The thoracic duct is the chief collecting trunk of the lymphatic system.
It drains lymph from all parts of the body except the right forelimb, right side of head neck and thorax.
It extends from about the level of the first lumbar vertebra forwards a little beyond the thoracic inlet.
Its beginning is in the form of a reservoir, the cisterna or receptaculum chyli. This reservoir is formed by the meeting of the two lymphatic trunks, The lumbar trunk and gastro intestinal trunk.
The cisterna chyli is in the form of elongated dilatation and lies in an oblique manner upwards and forwards a little above and in front of the level of the hiatus aorticus, between the first and second lumbar arteries and on the right face of the right crus of diaphragm.
It may also be found lying a little lower down or behind the right crus of the diaphragm when its position will be to the left of the posterior vena cava.
The duct proceeds from the anterior aspect of the cisterna chyli is the thoracic duct, which passes forwards and slightly upwards, enters the thoracic cavity through the hiatus aorticus and crosses the right face of the terminal part of the thoracic aorta to gain its dorsal face.
The thoracic duct from the cisterna chyli passes forwards on the right part of the dorsal face of the aorta covered by pleura and fat. The thoracic duct may remain single in some cases. But generally divide into two branches At the 6th or 7th dorsal vertebra it inclines ventrally crosses obliquely over the left face of the oesophagus, passes forward on the left face of the trachea to the thoracic inlet. The extra thoracic part is on the deep face of the left scalenus muscle.
It curves backwards and inwards under the bicarotid trunk and opens on the dorsal face of the left jugular confluence or anterior vena cava. In its course it receives efferents of the intercostal, mediastinal and bronchial glands and at thoracic inlet it receives the duct from the left posterior cervical and left tracheal duct.
The chief tributaries are gastro intestinal trunks, efferents of the mediastinal lymph glands and in its termination, the efferents of the posterior cervical, left costocervical, prescapular and sternal lymph glands and left tracheal duct.
Lumbar trunk
The lymph conveyed by the lymphatics from the hind limb, pelvic walls and organs, inguinal structures and the abdominal walls, finally reaches either the lumbar lymph glands placed in the sublumbar region or the efferents of these glands.
The efferents leaving these lumbar glands from two lumbar trunks, which unite to form single lumbar trunk. This in its turn joins the gastrointestinal trunk to form the cisterna chyli.
The lumbar trunk is formed by the union of the efferents from the iliac lymph glands. It also receives efferents from the lumbar and renal glands.
The gastro-intestinal trunk is formed at the ventral face of the posterior vena cava just behind the dorsal border of the liver by the confluence of the radicles gastric and intestinal branches. The gastric trunk is on the left of the coeliac artery.
The intestinal trunk follows on the anterior mesenteric artery.
The trunk formed by the union of these two trunks, bends dorsally between the aorta and posterior vena cava and unites with the lumbar trunk.
