more copious in amount than in the former instance. A further
difference is that this lymph is more concentrated, a feature
which always characterizes liver lymph. Ligature of the aorta
may or may not cause a rise in the liver capillary pressure, and
it has been shown that if the pressure rises there is an increased
lymph-flow from the liver and conversely. The increase of
lymph comes entirely in this instance also from the liver. It is
in fact but a special instance of the former experiment. From
these results it has been argued that lymph formation is simply
a filtration fundamentally, and the lymph-flow is determined
mainly by the capillary pressure. Variations in the quantity of
lymph issuing from different organs have been on this theory
ascribed to differences in the permeability of the capillaries of
the organs. Thus as liver lymph is richest in protein content
and is produced in greatest amount, it has been concluded
that the liver capillaries possess the highest permeability. The
intestines stand next in producing a concentrated lymph, and
their capillaries are therefore assumed to stand second as regards
permeability. Lastly, the lymph coming from limbs and other
organs is much poorer in solids and much less copious in amount.
Hence it is argued that their capillaries show the least permeability.
It is, however, very unsafe to compare the liver capillaries
with those of other organs, since they are not in reality
capillaries but rather venous sinuses, and their relation to the
liver cells is characteristically different from that of ordinary
capillaries. If an animal is at rest, no lymph flows from the hind
limbs. To obtain a sample of limb lymph it is necessary to
massage the limb. If, however, the veins to the limb be ligatured,
we obtain a flow of lymph. The ligature of course causes
a rise of the capillary pressure, and it has been argued that
this rise of pressure starts a filtration through the capillary wall
and hence a flow of lymph. But the stoppage of the blood-flow
also damages the capillary wall and tissue cells by asphyxiation,
and the resulting lymph-flow is in all probability the resultant
of many complex processes. This case is analogous to the production
of oedema in cases of heart disease where the circulation
is feeble and the oxygen supply to the parts deficient. The
results of these experiments form the main evidence in support
of the filtration theory of lymph formation. They were first
systematically studied by Heidenhain, to whom we owe so much
of our knowledge of lymph formation. He did not, however,
conclude that they established the filtration theory.
In continuing his observations Heidenhain next studied the results following the injection of a number of substances into the blood. He found many which on injection gave rise to an increased lymph-flow from the thoracic duct, and arranged them in two classes. As instances of lymphogogues of the first class we may mention extract of mussels, leech extract, peptone, extract of crayfish muscle, extract of strawberries, of raspberries and many other like substances. Lymphogogues of the second class comprise neutral salt solutions, urea, sugar, &c. Considering the latter class first we may take as a type a solution of sodium chloride. Injection of such a solution causes a large increase in the lymph-flow, and it has been proved that the lymph comes from the liver and intestines only—chiefly from the former. It is especially to be noted that there is no lymph-flow from the limbs, and the same is true for all lymphogogues of this class. As indicated above, the injection of a saline solution leads to a large and rapidly effected transport of fluid from the blood into muscle tissue, but though there is this large increase in tissue fluid, no lymph flows from the tissue. This result very powerfully disfavours the filtration theory of lymph formation. It practically refutes the idea that lymph formation is solely dependent upon such processes as filtration, osmosis and capillary permeability only. It brings out quite clearly that the exchange of fluid and dissolved salts, &c., between the blood and a tissue, and the flow of lymph from that tissue, are two separate and distinct processes, and especially that the first does not determine the second. Also it is to be noted that the injection of a strong salt solution also excites a flow of duct lymph, again arising from the liver and intestines, but none from the limbs. In this instance, as previously stated, the muscles of the limbs are losing water, and so presumably are the liver and intestinal cells. This independence of tissue-blood exchange and lymph-flow is distinctly in favour of the view, which is rapidly gaining ground from histological observations, that in all instances the lymphatics commence in a tissue as closed capillary vessels.
Turning, in the next place, to the lymphogogues of the first class, it has been proved that the origin of this increase of flow is again from the liver. Very many of the substances of this class are bodies which may when taken cause urticarial (nettle-rash) eruptions, a state which is generally regarded as being due to an action upon the capillary endothelium. Their action as lymphogogues is also generally ascribed to an effect upon the capillary wall rendering it according to some more permeable, according to others leading to a direct secretory action on the part of the endothelium. We also know that many of the bodies of this class act upon the liver in other directions than in exciting an increased lymph production. Thus they may cause an increase in bile secretion, or, as in the case of peptone, the liver cells may be excited to produce a new chemical material, in this instance an antithrombin.
We have now to consider the effect of throwing an organ into activity upon the lymph-flow from the organ. In all cases in which it has been examined it is found that increased activity is accompanied by increased lymph-flow. Thus, to take the instance of the submaxillary gland, which at rest does not discharge any lymph, stimulation of the chorda tympani is followed by a flow of lymph accompanying the flow of saliva simultaneously excited. The stimulation of the nerve also produces dilatation of the blood-vessels and therefore a rise in capillary pressure. But that this vascular change is not the factor determining the lymph-flow is proved by the administration of a small dose of atropine, which arrests the secretion without influencing the vascular reaction following chorda stimulation. After the atropine no lymph-flow occurs on stimulating the nerve. Many other instances of a similar kind might be adduced. Thus, we have seen that peptone specifically excites the liver cells and also causes an increased lymph-flow from the liver; or, as a last instance, the injection of bile salt excites a flow of bile and also excites a flow of lymph from the liver. The supporters of the filtration theory have argued that as activity of a tissue is necessarily accompanied by the discharge of metabolites from the active tissue cells, and as these are of small molecular size, they must set up an osmotic effect. Water is therefore drawn into the tissue spaces, and this rise in fluid content results mechanically in a flow of lymph from the organ. The lymph simply drains away along the open lymphatics. This argument, however, loses all its force when we recall the fact that we may set up an enormous flow of fluid and salt into a tissue and its tissue spaces without causing the least flow of lymph. Further, there is no reason to suppose that the metabolites discharged from a tissue during activity are produced in large quantities. The chief metabolite is undoubtedly carbonic acid, and this diffuses very rapidly and is quickly carried away by the blood. If, moreover, as is probably the case, the lymphatics commence as closed capillaries, we have a further difficulty in explaining how the fluid is driven through the lymphatic wall. Either we must imagine the wall to be porous or there must be a greater pressure outside than inside, and it is very difficult to conceive how this is possible. As a general conclusion, then, it seems much more probable that we are here dealing with a secretory process, and that the active tissue produces some substance or substances—it may be carbonic acid—which throws the lymphatic capillary cells into activity.
To sum up in a few words the present state of our knowledge as to lymph formation we may say that the exchange of water and salts between the blood and the tissues is probably entirely determined by processes of filtration and osmosis. Further, that the physical condition of the capillary cells is frequently altered by many chemical substances, and that in consequence it may permit exudation into the tissue spaces much more freely. In the next place, the flow of lymph from a tissue is not solely determined by the amount of the tissue fluids. The lymph
