Lymph is the reservoir of nutriment upon which every cell in the body draws. It is improbable that in health and under normal conditions the activity of any organ is ever restricted for want of sufficient food. As food is removed from lymph, it is instantly replaced by fresh food from the blood. There is some evidence—not very clear—that the removal of waste products offers greater difficulty than the renewal of supplies of food. When the activity of muscles has been excessively prolonged they ache. It has been supposed that their unwillingness to do more work is due, not to the exhaustion of the food which they use up when contracting, but to the inadequacy of the lymph and blood to carry off all refuse. This, at least, is the explanation of fatigue which is usually offered, although it is difficult to understand why the arrangements for removing waste products which have worked to perfection for eight hours should during the ninth hour become rapidly ineffective.
If a frog’s muscle, cut out of the body, has been made to contract until it refuses to work any longer, it again responds to stimulation after a solution of salt has been passed through its bloodvessels. The salt-solution brings no food; the only thing it can do is to wash away waste products. But this experiment upon a tired, isolated muscle does not necessarily throw light upon the nature of fatigue in muscles under normal conditions. The isolated muscle is using up, in contracting, food which it has stored. Cut off from the circulation, it has no means of getting rid of the lactic acid and other products into which food is changed. They may well have accumulated to a poisonous extent long before all the food has been used up. Hardly more cogent is the argument based upon the benefit which a tired man experiences from hot baths, massage, and the like. They take away the feeling of tiredness, but it does not follow that this result is due to the removal of waste products. Quickening the circulation of blood brings about renewal of the lymph. Renewal of lymph means fresh supplies of food as well as removal of waste products. Even human muscles are not perfect as machines. They will not work for an unlimited spell. There comes a time when they must have rest. Something goes wrong in the admirable adjustment which has hitherto provided exactly the right amount of food and exactly the necessary freedom from the products of action. A feeling of fatigue is the signal that the apparatus is not in a condition to work longer; but whether this feeling is due to a dislocation of the balance of supply and loss, or to some deterioration of the apparatus which calls for rest and renovation, it is at present impossible to say. It is not due to the exhaustion of muscle food. A more powerful stimulus, the urgency of fright or some other strong emotion, or an electric current applied directly to the muscle or its nerve, will still induce vigorous contraction. The muscles of a hare that has been coursed until it can run no farther still contain glycogen, muscle food.
Glycogen is stored in the liver. Fat, if it is assimilated in excess of the needs of the body, accumulates in the connective tissues. Proteins, if in excess, are either destroyed by oxidation, or partly destroyed and partly converted into fat. Increasing the amount and richness of the food does not, if nutrition is already at its best, improve the quality of the blood. The surplus of food is either stored or burnt. The composition of lymph is unaffected. Its quality is not improved by taking more food than enough. A perfect balance is maintained. Every cell is able, when conditions are normal, to obtain as much nutriment as it needs. It cannot get more. It cannot lay by food and shirk work. If it did it would grow. Reaching its optimum size, it would divide. Additional tissue would be formed. But when it does more work it needs more food; and it is a matter of common experience that the system is so adjusted that food is supplied to the tissues, not reluctantly, but with a slight tendency towards generosity. Working harder than usual, they find the lymph by which they are bathed somewhat richer in the materials that they need than the necessities of the case demand. They are able not merely to obtain all they want, but a little more. Activity favours growth.
Many attempts have been made to show that if a part of the body has more than its share of food it grows to an excessive size. John Hunter grafted a cock’s spur into its comb. It grew to monstrous dimensions. Such a result favours the view, but it is not quite conclusive. Undoubtedly the comb was richly supplied with blood, but it does not follow that the cells of the spur were able in their new situation to take advantage of this supply. Besides, the spur when projecting from the head was not subject to the accidents to which it was exposed whilst on the leg. Its size was not kept down by friction. Nor was it as hard and compact as it would have been in its normal situation. It is scarcely possible to devise any experiment that would be satisfactory now that the relations between blood and lymph and lymph and tissues are understood. In certain pathological conditions, however, hypertrophy is the result of the hyperæmia of chronic inflammation; and there is little doubt that, if we could arrange for a certain group of cells to receive lymph richer in food and freer from waste products than the perfect adjustment of supply to needs normally allows, the cells would grow.
