2 SS PROD UCTION AND ABSORPTION OF L YMPH. 
blood pressure, carried out by the aid of the mercurial manometer of 
Ludwig, these authors concluded that the chief factor in the forma- 
tion of lymph was the pressure of the blood in the capillaries, and 
that in fact the lymph was essentially only the fluid part of blood 
which had filtered through the vessel wall into the surrounding 
tissues. On arriving in the tissues, this lymph or blood filtrate was 
still under a certain pressure, derived from the blood pressure, and 
it was this pressure which occasioned the movement of the lymph 
into and along the lymphatics. Ludwig concluded that the flow and 
composition of the lymph must be explained not only by filtration 
of the fluid parts of the blood, but also by processes of osmosis taking 
place between the tissue juices and the blood. He summarises his theory 
in the following words : — "The blood which is contained in the vessels must 
always tend to equalise its pressure and its chemical constitution with 
those of the extravascular fluids, which are only separated from it by the 
porous blood-vessel walls. If, for example, the quantity of blood in the 
vessels has increased, the mean blood pressure is also increased, and at 
once a portion of the blood is driven out into the tissues by a mere 
process of filtration. The same result is brought about when the con- 
stitution of the blood is altered by the absorption of food or by increased 
excretion by the kidneys, blood, or skin, or when the composition of the 
tissue fluids is altered in consequence of increased metabolic changes 
taking place in the tissues. In the latter case, the changes brought 
about in the lymph are effected by processes of diffusion." Since it is a 
condition of the maintenance of life that these chemical changes in the 
tissues should go on, and that the waste products should be continually 
excreted by the kidneys, lungs, and skin, there must be at the same 
time constant changes in the amount and composition of the lymph 
produced. 1 
The testing of this, the mechanical theory of lymph formation and 
the lineal descendant of the theory propounded two hundred years 
previously by Bartholin, has been the object of all subsequent investiga- 
tions dealing with this question. Although we cannot claim to have 
arrived at a final decision on the matter, I shall endeavour to show in 
the following pages that the two processes — filtration and diffusion- 
described by Ludwig, will probably account for the lymph flow and 
composition in all the cases which have been sufficiently investigated. 
It was shown many years ago by Magendie and others, that chemical 
differences between blood and lymph provoked a transference of the 
substance that was in excess from one side of the vessel wall to the 
other. Thus, if colouring matters, salts, or sugar be injected into the 
blood, they are very shortly afterwards found in the lymph in various 
parts of the body. If, on the other hand, these substances be injected 
into the tissue spaces or into the pleural or peritoneal cavities, their 
existence can very soon be detected in the blood, whence they make 
their way into the urine. Other instances of the extreme rapidity with 
which osmotic interchanges take place between the blood and lymph 
will be mentioned later on in dealing with the action of lymphagogues. 
Since these interchanges take place after the introduction of abnormal 
as well as normal substances into the body, we must assume the general 
applicability of the results, and look upon processes of diffusion or 
osmosis as one of the factors in regulating the composition of the lymph. 
1 "Lehrbuclider Physiologie," 1861, Aufl. 2, Bd. ii. S. 562. 
