The Body at Work: A Treatise on the Principles of Physiology

by Alex Hill

Fig. 3.—A Ductule and Two Acini of a Mucous Gland of the Mouth, with a
Muscle-fibre cut Longitudinally; Capillary Bloodvessels and Connective Tissue.

Stellate connective-tissue cells form a labyrinth of intercommunicating lymph-spaces which separate the gland-cells and the muscle-fibre from the walls of the capillary bloodvessels. The capillaries contain circular red blood-corpuscles and nucleated leucocytes. Some of the leucocytes are squeezing their way either out of a capillary into a lymph-space or vice versa. A granular leucocyte is to be seen in a lymph-space at the bottom of the picture.

The facility with which the constituents of blood pass out to the lymph, and the constituents of lymph pass into the blood, depends upon the condition of the walls of the capillary vessels. Water and substances dissolved in water might pass through the wall of a capillary vessel in either of three ways—by filtration, by osmosis, or by secretion. A filter is a porous barrier, which allows water and all substances dissolved in water to traverse it. The solution passes through unchanged in composition. Only solid particles are kept back. The rapidity with which fluid passes through a filter varies as the difference between the pressure on the one side and the pressure on the other. A membrane does not allow of filtration. Water and things dissolved in water pass through it by osmosis. Some things it will not allow to pass; such, for example, as gum, mucin, white of egg. To others it offers resistance in varying degrees. Most of the things that can diffuse through a membrane are capable of crystallization; but the membrane exercises some control over the passage of even crystallizable substances when in solution. If a membranous tube containing water in which proteins, sugar, and various salts are dissolved is hung in a basin of pure water, the proteins remain in the tube; the sugar and the salts pass through its wall into the surrounding water. But they pass at different rates. Those of small molecular weight pass more quickly than those whose molecule is heavy. After a time a condition of equilibrium is established. No more salts pass out of the tube. If now the contents of the tube and the contents of the basin are analysed, it will be found that the tube contains all the proteins, some of the sugar, and some of each of the salts, although not in the proportions in which they were present at the commencement of the experiment. The water in the basin contains some sugar and some of each of the salts, but not in the same proportions in which they are found in the tube. As a matter of fact, the same number of molecules would be present, per unit volume, on each side of the membrane—in the tube and in the basin. In this respect the percentage composition of the two solutions would be the same. But some of the molecules being heavy, others light, the weight of salts which unit volume of the solution in the tube would contain would not be the same as the weight of salts in unit volume of the solution in the basin. A membrane exerts a discriminating action on the substances which pass through it. Secretion is osmosis in disguise. It may be even filtration in disguise. A gland-cell (like an amœba) takes things up and passes them out without regard to their osmotic equivalent. It seems to exercise a choice. It seems to act in disregard of the laws both of filtration and of osmosis. So, at least, it appears to us when we are looking at the result in ignorance of what has happened inside the living cell. The passage from blood to lymph and vice versa through the wall of a capillary vessel is in certain situations or at certain times a mere process of filtration; at others a process of restricted filtration. If the wall is behaving as a perfect membrane, it is a process of diffusion, or osmosis. It seems unnecessary to regard it, in any case, as a process of secretion. The more widely the capillaries are dilated, the less resistance do they offer to exudation. The narrower their calibre, the greater is the restraint which they place on the escape or entrance of fluid. When the skin of the palm of the hand is not sufficiently thick to protect the soft tissues beneath it from the injurious effects of the prolonged pressure of an oar or an axe, the capillary vessels of the under-skin dilate; more lymph transudes; the skin is raised up as a blister. The same thing happens when the capillaries are dilated and paralyzed by scalding water. The fluid of a blister has much the same constitution as blood-plasm, except that it contains less proteid substance. These results might be regarded as purely mechanical—the direct effects of pressure or heat upon the membranous capillary wall. But the “vital” element is more important. The capacity of endothelium to act as a barrier depends upon its nutritive condition—its vital integrity, as it might be termed; which no doubt in the last resort means its chemical relation to the fluids which bathe it. Now and again blebs, like blisters, are formed on the skin—the herpes which appears about the mouth; urticaria, which is more generally distributed; and various other cutaneous disorders. Frequently a connection can be traced between these eruptions and the consumption of a particular food. An attack of urticaria results not uncommonly from eating lobster, mussels, rook-pie, or some few other articles of diet. Various things—bad fish, for example—may produce the same effect; but shell-fish have an especially evil reputation. If extract of lobster or of mussels be injected into the blood of an animal, the amount of lymph which leaves the blood is markedly increased. The extract acts as a poison upon the endothelium of the capillary walls. It increases its permeability in all conditions in which lymph escapes in undue quantity from the blood-stream, or escapes more rapidly than it is absorbed; the nutritive condition of the endothelium is disturbed. Its unusual permeability is due in part, no doubt, to the dilatation of the capillary tube, the stretching of its membranous wall; but it is due also to the diminished vigour of the endothelial cells. They have lost to a certain extent their capacity for holding their edges in perfect apposition.

When the circulation is sluggish, owing to the inefficiency of the heart, the tissues become œdematous. In other words, lymph accumulates in the tissue-spaces. When the skin of a healthy person is pressed, it returns to its natural position as soon as the pressure is removed. If there is a tendency to dropsy—for ages the term “hydropsia” has been thus familiarly clipped—the finger leaves a pit behind it when pressed upon the skin. It is some little time before the lymph in the connective-tissue sponge readjusts the surface. Excessive escape of lymph from the blood, or its insufficient return into the blood, may also be the result of obstruction to the flow in the great veins. When the veins of the leg are varicose, the weight of the column of blood in the distended vessels impedes its circulation. After standing, the tissues about the ankle become œdematous. The œdema disappears on lying down. A hardening (cirrhosis) of the liver impedes the circulation of the blood which comes to it through the portal vein from the walls of the alimentary canal. The capillaries of the stomach and intestine are distended. Lymph accumulates in the abdominal cavity, producing ascites, another form of dropsy.

