The thoracic duct begins with a dilation, the cisterna chyli, which lies in the aortic hiatus (Figures 24: 12; 27: 9). Its cranial end narrows to the actual thoracic duct, which is initially located to the right of the median plane, more specifically on the right dorsal border of the thoracic aorta (Figure 18: 11), up to the 5th or 6th thoracic vertebra. At this point, it gradually crosses to the left side and extends into the precardial mediastinum, connecting to the thoracic inlet (Figure 17: 15), where it then joins the venous system (Figures 2 to 7: i, i’, i’’, i’’’; 17: 15).
A. Cisterna Chyli (Pars Abdominalis of the Ductus Thoracicus)
The cisterna chyli (Figures 24: 12 and 27: 9) is usually located to the right of and dorsally to the aorta, and usually either extends from the 4th to the 1st lumbar vertebra, less commonly from the 3rd or 2nd lumbar vertebra to either the 1st lumbar vertebra or last thoracic vertebra, or from the 4th lumbar vertebra only to the 2nd lumbar vertebra. Sometimes, the cisterna chyli is located more to the dorsal side of the aorta, sometimes more to the right side, and sometimes even ventrally. The cisterna chyli is shaped like an elongated sack, widest in the middle and tapering towards the caudal and cranial ends, though it can also have irregular shapes. The usually doubled, or reticulated, pelvic lymphatic trunk flows into the caudal end of the cisterna chyli (Figures 24: 11; 27: 10), while from the cranial end, the thoracic duct emerges as a gradual narrowing of the cisterna chyli, without a sharp distinction between it and the cisterna chyli (Figure 18: 11). If there is only one wide pelvic lymphatic trunk, it merges into the cisterna chyli without being sharply distinct from it. The shape of the cisterna chyli is very variable, particularly due to the varying behaviours of the intestinal trunk, which opens into the middle part of the cisterna chyli (see intestinal trunk). If the cisterna chyli is extremely reticulated, as is often the case, it is not uncommon for it to seem as though the cisterna chyli delimits islands through which the corresponding lumbar arteries pass (Huber ). In addition to the above description of the location of the cisterna chyli, it should be added that it is located on the right and dorsal sides of the aorta, between the crura of the diaphragm and the lumbar muscles, on the ventral side of the respective vertebrae, dorsal to the renal artery and vein, and, in its caudal part, to the left of the caudal vena cava (i.e. between the caudal vena cava and the aorta).
B. Thoracic Duct Proper
The thoracic duct emerges from the cisterna chyli without much distinction (see above) at the 1st lumbar vertebra. It initially runs cranially (Figure 18: 11) on the right-dorsal border of the thoracic aorta, and the ventral border of the azygos vein to the 6th thoracic vertebra, then gradually passes to the left side between the azygos vein and the aorta and then between the aorta and the 5th thoracic vertebra, to run in the precardial mediastinum to the thoracic inlet (Figure 17: 15).
The initial part of the thoracic duct located on the aorta (Figure 18: 11) is rarely just a simple vessel; in most cases it forms coarse networks, leading to the formation of islands through which the intercostal arteries often pass. The initial part of the thoracic duct is infrequently doubled, and if a second duct is present, it lies on the left dorsal border of the thoracic aorta and either emerges separately from the cisterna chyli or branches off from the main duct shortly after its origin. It then reunites with the main duct when it crosses from the right to the left side (in rare cases, the ducts merge more cranially). If there are 2 ducts, they are always connected to one another by numerous transverse branches, which also leads to island formation. The branching can be so extensive that the entire thoracic duct consists of a coarse network of lymph vessels. In 1 case, the thoracic duct was observed to emerge as 3 ducts from the cisterna chyli. The terminal portion of the thoracic duct (Figure 17: 15), located in the precardial mediastinum, lies between the M. longus colli (Figure 17: 2) and the left subclavian artery (Figure 17: 11) on the left side of the esophagus and crosses near and inside the thoracic inlet on the medial side of the costocervical artery and vein (Figure 17: 14) and the vertebral artery (if it arises independently and not from the costocervical trunk). Immediately cranial to the thoracic inlet, the thoracic duct crosses the lateral side of the left common carotid artery and the medial side of the omocervical trunk (currently termed the superficial cervical artery). The thoracic duct drains into the venous system, just cranial (1 to 3 cm in large dogs) to the 1st rib (see below). The terminal portion of the thoracic duct consists of a simple vessel in about half of all cases, and in the other half of cases, the terminal portion of the duct consists of a coarse network of branches delimiting islands, although not as extensive of a network as present in the initial portion of the duct. The structure of the most terminal portion of the thoracic duct is so highly variable that all variations cannot be described; some of them are shown in Figures 2, 3, 4, 5, 6, 7 and 17. The figures show that the terminal portion of the thoracic duct is rarely simple and undivided (Figures 4, 7). In this case, the thoracic ducts may open either into the left common jugular vein, or at the border of the cranial vena cava with the left common jugular vein, usually on the dorsal or lateral, but occasionally on the ventral or medial, side. However, the thoracic duct most commonly opens at the angle where the common jugular vein branches into both left jugular veins on the dorsal side.
There is usually an ampoule-shaped widening of the thoracic duct (shown in Figure 7: i) shortly before it opens into the venous system, but this widening narrows down again so that the opening of the duct into the vein is always very small (about 1 mm in large dogs). In most cases, the terminal portion of the thoracic duct is divided into 2, even 3, or 4 branches (Figures 2, 3, 5, 6 and 17), which all open separately into the venous system. One of these branches behaves as described above for the undivided thoracic duct, while the other terminal branch or branches enter the venous system separately, either at the point where the cranial vena cava divides into the axillary vein and the short left common jugular vein, or into the left common jugular vein, or in the angle between the axillary vein and the left common jugular vein, as shown in Figures 2, 3, 5 and 6 by i, i’, i’’ and i’’’. In 1 case, 1 terminal branch opened into the trunk of the right common jugular vein. It is not uncommon for the individual terminal branches to be connected to one another by transverse vessels, forming islands (Figure 6). The terminal branches can also widen into an ampoule shape just before they enter the venous system. The efferent vessels of the axillary (Figures 2, 3, 4, 5: e) and cranial mediastinal (Figure 17: a) lymph nodes, and usually those of the caudal cervical (Figure 6: c), middle cervical and superficial cervical (Figures 2, 4, 6: g) lymph nodes, as well as the left tracheal duct (Figures 2, 3, 4, 5 and 6: f), drain either into the end of the thoracic duct or its terminal branches. In most cases, a small valve was either found at the opening of the thoracic duct into the venous system, or the duct opened into the pocket of a larger venous valve. The connection between the thoracic duct and the venous system seems to always be tightly sealed (Huber ).
Width of the thoracic duct
In the case of a simple vessel, the width remains mostly constant throughout its entire length and is about 3 to 4 mm in large dogs. When coarse networks with islands are formed, the widths of the vessels are smaller.
Valves in the thoracic duct
The duct has many valves, which are separated by a distance of about 1 to 3 cm in large dogs. Valves are absent in the portion of the thoracic duct that is located between the 11th to 12th thoracic vertebra and the pelvis. The valves are usually fully closed, even in dead animals.