COMPOSITION AND FORMATION OF LYMPH. 477 



Summary of the Factors Controlling the Flow of Lymph.— 



We may adopt, provisionally at least, the so-called mechanical 

 theory of the origin of lymph. Upon this theory the forces in 

 activit}^ are, first, the intracapillary pressure tending to filter 

 the plasma through the endothelial cells composing the walls 

 of the capillaries, and second, that form of molecular energy which 

 gives rise to the phenomena of diffusion and osmotic pressure. 

 By the action of this force, the flow of water from one place to 

 another is influenced in accordance with the difference in concen- 

 tration of the dissolved substances. These two forces acting 

 ever^-where control primarily the amount and composition of the 

 lymph; but still another factor must be considered; for when we 

 come to examine the flow of l5anph in different parts of the body 

 striking differences are found. It has been shown, for instance, 

 that in the limbs, under normal conditions, the flow is extremely 

 scanty, while from the liver and the intestinal area it is relatively 

 abundant. In fact, the lymph of the thoracic duct may be con- 

 sidered as being derived almost entirely from the latter two 

 regions. Moreover, the lymph from the liver is characterized by 

 a greater percentage of proteins. To account for these differences 

 Starling suggests the plausible explanation of a variation in permea- 

 bility in the capillary walls. This factor is evidently one of great 

 importance, although it is not possible to state the character of the 

 changes supposed to occur. The idea that the permeability of the 

 capillaries may vary under different conditions has long been used 

 in pathology to explain the production of that excess of lymph which 

 gives rise to the condition of dropsy or edema. The theories and 

 experiments made in connection with this pathological condition 

 have, in fact, a direct bearing upon the theories of lymph forma- 

 tion.* Under normal conditions the lymph is drained off as it is 

 formed, while under pathological conditions it may accumulate in 

 the tissues owing either to an excessive formation of lymph, or to 

 some diminution in the process of absorption, or to some interrup- 

 tion in its circulation. 



From the foregoing considerations it is evident that changes 

 in capillaiy pressure, however produced, may alter the flow of 

 lymph from the blood-vessels to the tissues, by increasing or 

 decreasing, as the case may be, the amount of filtration; changes in 

 the composition of the blood, such as follow periods of digestion, 

 will cause diffusion and osmotic streams, tending to equahze the 

 composition of blood and lymph; and changes in the tissues them- 

 selves following upon physiological or pathological activity will 

 also disturb the equilibrium of composition, and, therefore, set up 



* Consult Meltzer, "Edema" ("Harrington Lectures"), "American Medi- 

 cine," 8, Nos. 1, 2, 4, and 5, 1904. 