It is almost hopeless to attempt to disentangle the various factors which disturb the balance between blood and lymph—excessive outflow from blood, deficient inflow from lymph, stretching of the endothelium of the capillary tubes, imperfect nutrition and consequent imperfect apposition of the endothelial scales, increased permeability of the scales. The exudation which accompanies inflammation would seem to be due to the diminished vitality of the endothelium rather than to a mechanical factor, such as increased blood-pressure in the capillaries, and their consequent distention. Ascites is, apparently, a purely mechanical result of the resistance offered to the passage of blood through the liver; but pleurisy, the accumulation of lymph in the space between the lungs and the chest-wall, cannot be explained in the same way. There is no undue pressure on the vessels in which the blood circulates through the inflamed pleura (the investing membrane of the lungs and lining membrane of the chest), yet the walls of the capillaries fail to maintain a proper balance between blood and lymph.

Hitherto we have spoken of the lymphatic system as a labyrinth of communicating spaces containing stagnant fluid, which is kept in a fitting state by egress and ingress out of and into blood. Such a mental picture is substantially correct. But the system is complicated by the presence of lymphatic vessels. Cells of the connective-tissue sponge-work arrange themselves side by side. They flatten into endothelial scales. The borders of the scales close up. They form lymphatic channels, wider than blood-capillaries, but strictly comparable in every other respect. The lymph capillaries unite into larger vessels. The larger vessels are connected by cross-branches; they form plexuses. Their walls are strengthened with fibrous tissue. Like the veins, they are abundantly provided with valves, which check any tendency to a backward flow on the part of the fluid which they contain. Lymphatic plexuses surround and accompany the larger bloodvessels. They are disposed on the surface of muscles and glandular tissues. They are abundant beneath the skin. Nearly three centuries ago the lymphatic vessels of the mesentery, which collect products of digestion, especially fat, from the walls of the alimentary canal, were recognized owing to the milkiness of their contents after a meal. They were, on this account, termed “lacteals.” Other lymphatic vessels, owing to their transparent walls and colourless contents, are not easily seen; but they are readily injected with mercury or other fluids which render them conspicuous. In the upper part of the thigh, in the armpit, or in the neck, they are about large enough to admit a crow-quill. Those from the lower limbs, from the viscera, and from the walls of the abdomen converge to a receptacle which lies in front of the spinal column. The receptaculum chyli is continued upwards as the thoracic duct, which pours the lymph into the great veins of the left side of the neck and of the left arm just where they join together.

The thoracic duct provides for the overflow of lymph from the spaces of the body. There is no circulation of lymph. Lymph from the liver and from the intestines is constantly draining into the thoracic duct, and thus returning to the blood-stream by a short direct route, entering it without the necessity for reabsorption through the walls of capillary vessels. By no means all of this fluid has exuded from the blood-stream. Much of it is water which was poured into the stomach as gastric juice, and into the intestines as the secretions of the pancreas and other glands, or imbibed through the mouth and absorbed by the lymphatics of the alimentary canal. The remainder of the water taken up from the alimentary canal enters its bloodvessels. The diluted blood flows to the liver, loaded with digested products which the liver will store. As the blood parts with them the additional water which has served for their transport exudes from the capillaries of the liver into lymphatics, which empty it into the thoracic duct. Large quantities of water are used in washing out digested products. Secreted into the alimentary canal by the digestive glands, it passes out through its wall as the vehicle of digested products. Collected by lymphatic vessels, it is either carried directly into the thoracic duct, or passed from lymph into blood, carried by blood to the liver, again transferred from blood to lymph, and borne by the lymphatic vessels of the liver to the thoracic duct.

Water exuded from blood into lymph may be reabsorbed into the blood near the place where it was poured out, or it may reach the blood via the thoracic duct. It would seem that the former is the natural, the latter the emergency route; the former the course taken when an organ is tranquil, the latter a necessity when the organ is active. If the large lymphatic vessels of a limb are cut, no lymph escapes from them so long as the limb is at rest. When the muscles contract lymph begins to flow. If the limb is flexed and extended by hand, lymph flows. If the muscles are squeezed or massaged, lymph flows. As the flow is set up both by active contraction of the muscles and by passive movements in which the muscles do not take part, it clearly must be due to external pressure on the lymphatic vessels. As they are provided with valves, squeezing them converts them into pumps. The fluid which they contain is bound to go forwards. Additional fluid is squeezed into them from the tissue-spaces. To a large extent, therefore, the outflow of lymph from contracting muscles is to be explained as the result of the pressure which the swelling muscles exert upon the lymphatic vessels within their sheaths. But there is another factor which must not be overlooked, although it cannot readily be estimated. When a muscle is actively contracting its bloodvessels dilate. There is a greater exudation of lymph; and reabsorption by blood is not equal to the exudation. The surplus leaves the limb by the lymphatic vessels. A gland is never at rest. In the intervals between the ejection of its secretion its cells are preparing materials for the next outflow. Lymph is always flowing from a gland; its amount increases as the activity of the gland increases. More lymph leaves the blood when the gland is exceptionally active than when it is relatively quiet. Some of it is not reabsorbed into the blood. A certain proportion of the waste products of the active gland are hurried away by the overflow system in the direction of the thoracic duct.