Under perfectly healthy normal conditions growth can be induced only by use. Nature supplies the fuel which is used during activity, and a balance of food available for the construction of additional machinery. The muscle which is called upon to do work develops a greater capacity for work.
When nutrition is not at its best, the growth of muscle may be favoured by external pressure which squeezes lymph out of its tissue-spaces, and therefore leads to increased exudation from the blood. It is not improbable that in badly nourished tissues the circulation of blood is somewhat torpid and the lymph stagnant. A feeble circulation usually results in some œdema. The muscles, or rather the connective tissue which envelops and penetrates them, feels doughy, instead of being, as it should be, firm and elastic. Under these conditions massage is undoubtedly of service. Squeezing the muscles displaces lymph, and, if the pressure is properly directed, drives it along the lymphatic vessels. Fresh lymph exudes from the capillary bloodvessels, and the muscle-fibres, surrounded with a more abundant supply of nutriment, benefit, as, in a vigorous person, they benefit from use.
Lymph is an exudate from blood. Its composition therefore depends upon that of blood-plasma, but it tends to differ from it owing to the influence of two causes. In the first place, the walls of the capillary bloodvessels restrict exudation. Red blood-corpuscles cannot pass through them. Proteins which are non-diffusible are, according to the circumstances of the tissues, held back to a greater or to a less extent. The pseudo-capillaries of the liver let them pass, as has already been said. The capillaries of the limbs restrict their passage to such proportions as, it may be supposed, are absolutely necessary for the nutrition of the tissues. In the second place, tissues remove food from lymph and add to it waste products. Hence the lymph issuing from a limb, after full contact with the tissues, contains less of the former and more of the latter—less sugar, for example, and rather more oxidized nitrogenous substances, lecithin and other things termed collectively “extractives,” because they can be extracted from dried blood or lymph by ether. The reaction of lymph is alkaline. After a time it coagulates, but coagulation is slower, and the clot less firm than in the case of blood.
As the composition of lymph depends upon the source from which, and the conditions under which, it has been obtained, it is unnecessary to state the results of a chemical analysis. It suffices to say that lymph contains all the substances which are present in the plasma of blood, but not necessarily in the same total amount or in the same relative proportions. Speaking generally, leucocytes are present in about the same numbers as in blood—6,000 to 8,000 to the cubic centimetre; but leucocytes are everywhere present: in blood, in the lymph, in lymph-vessels, in the tissue-spaces. As they are not passively floating bodies like red blood-corpuscles, but active migratory organisms, they tend to accumulate in one situation and withdraw from another, in accordance with the opportunities which the different localities afford. They desert effused lymph, blisters, ascitic fluid, and the like. They are not found in the lymph in the pericardium. There are fewer in the lymph coming from the intestines after a meal than in the same lymph during the intervals between meals. Their departure from effused lymph might easily be explained. It is not so easy to account for their comparative absence from the lymph in the lacteals when it is heavily charged with fat and other products of digestion. Such leucocytes as are present at this time are loaded with fat granules which they have stolen from the chyle, as the lymph in the lacteals is usually termed. One would need to be very intimate with a leucocyte before one ventured to give reasons for all its movements. Lymph contains the same proteid substances as blood, and in the same relative proportions, but usually in smaller quantity.
