The rest of the cervical vertebrae were free. The brachial plexuses on both sides were formed by the union of the ventral rami of the lower six cervical nerves and the first thoracic nerve. These roots fused with each other to form three trunks on the left side and four trunks on the right side. On both sides, these trunks fused into one band-shaped fascicle. At the axilla, the fascicle radially gave off many nerve branches. The phrenic nerve and the suprascapular nerve arose from the cranial margin of the plexus. The pectoral nerves arose from the ventral surface of the brachial plexus. In contrast, the subscapular, the axillar, the radial, and the thoracodorsal nerves arose in turn from the dorsal surface of the brachial plexus. The median and ulnar nerves reached the forearm as a common trunk and gave off many cutaneous branches for the skin of the palmar surface of the flipper, and terminated as the palmar digital nerves. The costo-coracoid muscle was innervated by branches from the pectoral nerve and the phrenic nerve and, on occasion, a branch divided directly from the brachial plexus. The present study and a survey of the literature suggest that the more compressed in length the cervical vertebrae, the greater the increase in number of the ventral rami of the spinal nerves which form the brachial plexus.
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