Please see question 3.

Please see question 3.

The lymphatic system, like the remainder of the vascular system, is thought to arise from the mesoderm. The lymphatic system arises from six primordial lymph sacs that are outgrowths of venous channels, paralleling the six aortic arches from the branchial apparatus. The first primordial lymph sacs arise at the jugular–subclavian confluences and extend into the upper extremities and head. Two unpaired sacs appear along the mesenteric and celiac trunks and form the cisterna chyli. Finally, paired femoral sacs appear and extend into the lower extremities. These six sacs become linked by a rich network of lymphatic vessels, ultimately leading from the peripheral tissues to the central veins. Lymph nodes develop at a later stage at places where lymphatic channels join.

The right and left thoracic ducts are derived from the jugular–subclavian lymph sacs. Originally, they both connect to the cisterna chyli and drain lymph towards the subclavian vessels. Later in development the distal portion of the right thoracic duct crosses over the midline behind the esophagus to join the proximal part of the left thoracic duct (T5–T7), thus forming a single duct. The caudal aspect of the left thoracic duct involutes, while the proximal part of the right thoracic duct persists and drains the lymph from the right side of the head and neck, right arm, and upper thorax into the right jugular–subclavian confluence. The thoracic duct crosses the diaphragm through the aortic hiatus. As the thoracic duct ascends though the thorax, it receives multiple intercostal branches [1].

The cisterna chyli is an elongated lymphatic sac located over T12 to L2. It receives the lymphatic drainage of the right and left lumbar trunks carrying the lymph from the lower extremities. In addition, it receives the intestinal and intercostals trunks, carrying the lymph from the GI tract and lower chest wall, respectively (Fig. 23.1).