L YMPH. 75 



the direct application of the laws of filtration and diffusion to the explanation 

 of the composition of lymph, but it is a point upon which more information 

 is necessary before it alone can be accepted as a basis for a secretion theory. 

 Meanwhile it seems evident that in spite < » t* the very valuable work of 

 Heidenhain, which has added so much to our knowledge of the conditions 

 influencing; the formation of lymph, the existence of a definite secretory 

 activity of the endothelial cells of the capillaries has not been proved. 



Summary of the Factors Controlling - the Flow of Lymph. — We may, 

 therefore, adopt, provisionally at least, the so-called mechanical theory of the 

 origin of lymph. Upon this theory the forces in activity are, first, the intra- 

 capillary pressure tending to filter the plasma through the endothelial cells 

 composing the walls of the capillaries; second, the force of diffusion depend- 

 ing upon the inequality in chemical composition of the blood-plasma and the 

 liquid outside the capillaries, or, on the other side, between this liquid ami 

 the contents of the tissue-elements; third, the force of osmotic pressure. 

 These three forces acting everywhere control primarily the amount and com- 

 position of the lymph, but still another factor must be considered. For when 

 we come to examine the flow of lymph 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 considered as being derived almost entirely from the 

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

 a greater percentage of proteids. To account for these differences Starling 

 suggests the plausible explanation of a variation in permeability in the capil- 

 lary walls. The capillaries seem to have a similar structure all over the 

 body so far as this is revealed to us by the microscope, but the fact that the 

 lymph-flow varies so much in quantity and composition indicates that the 

 similarity is only superficial, and that in different organs the capillary walls 

 may have different internal structures, and therefore different permeabilities. 

 This factor is evidently one of great importance. From the foregoing con- 

 siderations it is evident that changes in capillary pressure, however produced, 

 may alter the flow of lymph from the blood-vessels to the tissues, by increas- 

 ing 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 equalize the composition of blood 

 and lymph; and changes in the tissues themselves following upon physio- 

 logical or pathological activity will also disturb the equilibrium of composi- 

 tion, and, therefore, set up diffusion and osmotic currents. In this way a 

 continual interchange is taking place by means of which the nutrition of the 

 tissues is effected, each according to its needs. The details of this interchange 

 must of necessity be very complex when we consider the possibilities of local 

 effects in different parts of the body. The total effects of general changes, 

 such as may be produced experimentally, are simpler, and, as we have Been, 

 are explained satisfactorily by the physical and chemical factors enumerated. 



