This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The nerves in the posterior cervical triangle are the spinal accessory, branches of the cervical plexus, and the brachial plexus. The position of the spinal accessory is important because it is frequently encountered in operations for the removal of enlarged lymphatic nodes. It enters the under surface of the sternomastoid muscle from 3 to 5 cm. below the tip of the mastoid process and emerges at the posterior edge about its middle or a little above. It is about at this point that the external jugular vein reaches the posterior border of the sternomastoid, and the cervical plexus, formed by the anterior divisions of the four upper cervical nerves, reaches the surface. From this point also the occipitalis minor runs upward along the posterior edge of the sternomastoid and the auricularis magnus runs upward over the sternomastoid direct to the external ear. The superficial cervical runs directly across the muscle towards the median line and the descending branches - the sternal, clavicular, and acromial - pass down beneath the deep cervical fascia to perforate it just above the clavicle and become cutaneous. Care should be taken not to mistake them for the spinal accessory. Still deeper are the cords of the brachial plexus. These cords, sometimes two, at others three in number, are beneath the deep fascia and lie above the subclavian artery. They can be felt and in a thin person, if the head is turned to the opposite side, the prominence which they form under the skin can even be seen.
The lymphatics of the posterior cervical triangle are numerous and being often enlarged are frequently operated on. They lie along both the outer side of the internal jugular vein and under the posterior edge of the sternomastoid muscle, which they follow clear up to the base of the skull. They also follow the edge of the trapezius muscle and lie in the space between it and the sternomastoid; they extend downward under the clavicle and become continuous with the axillary lymphatics. The right and left lymphatic ducts empty into the venous system at the junction of the innominate and internal jugular veins. That on the left side is called the thoracic duct; it begins as the receptaculum chyli on the body of the second lumbar vertebra and is about 45 cm. (18 in. ) long. It drains all the left side of the body and the right as far up as and including the lower surface of the liver.
The duct on the right side is called the right lymphatic duct; it is only 1 or 2 cm. in length and drains the right side of the head and neck, the right upper extremity, and the right side of the chest as far down as and including the upper surface of the liver.