They were for this reason the first to be detected, and were known even in ancient times to anatomists. The milky fluid in them was called "the chyle." Its milky appearance is due to the same cause as the white opaque appearance of milk, namely, to the presence of an immense number of excessively small particles of oil (fat) and a certain proportion of larger globules of the same nature. It was thus not difficult for the old anatomists to trace the fine branches of the lacteals uniting branch to branch, and at last forming a large trunk--called the thoracic duct--about a quarter of an inch thick, which runs up the inner face of the backbone to the neck, where it joins the great left subclavian vein, and pours its contents into the blood-stream which is there nearing the heart. A small trunk formed by the union of lymphatic vessels from the right side of the head and neck and the right upper limb opens into the right subclavian vein. It took some time to discover this smaller trunk, since it is not brought to view by milky contents. Gradually it was made out that there are innumerable transparent branches opening into the thoracic duct from the whole of the body, besides the milky-looking lacteals: branches which bring "limpid" clear fluid, or "lymph," from all the viscera, from the muscles, and from the deeper layers of the skin in every region of the body, even from the toes, fingers, and tongue tip. In fact, wherever the blood-vessels take blood there are also vessels of the lymphatic system bringing back to the heart the liquid exudation which escapes into the tissues from the finest blood-vessels (Fig. 43). [Illustration: FIG. 43.--The fore-arm of man, with the skin removed so as to show the large superficial lymphatic vessels resting on the muscles. They are represented as white knotted cords. On the palm of the hand (8, 8) and on the fingers a closer network of these vessels is represented, but the smaller lymphatic capillaries and spaces are not shown.]