The sac that often forms the initial portion of the thoracic duct is:
a. Lacteal
b. Right lymphatic duct
c. Cisterna chyli
d. Lymph sac

Verified step by step guidance

1. Step 1: Understand the anatomy of the thoracic duct, which is the main lymphatic vessel that drains lymph from most of the body into the venous system.
2. Step 2: Recognize that the thoracic duct begins in the abdomen and ascends through the thorax to empty into the venous circulation near the junction of the left subclavian and internal jugular veins.
3. Step 3: Identify the structure that forms the initial dilated portion of the thoracic duct, which acts as a sac collecting lymph from the lower limbs and intestinal trunk.
4. Step 4: Recall that this initial sac is called the "cisterna chyli," a key anatomical landmark in the lymphatic system.
5. Step 5: Conclude that among the given options, the correct answer is the cisterna chyli, as it is the sac that often forms the initial portion of the thoracic duct.

Key Concepts

Thoracic Duct

The thoracic duct is the largest lymphatic vessel in the body, responsible for transporting lymph from most of the body into the venous system. It begins in the abdomen and ascends through the thorax to drain into the left subclavian vein.

Cisterna Chyli

The cisterna chyli is a dilated sac at the lower end of the thoracic duct, located in the abdomen. It collects lymph from the intestinal and lumbar lymphatic trunks before the lymph is transported upward through the thoracic duct.

Lymphatic System Anatomy

Understanding the structure and function of lymphatic vessels, including lacteals, lymph sacs, and ducts, is essential. Lacteals absorb fats in the intestines, lymph sacs are embryonic structures, and the right lymphatic duct drains lymph from the right upper body.