LYMPH. the fluid contained in the lym phatic glands and in the lacteals (q.v.). It is elaborated primarily by the assimilation of food, and is also obtained from the blood and tissues, the system of vessels in which it is contained ramifying throughout the bodies of vertebrates. The clearest and simplest view of the nature and functions of the lymphatic vessels is that which considers them as forming a connecting link between the digestive and circulatory sys tems. It is to be understood that the largest portion of all the cells in the body never comes in direct contact with the blood, which itself touches only the linings of the veins and arteries and the cells of the pulp of the spleen. i What builds up the tissues of the body is not therefore the blood, but the lymph. Also the waste matters which are formed in the tissues are collected by the lymph, which turns it over to the blood, from which it is carried to the lungs, kidneys, skin, etc., from which it is excreted. The matters absorbed from the alimentary canal and from the blood and tissues are converted in the lymphatic glands into lymph, which supplies initial and essential ele ments of the blood. The lymphatic system, con cerned in absorption, is also called the ab sorbent system. None of the invertebrates have such a defined set of vessels; in the lower ani mals matters pass from the digestive system into the blood-system directly and without the intervention of any absorbent vessels. The lymphatic vessels constitute a distinctive char acter of the highest subkingdom of animals. Fresh supplies of nutritive matter are poured from the lymphatic system into the current of circulation, but it is difficult, if not impossible, to determine where the function of circulation ends and that of absorption begins. Hall, ex perimenting upon a patient from whom an acci dent permitted the chyle to be taken, found that the fat contained in it varied according to the diet, while the protein and sugar varied very little.
The lymph as it exists in the lymphatic vessels of a fasting animal is a colorless, trans parent fluid, odorless, with a slightly saline taste and an alkaline reaction. Four kinds of lymph are recognized: (1) the tissue lymph which fills the spaces between the cells in all parts of the body; (2) circulating lymph which flows along the lymph vessels and returns to the blood system; (3) chyle, which is a circu lating lymph in the intestines, and (4) the serous lymph which the serous cavities con tain. When microscopically examined it is seen
to be a clear plasma containing corpuscles. It resembles blood in being an alkaline fluid, and in that it coagulates or clots by the separation of the fibrinous part of the plasma; but it differs from blood in that its corpuscles are nearly all colorless, and because its solid mat ters are present in very small proportions. Ex cepting chyle, all lymph is about 95 per cent water, the rest being made up of about 3.75 per cent of proteins, 0.1 per cent of extractives and a very small proportion of inorganic mat ter. In chyle the water is about 90 per cent. The lymph-corpuscles, when passed into the current of the blood, by a simple change of medium become the white blood-corpuscles, and when partially altered they give origin to red corpuscles. The lacteals absorb the chyle di rectly from the alimentary canal, and pour it, as rudimental blood, into the current of the circulation. Then from the body generally the ordinary lymphatic vessels bring the lymph, which is further elaborated in the lymphatic glands, and pour it into the current of the blood. The actual termination of the lymphatic vessels is in the large veins in the neighbor hood of the heart. The lymphatic circulation may thus be regarded as corresponding in its general direction to the course of the venous blood.
The lymphatic vessels resemble small veins in their general structure. They are provided with valves permitting the flow of the lymph only toward the large veins near the heart and into which the lymph is poured. The only structures in which lymphatic vessels do not exist are the non-vascular parts, such as the hair, nails, outer skin and cartilaginous tissues. The flow of lymph toward the heart is induced partly by the general pressure and action of the muscles of the body, the valvular structure aid ing its propulsion as in the veins; and to this, as well as to the absorptive power, must be added the action of the contractile muscular fibres of the lymph-vessels themselves. It is not known to what extent the flow of lymph may be under the direct control of the nerves.
The fluid employed in vaccination (q.v.) is also called lymph, of which two varieties are distinguished, human and bovine. The term has likewise been applied to various serums made from bacterial cultures for preventive or cura tive use in certain diseases, especially to those serums known as antitoxins (q.v.). See IM MUNITY; KOCH, ROBERT; SERUM, THERAPY; TUBERCULOSIS.