The =thoracic duct= (Ductus thoracicus) is the chief collecting trunk of
the lymphatic system. It begins as an elongated irregular dilatation,
the =cisterna= or =receptaculum chyli=, which is situated between the
right side of the aorta and the right crus of the diaphragm at the first
and second lumbar vertebræ. The duct enters the thorax through the
hiatus aorticus and runs forward on the right of the median plane
between the vena azygos and the aorta, covered by the pleura. At the
sixth or seventh thoracic vertebra it inclines somewhat ventrally,
crosses obliquely over the left face of the œsophagus, and passes
forward on the left side of the trachea to the inlet of the thorax. The
extrathoracic terminal part passes downward and forward a variable
distance (3 to 4 cm.) on the deep face of the left scalenus muscle,
bends inward and backward under the bicarotid trunk, and opens into the
upper part of the origin of the anterior vena cava just behind the angle
of junction of the jugular veins. The terminal bend is ampullate and
sometimes divides into two very short branches which open close
together.
Since the duct develops from a plexus of ducts in the embryo,
considerable variation from the more usual course occurs. There is
often a left duct which arises at the cisterna or at a variable point
from the right duct, runs across the left intercostal arteries
parallel to the latter, and unites with it over the base of the heart
or further forward. The two are connected by cross-branches. In some
cases the left duct is the larger, and there may indeed be none on the
right side. Other variations are common.
The chief tributaries of the thoracic duct are as follows:
1. The two =lumbar trunks= (Trunci lumbales) are formed by the
confluence of the efferent ducts of the lumbar glands, and commonly
unite with each other and with the posterior intestinal trunk before
opening into the cistern.
2. The =intestinal trunks= (Trunci intestinales), two or three in
number, receive the efferents of the lymph glands of the intestine,
stomach, liver, and spleen.[176]
In its course through the thorax the thoracic duct receives efferents
from the intercostal, mediastinal, and bronchial glands. At the thoracic
inlet it is joined by ducts from the prepectoral and right axillary
glands, and by the left tracheal duct. The duct is provided with several
pairs of valves; the best developed are at its termination.