Thoracic duct anatomy must be understood in the context of its embryology.
The anlage of the thoracic duct appears in the sixth to seventh week of fetal life as lymphatic clefts surrounded with mesenchyme near large veins. Connecting with each other, the clefts form the jugular and retroperitoneal lymph sacs and a well branching network of canals. Further development of the thoracic duct is connected with the lymph nodes formation; their germs appear on the 9th-10th week along the course of the left trunk, as well as along the ductal branches and anastomoses. The formation of the lymph nodes results in reduction of some trunks and plexuses of the thoracic duct. Disturbances in the formation processes of the lymph nodes can result in various structural variants of the thoracic duct in children and adults. The thoracic duct wall and the lymph nodes formation are not completed by birth. [1, 2, 3]
The first lymph sacs to develop are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. The jugular lymph sacs communicate inferiorly with the single retroperitoneal lymph sac at the root of the mesentery of the intestine and with the cisterna chyli, a lymph sac below the developing diaphragm on the posterior abdominal wall. The retroperitoneal lymph sac develops from mesonephric (primitive kidney) veins and the primitive vena cava. Capillary plexuses and lymphatics expand from the retroperitoneal lymph sac to the abdominal viscera and diaphragm. Channels that join the jugular lymph sacs to the cisterna chyli become the thoracic duct (or left lymphatic duct) and the right lymphatic duct. The retroperitoneal sac establishes connections with the cisterna chyli but loses its connections with the nearby veins.
The thoracic duct is a tubular structure that is 2-3 mm in diameter, varies in length from 38-45 cm, and extends from the second lumbar vertebra to the root of the neck (see the following image). It begins in the abdomen by a triangular dilatation, the cisterna chyli, which is situated on the front of the body of the second lumbar vertebra, to the right side of and behind the aorta, by the side of the right crus of the diaphragm. It enters the thorax through the aortic opening of the diaphragm between the aorta and the azygos vein. In the posterior mediastinum, the thoracic duct lies anterior to the vertebral column, the right intercostal arteries, and the hemiazygos veins as they cross to open into the azygos vein. Anterior to it are the diaphragm, esophagus, and pericardium. The pericardium is separated from it by a recess of the right pleural cavity. [2, 3, 4, 5, 6]
The structure of the thoracic duct is considered to be similar to that of a vein. However, the duct is more muscular, and its adventitia and media are less demarcated. The thoracic duct also has also an internal elastic lamina that is more prominent in the thoracic portion. Longitudinal smooth muscle fibers are also described in the subendothelial layer of the midthoracic section. The cervical part is less muscular.
Thoracic duct valves
At the lymphovenous junction, 3 types of valves are described, as follows (see the following image):
Venous valves at the internal jugular vein (IJV) (with 2 big semilunar valves) and the subclavian vein (unicuspid): The cusps of these valves close when the refluxing blood fills them
The ostial valves are bicuspid and extend obliquely across the junction; they prevent blood from entering the thoracic duct
The third type of valves are the ordinary type found in the rest of the thoracic duct; their cusps are closed by the refluxing lymph; however, the last 5 mm of the duct is devoid of any valves
Single thoracic duct that passes cephalad in the thorax on the right side of the aorta; it crosses to the left side at the level of the fifth cervical vertebra and opens in the venous system as described in Gross Anatomy; this is the most common course as described in most anatomy texts, with an incidence of 60-65%.
The thoracic duct may be partially doubled and opens in the left venous system; this occurs in 15-20% of cases
The thoracic duct is described as double throughout its entire course, one duct on each side of the aorta; the ducts open in the venous system of the corresponding side; this anatomic variation occurs in 12-15% of cases.
The thoracic duct may lie on the right side of the aorta in its entire length and open in the right venous system in 4% of cases
The thoracic duct may lie on the left side of the aorta in its entire length and opens in the left venous system in 4% of cases
The anatomy of the duct may vary in children with complex congenital heart disease. In patients with dextrocardia, left periaortic mass ligation should be considered in patients with chylothoraces that persist after the right-sided thoracotomy. 
The termination of the thoracic duct in the venous system also has many variations. It could end as a single duct in the left internal jugular vein (IJV), left subclavian, left angulus venosus, left innominate, or end as a double, triple, or even quadruple mode in any of the aforementioned venous channels.
Chylous leak in the neck could occur as a complication of oncologic cervical surgery or during thoracic surgical procedures of the posterior mediastinum. The best treatment for chylous leak is to prevent its occurrence. The reported incidence of chylous fistula after neck dissection is 1-2.5%.
Keep in mind that thoracic duct injury and or chylothorax could occur due to other causes such as penetrating neck trauma, after surgery for congenital heart disease in newborns and infants, and rarely after left internal jugular venous cannulation. Chylothorax could lead to metabolic and immunologic disorders that can be life threatening, with a mortality rate reaching 50%.
The management of chyle leaks is dependent on the etiology and daily output. The right lymphatic duct enters the internal jugular vein (IJV) below the level of the clavicle; therefore, it is less likely to be traumatized during cervical surgery. The thoracic duct, on the other hand, opens in the left IJV above the clavicle and can extend up to 6 cm in the neck