THEORIES OFLUDWIG AND HEIDENHAIN. 293 
somewhat increased. Hence the only region of the body below the 
point of obstruction where the capillary pressure is not much diminished 
is the liver. Now we find thai the liver is also the sole source of the 
lymph obtained under these circumstances. If the hepatic lymphatics 
be ligatured, and the thoracic aorta be then obstructed, the flow of 
lymph from the thoracic duct is absolutely stopped. 
These three experiments show, therefore, that the lymph production 
in the organs of the abdomen is directly proportional to the capillary 
pressure in these organs, and not independent of them, as was imagined 
by Heidenhain. 
Hydrcemia and hydrcemic plethora. — In another series of experi- 
ments we find, as was predicted by Ludwig (cf. p. 288), that a marked 
increase in the lymph flow is produced by a general rise of capillary pres- 
sure in all the organs of the abdomen. Such a general rise of capillary 
pressure may be brought about by the injection of large cpuantities 
of normal saline fluid into the circulation, thus causing a condition of 
hydrsemic plethora. Under such circumstances the lymph may be in- 
creased from fifty to one hundred times in amount, and may in some 
cases run from the cannula in the duct in a steady stream. Now, in 
hydraemic plethora there are two changes in the circulation which might 
possibly be responsible for the increased production of lymph — first, 
the change in the composition of the blood, and secondly, the increased 
pressure in the capillaries of the abdominal viscera, "We can decide 
which of these two factors is responsible for the increased lymph flow 
by a very simple experiment. Previously to injecting 300 c.c. of normal 
saline, we bleed the dog to 300 c.c, so that after the injection the total 
amount of circulating fluid is the same as at the beginning of the 
experiment. In this way we entirely avoid any rise of capillary 
pressure, while we have diluted the blood to an even greater extent 
than in the experiments in which hydraemic plethora was produced. 
The effect of such a simple hydra?mia is to increase the lymph flow 
from 3 c.c. in ten minutes to 4 or 6 c.c. in ten minutes ; whereas, if 
hydraemic plethora were produced, the lymph would be increased from 
3 c.c. to 30, 50, or 100 c.c in ten minutes. It is evident, therefore, that 
in the production of this increased lymph flow the all-important factor is 
the rise of capillary pressure ; although the slight increase in the lymph 
flow observed as the result of simple hydrsemia shows that, as might be 
expected, a watery plasma gives rise to a transudation of lymph more 
easily than does the normal more concentrated plasma. 
JTeidenJioi/i's second class of lymphagogues. — In a precisely similar 
manner we may explain the mode of action of the substances which 
were described by Heidenhain as the second class of lymphagogues. 
These include bodies such as salt, sugar, potassium iodide, etc. The 
injection of a strong solution of dextrose (30 grms. in 30 c.c. water) into 
the veins of an animal causes a considerable increase in the lymph flow 
from the thoracic duet. The lymph at the same time becomes more 
watery than at the commencement of the experiment. Heidenhain 
ascribes this effect to a specific excitation of the secretory activities of 
the endothelial cells. The effect, however, can be explained in a much 
more simple fashion. All these solutions have an osmotic pressure 
which is considerably higher than that of normal blood plasma. A 
solution of dextrose that should be isotonic with the blood plasma would 
contain from 5 to 6 per cent, of this body. When we inject a solution 
