LYMPH  and LYMPH FORMATION . Lying

close to the blood-vessels of a limb or organ a further set of vessels may be observed . They are very pale in colour, often almost trans-parent and very thin-walled . Hence they are frequently difficult to find and dissect . These are the lymphatic vessels, and they are found to be returning a fluid from the tissues to the blood-stream . When traced back to the tissues they are seen to divide and ultimately to form minute anastomosing tubules, the lymph capillaries . The capillaries finally terminate in the spaces between the structures of the tissue, but whether their free ends are closed or are in open communication with the tissue spaces is still undecided . The study of their development shows that they grow into the tissue as a closed system of minute tubes, which indicates that in all probability they remain permanently closed . If we trace the lymphatic vessels towards the thorax we find that in some part of their course they terminate in structures known as lymphatic glands . From these again fresh lymphatic vessels arise which carry the fluid towards the main lymph-vessel, the thoracic duct . This runs up the posterior wall of the thorax close to the aorta, and finally opens into the junction of the internal jugular and left subclavian veins .