CHAPTER XXXIII

THE LYMPH AND THE LYMPHATIC SYSTEM

MOST people do not know even of the existence in their own bodies of a fluid called "the lymph," and of a system of vessels and spaces containing it which ramify like the blood-vessels into every part of the body. This arises from the fact that the lymph is translucent and colourless. You can see the finest blood-vessels when the body of a dead rat, sheep, or man is opened, because they are filled with the beautiful red blood, and appear as a rich, coloured network. But the lymph and the lymph-vessels escape notice, and, indeed, are invisible except the largest, because they are colourless. They remained unknown to anatomists long after arteries and veins, and the fine networks of hair-like vessels or capillaries connecting them, were thoroughly well studied. It is, when one thinks of it, a very noteworthy fact, tending to convince us of the readiness with which we may (in the absence of careful examination and attention) overlook the most weighty things, that here is a great system of vessels and spaces in the human body and in that of other animals, carrying on most important operations in our daily life, and yet most of us have never seen any evidence of its existence, and never hold it in our mind's eye as part of the great mechanism of the animal body.

The lymph is a clear, colourless fluid, with "corpuscles"--minute nucleated cells or particles of protoplasm--floating in it. The liquid part is closely similar in its properties and chemical constitution to the liquid part of the blood. It, indeed, consists largely of the liquid part of the blood which exudes from the finest hair-like blood-vessels or capillaries as they traverse the various tissues, and it is the chief business of the "lymphatics" or lymph-holding vessels to return this exuded liquid to the blood system, which they do by joining--like the rivulets of a river system--to form two large trunks which open into the great blood-holding veins at the region where they approach the heart. The total amount of lymph in the lymphatic system is difficult to estimate, but it is larger in quantity than the blood in the entire blood-vascular system. A large number of the delicate vessels of the lymphatic system take their origin just below the lining layer of the intestine, and ramify through the transparent membrane, which holds the coils of intestine together, and is called the mesentery. The fatty or oily materials of food pass through the lining "cells" of the intestinal wall into these "lacteal" or milky lymphatics, and consequently in an animal killed and examined after a meal, the fluid in them has a milky appearance, and renders this kind of "lymphatics" visible.

They were for this reason the first to be detected, and were known even in ancient times to anatomists. The milky fluid in them was called "the chyle." Its milky appearance is due to the same cause as the white opaque appearance of milk, namely, to the presence of an immense number of excessively small particles of oil (fat) and a certain proportion of larger globules of the same nature. It was thus not difficult for the old anatomists to trace the fine branches of the lacteals uniting branch to branch, and at last forming a large trunk--called the thoracic duct--about a quarter of an inch thick, which runs up the inner face of the backbone to the neck, where it joins the great left subclavian vein, and pours its contents into the blood-stream which is there nearing the heart. A small trunk formed by the union of lymphatic vessels from the right side of the head and neck and the right upper limb opens into the right subclavian vein. It took some time to discover this smaller trunk, since it is not brought to view by milky contents. Gradually it was made out that there are innumerable transparent branches opening into the thoracic duct from the whole of the body, besides the milky-looking lacteals: branches which bring "limpid" clear fluid, or "lymph," from all the viscera, from the muscles, and from the deeper layers of the skin in every region of the body, even from the toes, fingers, and tongue tip. In fact, wherever the blood-vessels take blood there are also vessels of the lymphatic system bringing back to the heart the liquid exudation which escapes into the tissues from the finest blood-vessels (Fig. 43).

Whilst we distinguish in an animal body various "tissues" which have special properties and activities, and can be dissected out and delimited--as we could dissect and distinguish the "tissues" (flannel, silk, leather, whalebone, wadding, gold-thread, etc.) making up an elaborate padded, stiffened brocaded, lined, and decorated costume--we find that, unlike what is usual in a man-made costume, all the parts of an animal body (viscera, and their lobes and sub-divisions, the blood-vessels, nerves, muscles, bones, etc.), are covered and separated from one another, and, at the same time, held together by a ubiquitous soft, spongy tissue, consisting of delicate threads and bands, enclosing spaces--some excessively minute and narrow, others larger--in which is a liquid. This is the great packing tissue of the body, and is called "the connective tissue." Its threads and bands have delicate, usually flat nucleated corpuscles (so-called "cells") of transparent protoplasm resting upon them and bathed by the liquid in the fine spaces. The threads and bands are, indeed, the product of the protoplasmic cells, built or "spun" by them, laid down by them as a snail leaves a slimy smear behind it as it crawls. It is not difficult to cut out transparent pieces of this "connective tissue" from a recently killed animal and to examine it with a very high power of the microscope. You may then see the living protoplasmic corpuscles slowly "streaming" and changing shape, and sometimes dividing (one into two) so as to form new corpuscles.

