This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The Cardiac Nerves, derived from these sources, converge from both sides upon the origin of the aorta and pulmonary artery, and form the cardiac plexuses, which, dividing into the right and left coronary plexuses, surround and accompany the coronary arteries and their branches.
The Superior Cardiac Nerve arises from the superior cervical ganglion of the sympathetic, or from the communicating branch which connects this ganglion with the middle; it is joined by one or two filaments from the pneumogastric nerve.
The Middle Cardiac Nerve arises from the middle cervical ganglion; but when this ganglion is absent, the nerve arises from the trunk of the sympathetic itself. Scarpa has called this the great cardiac nerve, from its frequently being the largest of the three: sometimes, however, it is absent altogether.