Besides blood vascular system, the lymphatic system is also found in chordates. It comprises lymph, lymph vessels and lymph nodes.
Lymph:
Blood including RBCs never leaves the blood vessels, but plasma and leucocytes (WBCs) of the blood escape into the intercellular space of the tissues from blood capillaries. This colourless part of the blood (without erythrocytes and plasma-proteins) is called the lymph or tissue fluid.
The lymph is also a transportable media in the body like that of the blood. It carries food and oxygen from the blood to the tissues and receives substances from the tissues to re-enter blood by means of lymphatic vessels. Some lymph also enters the venous capillaries by osmosis.
Lymph Vessels:
The lymphatic vessels are formed by the union of lymph capillaries. Lymph capillaries are formed of simple epithelium permeable even to protein molecules. Fluid pressure in capillaries is nearly zero. Lymph capillaries are not united with blood capillaries. Unlike blood capillaries their diameter is not uniform throughout. These are found in the intercellular spaces.
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Lymph vessels are thin-walled like veins and provided with valves. More lymph flows due to squeezing pressure of the surrounding muscles since lymph heart is not found in mammals.
Valves present in the lymph vessels allow the lymph flow only in one direction. All the lymph vessels of the posterior side of the body and from the left side of the head, neck and thorax communicate with the main lymphatic trunk called thoracic duct situated below the vertebral column.
The thoracic duct finally opens in the left subclavian at the place where it meets with the left external jugular vein. The lymph vessels from the right side of the head, neck, thorax, right arm open in the right lymphatic duct which opens in the right subclavian vein where it meets with the right external jugular vein.
Lymph Nodes:
Lymph nodes are formed of a meshwork of connective tissue fibres encased in a capsule. The lymph vessels pass through a lymph node at certain places and these are found only in mammals. In rabbit the lymph nodes are found in the head, neck, arm pits, near large blood vessels, groins, on the tonsils and Peyer’s patches on the intestine are formed by these nodes.
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The lymph hearts are not found in rabbit, therefore, the lymph flows in the lymph vessels with the help of body muscles, by pressure exerted to the small vessels by osmosis and absorption at tissue fluids. Though the lymphatic system is a closed system but it does not form a complete circuit of flow in the body.