I made my first acquaintance with them when I was a student at Vienna with the great microscopist Stricker. We used the glass-clear connective tissue which forms the "cornea" of the eye, cut from a freshly killed frog. In those days the part taken by these cells in inflammation was being discovered, the name "phagocyte" had not been invented, the part played by them and by bacteria in disease and the suppuration of wounds was unknown, and I had the privilege of introducing Lister's earliest researches on aseptic surgery and on the coagulation of the blood to the notice of my friend and teacher.

This ubiquitous "connective tissue" underlying the skin, pushing its way into and around every part of every structure in the body, is the "source"--the reservoir, as it were--from which the lymph stream and the finest lymphatic vessels take their origin. The question may very naturally be asked, "How is it that the lymph flows along the channels provided by the transparent lymph vessels and is poured through 'the thoracic duct' into the great vein near the heart?" If we inject a suitable coloured fluid by means of a needle-pointed syringe into any mass of connective tissue, we can see the fluid pass into the numerous lymph vessels previously invisible, and if we inject into them a weak solution of silver nitrate we can, subsequently by aid of the microscope, make out the structure of the walls of the lymphatics and the lining pavement cells which become stained of a brown colour by the silver when exposed to light. But there is no muscular envelope, nothing like "a lymph-heart" in mammals, to drive the lymph along. There are valves or flexible flaps in the walls of the lymph-vessels, as there are in the veins, and the lymph is driven to the heart by the intermittent pressure upon these valved tubes, caused by the movements of the muscles and of the body generally. The valves, like those of the veins, prevent the flow of the lymph backwards, but allow it to pass forward towards the heart. This is shown by the examination of a narcotized mammal (killed immediately after the examination has been made). A glass tube is placed in the thoracic duct, and about a dozen drops of lymph (which would have been delivered into the great vein) pass from it in a minute. If, however, the animal's legs are moved, as though in running, or if "massage" is applied to the limbs--the pressure being directed from the extremities towards the heart--then a greatly increased flow of lymph is observed, as much as sixty drops in a minute! This is the chief explanation of the value to our health of exercise, and also of the importance of "massage" as a treatment in disease. Either exercise or massage entirely revolutionizes the rate of flow of the lymph, quickening it so greatly that the physiological effect on the general chemical processes going on in the body cannot fail to be most important.

Curiously enough, whilst mammals have to depend entirely on pressure and exercise for anything but the slowest flow of the lymph, the cold-blooded vertebrates, fish, amphibia and reptiles (and even some birds), have remarkable, rhythmically contracting, muscular sacs, which pump the lymph from large lymph-vessels into large veins, and are called "lymphatic hearts." The eel and other fish have them in the tail, but they are best seen in the common frog. There is an anterior pair, one under each shoulder-blade, and another pair, one on each hip. Each opens at one end into a large "collecting" lymph-vessel, and at the other end into a large vein. They "beat" like a heart, but do not keep time with one another. Their muscular walls are formed by what is called "striated" muscular tissue (as are those of the blood-heart), and they are under the control of branches of the spinal nerves. The movement of the hinder pair in a frog can be seen through the skin.

In man and all vertebrate animals the intestines, stomach and liver, heart and lungs (or swim-bladder) lie loose, except for a fibrous band of attachment, in a great cavity (often divided into two or more chambers), which they fit fairly closely. The small space between them and the walls of the cavity is occupied by a liquid. This is lymph, and the great cavity is a lymph-space. When this cavity is in its primitive form it is called the body cavity, or "cœlom." In man and mammals it is divided into four chief chambers--the peritoneal cavity (in which the stomach, intestine, and liver are loosely attached and have a certain mobility), the right pleural and left pleural cavity (one for each lung), and the pericardial cavity (for the heart). These great chambers are part of the lymph-system, and so is the lymph-holding space around and within the brain and spinal cord, and so are the great spaces beneath the frog's skin.

