LYMPH. 71 



contact with the tissues, to which it brings the nourishment and oxygen of 

 the blood and from which it removes the waste-products of metabolism. This 

 extravascular lymph is collected into small capillary spaces that in turn open 

 into definite lymphatic vessels. These vessels unite to larger and larger 

 trunks, forming eventually one main trunk, the thoracic or left lymphatic 

 duct, and a second smaller right lymphatic duct, which open into the blood- 

 vessels, each on its own side, at the junction of the subclavian and internal 

 jugular veins. While the supply of lymph in the lymph-vessels may be consid- 

 ered as being derived ultimately entirely from the blood-plasma, it is well to bear 

 in mind that at any given moment this supply may be altered by direct inter- 

 change with the plasma on one side and the extravascular lymph permeating the 

 tissue-elements on the other. The intravascular lymph may be augmented. 

 for example, by a flow of water from the plasma into the lymph-spaces, or 

 by a flow from the tissue-elements into the lymph-spaces that surround them. 

 The lymph movement is from the tissues to the veins, and the flow is main- 

 tained chiefly by the difference in pressure between the lymph at its origin in 

 the tissues and in the large tymphatic vessels. The continual formation of 

 lymph in the tissues leads to the development of a relatively high pressure in 

 the lymph capillaries, and as a result of this the lymph is forced toward the 

 point of lowest pressure — namely, the points of junction of the large lymph- 

 ducts with the venous system. A fuller discussion of the factors concerned in 

 the movement of lymph will be found in the section on Circulation. As would 

 be inferred from its origin, the composition of lymph is essentially the same as 

 that of blood-plasma. Lymph contains the three blood-proteids, the extractives 

 (urea, fat, lecithin, cholesteriu, sugar), and inorganic salts. The salts are found 

 in the same proportions as in the plasma; the proteids are less in amount, espe- 

 cially the fibrinogen. Lymph coagulates, but does so more slowly and less 

 firmly than the blood. Histologically, lymph consists of a colorless liquid con- 

 taining a number of leucocytes, and after meals a number of minute fat-drop- 

 lets ; red blood-corpuscles occur only accidentally, and blood-plates, according 

 to most accounts, are likewise normally absent. 



Formation of Lymph. — The careful researches of Ludwig and his pupils 

 were formerly believed to prove that the lymph is derived directly from the 

 plasma of the blood mainly by filtration through the capillary walls. Emphasis 

 was laid on the undoubted fact that the blood within the capillaries is under 

 a pressure higher than that prevailing in the tissues outside, and it was Hip- 

 posed that this excess of pressure is sufficient to squeeze the plasma of the 

 blood through the very thin capillary walls. Various conditions that alter 

 the pressure of the blood were shown to influence the amount of lymph 

 formed in accordance with the demands of a theory of filtration. More- 

 over, the composition of lymph as usually given seems to support such :i 

 theory, inasmuch as the inorganic salts contained in it are in the same concen- 

 tration, approximately, as in blood-plasma, while the proteids are in less con- 

 centration, following the well-known law that in the filtration of colloids 

 throueh animal membranes the filtrate is more dilute than the original solution. 