Incidental reference has been made to the great lymph-spaces—peritoneal, pleural, and pericardial. The brain and spinal cord are separated from their outer membranes by a lymph-space. There are also spaces within the brain—the ventricles—and a central canal in the spinal cord. The aqueous and vitreous humours of the eye are also lymph-spaces, although the latter contains some remnants of tissue. The joint cavities are lymph-spaces. So also are the bursæ which surround tendons or separate them from bones. It is not, however, justifiable to include all these cavities in a single category, either from the point of view of their purpose, their mode of formation, or the nature of their contents. The peritoneal, pleural, and pericardial spaces are parts of the great primitive body-cavity, or cœlom. The two first are potential rather than actual. Normally they contain just sufficient fluid to moisten the apposed surfaces of the endothelium which lines their walls and covers the organs which they contain. There is no fluid in them which can be collected and labelled “peritoneal” or “pleural” fluid. The purpose of the spaces is to allow of movement without friction—in the one case of the intestines, in the other of the lungs. It is possible to take a spoonful or so of fluid out of the space which surrounds the heart. It has the usual composition of lymph. It contains proteins, but is not spontaneously coagulable. Leucocytes are absent, a fact which probably accounts for its not clotting. The fluid inside the cerebro-spinal system is extremely dilute. Its principal salt—its principal constituent, indeed—is sodic chloride. It contains hardly a trace of proteins, and these in a modified condition—proteoses. It also contains pyro-catechin, a benzoic alcohol. This substance has long been recognized as a constituent of cerebro-spinal fluid, owing to the fact that, like sugar, it reduces copper salts when heated with them in an alkaline solution. It appears to be one of the products of proteid decomposition. Although exuded as lymph from the bloodvessels of the chorioid plexuses, the composition of cerebro-spinal fluid has been profoundly changed by the activity—it might almost be called the digestive activity—of the epithelium which lines the cerebro-spinal canal. There is a theory that the ancestors of all vertebrate animals were organized on a very different plan from that of their distant descendants. Our cerebro-spinal canal was their stomach and intestine. It would appear that the lining epithelium of these organs, although disused for millions of years, cannot resist the temptation to digest the lymph which they contain! The fluid in joints contains mucin (the essential constituent of mucus), or a substance resembling mucin. In this case the joint-membrane has added something to lymph without removing or destroying any of its other constituents.
Other illustrations might be given showing how the plasma of blood is altered in composition while it is passing out of, or after it has passed out of, capillary bloodvessels. Perhaps it would be more logical to start on the outer side of the walls of the capillaries; since blood may, very properly, be regarded as a tissue, dependent, like all other tissues, upon diffusion from lymph for the nutrient materials that it needs. In the wall of the alimentary canal it receives supplies via the lymph. It drops them in the liver, its garde-manger, to pick them up again as they are wanted. The torrent of lymph which the thoracic duct discharges into the veins of the neck conveys the fat which could not traverse the walls of the capillary bloodvessels, and much of the reserve of food which the blood had deposited in the liver. Only about one-quarter of the fluid of the body (one-thirteenth of the body-weight) is included within the blood-system; but this enclosed fluid, owing to the fact that it is kept in circulation by the heart, replenishes and purifies the much larger quantity which does not circulate. The unenclosed lymph has in particular situations a chemical composition which varies widely from that of the blood. Imagine a marsh through which a river flows—the vast plains of water-plants on the Nile above Fashoda, for example. There is a constant interchange between the flowing water of the river and the stagnant water of the marsh. In any given part of the marsh the quality of the water will depend upon what it has been able to take from, and what it has given back to, the river; upon what the water-plants have taken from it, and what they have added to it. Boats which cannot penetrate the walls of reed keep to the open channel of the Nile. Fish swim, now in the river, now in the narrow passages and open pools of the marsh. So it is, in a way, with the fluid in the spaces and cavities of the lymphatic system and in the bloodvessels which traverse them, and with its migratory inhabitants. In our extravagant analogy read leucocytes for fish. Fish have two reasons for wandering from river to marsh. Amongst the water-weeds they hunt for food; they seek quiet places in which to breed. In this matter the analogy holds good. A leucocyte may be overtaken with cell division anywhere—in the blood-stream or in a lymph-vessel. But cell division very rarely occurs except in certain favoured spots. The breeding-places chosen by leucocytes are sheltered situations in connective tissue where the blood-supply is abundant, and the eligibility of such a spot is much increased by its being near to a field where their services are likely to be called for. The nests of connective tissue made by the leucocytes are of three kinds, termed respectively diffuse adenoid tissue, lymph-follicles, and lymphatic glands. The connective tissue beneath the mucous membrane of the whole of the respiratory tract—trachea, bronchi, and bronchioles—is diffuse adenoid tissue. It presents no special structure, but its spaces are packed with leucocytes in various stages of cell division, and young leucocytes, or lymphocytes, as they are usually named. Some of the lymphocytes make their way into the blood or into the lymph. Others, acquiring their full dimensions, scour the epithelium which lines the respiratory tract for germs and other foreign bodies which are drawn into the tract with inspired air. They may be seen pushing aside the cells of the lower strata of the epithelium, on their way to the surface, or returning to the subepithelial connective tissue with germs, or particles of soot, or débris of epithelial cells which they have taken into their substance ([Fig. 4, B]).