If we look at the structure of an earth-worm or of one of the graceful marine worms (Nereis or Arenicola), we gain a good deal of light as to the nature of the lymphatic system of Vertebrates. Suppose you have killed a large earth-worm with chloroform! Then pin it out on a cork plate, and open it by a cut along the back with a fine pair of scissors. The point of your scissors passes through the muscular body-wall of the worm into a great chamber filled with a clear liquid. This chamber is the "cœlom," and is the same structure as the pleural and peritoneal chambers of the Vertebrate. But it holds (proportionately) more liquid. The liquid is "lymph," like that of the Vertebrate, and has numerous protoplasmic cells floating in it. There is comparatively little connective tissue in the earth-worm. The cœlom is free and unblocked--the great viscera lie in it. There are some delicate, transparent bands of connective tissue, but not much nor bulky. The wall of the cœlom itself is lined with connective tissue, and if that tissue grew greatly in bulk, and bound all the organs and muscles together, it would reduce the large cavity, filling it up with spongy tissue in the small interstices of which there would be lymph. And so we should get a lymph system resembling that of Vertebrates, instead of one large chamber.

But what about the opening of the lymphatics into the blood-vessels? This is one of the interesting differences between the earth-worm and the Vertebrate. The earthworm and many marine worms have a beautiful system of vessels, containing a bright red blood, and forming true capillaries, connecting arteries and veins. The heart is a long, rhythmically beating tube, extending along the whole length of the animal just above the intestine. There is no opening into it of the lymph-cavity. It is purely a respiratory blood-system, pumping its fluid, coloured red by oxygen-seizing hæmoglobin into every part of the body. It passes along the fine capillaries of the skin, where it seizes oxygen from the outside air or water and carries it to all the tissues. The fact is that the red respiratory element of the blood which we call the "hæma" or hæmal portion (the Greek word for red blood is αἷμα) is here kept separate from the nourishing and elaborating element, the lymph or lymphatic portion. So that we should, to be explicit, describe the blood of a vertebrate as "hæmolymph," a conjunction of hæma and lymph, which in the more primitive earth-worm and sea-worm have never effected a junction! In some closely allied marine worms, however, a junction of these two is effected in another way. We know that in the Vertebrates the red blood corpuscles are formed by detached bits of the same tissue, which becomes converted into capillaries, the finest blood-vessels. Now in several marine Chætopods or bristle-footed worms (Glycera, Capitella, etc.) the tissue which should form the blood-vascular system and its red liquid blood, changes its mode of growth; it never forms blood-vessels at all, but divides into free red (hæmoglobinous) cells or red blood corpuscles, which float in the lymph of the cœlom. There is no blood-vascular system produced in these worms, but the "cells" of the tissue which would in other worms form blood-vessels break up into red corpuscles, which, mixing with the lymph, bring it into the condition of "hæmolymph," identical with the blood of Vertebrates!

In the molluscs, snails, whelks, oysters, clams, and cuttle-fishes there is a further, variation. The same two fluids and two systems of spaces are present as in the earth-worm, but the cœlomic space and fluid have been nearly blocked up and obliterated by the swelling-up and great size of the proper hæmal vessels. Only in rare cases is the blood of molluscs coloured red by hæmoglobin, usually it is of a pale blue colour. There is still left a pericardial cœlom, a space around the heart, and from this some fine lymph-holding vessels ramify amongst the tissues, but the chief spaces in the body are dilated parts of the true hæmal system. In Insects and Crustacea (say cockroach and lobster) this process is carried still further. The great cœlom, so well developed in the Chætopod worms, and the Sea-urchins and Star-fishes, and retaining quite a large development also in the Vertebrates, is nowhere to be found. The swollen blood-vessels have squeezed it out of existence, except for certain sack-like remnants which enclose separately the ovaries, and the testes, and the kidneys, and have each its opening to the exterior conveying the products of those important organs to the outer world. Thus we gain a brief insight into the true history of the lymphatic system and its vicissitudes in the lower animals and in man.