The beginning of the thoracic duct is known as the:

The cisterna chyli, or the beginning of the thoracic duct, receives the right and left lumbar trunks. The cisterna lies on the bodies of the upper two lumbar veebrae between the right crus of the diaphragm and the abdominal aoa and represents a dilated receptacle for the lymph gathered from the lower pa of the body. Ref: Anatomy to Color and Study By Raphael Poritsky, 2nd Ed, page 58