The essential points to find out about the elusive ghostly lymphatics are their extent and relation to the tissue spaces of the body, which are not empty at all but instead are filled with various fluids, the more important of which are (1) plasma, confined within the closed haemal system of arteries, veins, and capillaries; (2) lymph, carried within the lymphatic vessels, which is much like plasma but is without red corpuscles; (3) tissue fluid, which is plasma that has seeped out of the main blood stream and bathes the cells of most of the tissues much as the waters of a swamp surround soil particles and keep them moist; (4) cerebro-spinal fluid in the subarachnoid spaces around the central nervous system which contains certain products of internal secretion and is different from the lymph in the lymphatic channels; and finally (5) the fluid in the aqueous chambers of the eye, and that in the inner ear which, although described by the terms endolymph and perilymph, has no direct connection with the lymphatic system.
The anastomosing network of the lymph channels, which are crocheted all around the other blood vessels (Fig. 312), is anatomically and physiologically distinct from the much more easily seen arteries, veins, and capillaries of the haemal system through which the red blood flows. They probably equal if they do not exceed the haemal system in total carrying capacity although they do not extend everywhere throughout the body.
To quote Dr. Sabin who is an outstanding authority in this phase of anatomy: “Lymphatics have not been demonstrated in the adult liver beyond the capsule and the connective tissue septa, nor in the spleen beyond the capsule. It is well known that lymphatics are abundant in tendons; but that they have not been demonstrated in striated muscle. On the other hand, it has been definitely shown, both in the embryo and in the adult, that there are no lymphatics in the central nervous system.”
The lymphatic vessels of the digestive tract, which project into the innumerable villi of the small intestine, are known as lacteals because the absorption by them of the fatty products of digestion results in an emulsion that gives to them a white milk-like appearance.
Lymphatic channels vary in size from minute varicose capillaries where transfer of materials takes place, to large ducts and sinuses. Under the loose skin of a frog there are extensive lymph reservoirs in the form of subcutaneous sacs (Fig. 313) which, like a wet blanket, protect the underlying tissues from excessive loss of water during the temporary excursions of this amphibian from water to land.
Unlike red blood, lymph always travels toward the heart, backsliding being prevented by numerous pocket-like valves like those in certain veins, which allow forward movement but prevent retreat. While the flow of lymph is accomplished in some degree by gravity, its advance is principally brought about by the muscular movements of the exercising body, by means of which it is squeezed along its one-way course toward the heart.
Since the lymphatic channels in the adult are not incorporated in the closed circuit of the haemal system, the pumping power of the heart, being an integral part of the haemal system, can have no effect upon lymph movement.
In the lower vertebrates the larger lymph channels may acquire muscular walls and become pulsating lymph hearts, thus supplementing by their contraction the body muscles in the forward movement of the lymph. Among amphibians the caecilians (Apoda) have a pair of lymph hearts for each of the numerous segments of their elongated wormlike bodies, while urodeles likewise possess a double row of several lymph hearts, situated along the posterior part of the lateral line on either side. Frogs, and their relatives, typically have four lymph hearts in adult life, situated at four busy centers with reference to the legs and arms (Fig. 314). The anterior pair are at the level of the third vertebra near the junction of the subclavian and jugular veins at the base of the arms. The posterior pair are at the base of the iliac veins in association with the blood supply of the legs on the dorsal side to the right and the left of the urostyle and can easily be seen throbbing under the skin of a live frog.
In reptiles only the posterior pair of lymph hearts typically persist, while in birds and mammals even these vanish after a reminiscent embryonic appearance.
Added to the regular lymphatic channels and their modifications in the form of enlarged sinuses and pulsating hearts, there are present through-out the lymphatic system, particularly in mammals, numerous lymph nodes, as well as certain localized tissues and organs of a predominantly lymphoid character, such as the bone marrow, the “fat bodies” of hibernating animals, tonsils, Peyer’s patches in the small intestine, and the spleen.
Lymph nodes are usually encapsuled, with an internal mesh of connective tissue in which leucocytes are lodged. They are supplied with incurrent and excurrent lymph vessels, the former being more numerous than the latter. In these nodular substations not only is the filtering of solid materials in the lymph accomplished, but also tarrying leucocytes may there undergo dissolution and removal, as well as renewal by mitosis.
The largest and most constant of the lymphatic organs is the spleen, which lies encapsuled in the body cavity of vertebrates. It attains an average weight of seven ounces in man and upon occasion may be extirpated without fatal results. The exact function of the spleen is so problematical that no statement with reference to it has ever gone uncontradicted. The ancients were quite at sea regarding it. Hippocrates, the Father of Medicine, says that “it draws the watery part of the food from the stomach.” The great Aristotle could only guess that it is a “prop for the stomach,” while Galen, who was the prime authority in anatomy for centuries, oracularly declared that its function is “to keep the body warm.” In more recent times, with almost equal vagueness, word goes forth that “it is generally believed that this organ shares with the lymph tissues in the formation of lymphocytes, and is also concerned in the destruction of waste red blood corpuscles.” The ambitious student with an itch for discovery may be gratified to know that there are still left many little known regions awaiting exploration beyond the horizon.
The largest of the lymphatic vessels is the thoracic duct, into which the others posterior to the diaphragm and on the left side of the anterior part of the body empty. It opens into the venous system at the junction of the left jugular and left subclavian veins. The right lymphatic duct, which receives all lymphatics from the right anterior part of the body, empties into the right subclavian vein. In mammals, although embryonically paired, only the left thoracic duct persists.
The lymphatic channels arise from the haemal blood vessels, as has been demonstrated by Dr. Sabin, and not independently in the interstices of the tissues as formerly thought. Endothelial buds from the walls of the capillaries grow out and make a centrifugal invasion of the entire body, eventually becoming disconnected from the closed haemal system except at the points where the thoracic duct and the right lymphatic duct connect with the veins in the neighborhood of the heart.
The lymphatic capillaries forming as outgrowths of the endothelial buds are thus closed at their distal ends and do not open freely into tissue spaces. The return of the lymph from these spaces to the thin-walled lymphatic capillaries, therefore, like its escape from the haemal capillaries, is by the process of seepage and diapedesis and not by direct entry into open ends of lymphatic vessels.