The forces concerned in the locomotion of the lymph are numerous. Recklinghausen believes the movement of the lymph to be mainly due to the difference between the arterial and venous blood-pressure. The greater this difference the more rapid its current. The lymph canalicular system is not in vascular continuity with the blood-capillaries, and consequently the force of blood-pressure can only be communicated to the column of lymph by the passage of the plasmatic fluid into the lymphatic system by peripheral transudation and endosmosis. These are favored by the single homogeneous walls of the lymphatic plexuses and the enormous absorbing surface. These forces are essentially vis a tergo, for the difference between the arterial and venous blood-pressure is the excess of the former over the latter. To these must be added other factors, not less important or necessary, derived from the contractility of the walls of the lymphatic vessels, from the compression of the surrounding and contiguous parts, from the movements of respiration, and from the absorption of chyle. Besides these, the slowness of the movement of the lymph as compared with the rapidity of the arterial and venous blood-currents; the varying amount of pressure in the lymph vascular system, and the absence of distension in a normal condition; the entrance of the lymphatic trunks into the veins near the confluence of large branches, where the venous blood-pressure is almost inappreciable and the current is most rapid; the marked effect of active muscular movement in accelerating the flow of lymph; and the contractility of the vascular walls,—are all conditions which cannot be omitted from a consideration of the forces concerned in the locomotion of the lymph.
The supply of valves is very abundant, and they are always more numerous where pressure from surrounding and contiguous parts is most effective, though not infrequently most irregular in its operation, and consequently where isolation of small sections of the column of the fluid is most needed. The valves prevent regurgitation only so far as the superimposed column of fluid is insufficient to impair their integrity, or where there is no solution of the continuity of the vascular walls, and distension is within the limits of ordinary and normal extensibility. In cases of lymphangiectasis it is not usually necessary to look beyond the nearest neighboring and connected gland for the cause of such distension. Nature has increased the number of valves in the afferent vessels as they approach the glands, as well to modify and direct the flow as to prevent regurgitation; but if from any cause the passage of the lymph through the glands is obstructed or prevented, dilatation of the afferent vessels will ensue as a consequence. Valvular insufficiency and dilatation may exist in opposite relations to each other, either as cause or effect. The dilatation of a vessel may result from thinning or loss of contractility of its walls, caused by increased resistance to the onward movement of the fluid, and by the lesser extensibility of the intima than of the adventitia.
Lymphatic varices usually have their beginning in the vessels and extend to the plexuses, but the plexuses may be alone affected. Varicosities always extend backward from the point where the flow of the contained lymph is arrested, and may result from a repletion of each proximal intervalvular section with valvular incompetency.
The propulsive power of the heart diminishes with increased distance, due to increased friction and increasing resistance from flexures, bendings, and anastomoses, but chiefly from the increased carrying capacity of the vascular subdivisions. Hence, as the current of the lymph is in reverse relation to the capacity of the vessels, flowing, as does the venous blood, from subdivisions into trunks of diminished aggregate capacity, the velocity of the current of the lymph should be faster in the trunkal vessels than in the subdivisions. Such is the fact, though farther removed from the left heart and peripheral plasmatic circulation; and yet it is much slower in the thoracic duct than the blood-current in the venæ cavæ, which are not supplied with valves. The movement of the venous blood is in a measure due to cardiac and arterial contraction, but that force is least where the current is most rapid—in the venæ cavæ. The increased rapidity of the venous blood-current as it approaches the heart must, therefore, be derived from some other source; and it is equally manifest that the velocity of the venous blood in the terminal trunks is transmitted to the column of lymph and chyle flowing from the thoracic duct into the blood-channels.
The foregoing reference to the anatomy and physiology of the absorbent system shows very conclusively the importance of its pathological relations. It is certainly concerned in the morbid processes of a variety of diseases. But not less important is the fact that it is the main channel for the diffusion of infections throughout the body. Disease may be conveyed by the lymph from a single focus to many and distant parts, whilst the intervening channel of communication may remain free from injury. Along the course of the current every gland may become an additional focus, intensifying the infectiveness of the noxious material and increasing the area of its diffusion. This is alike true of poisons introduced from without and of those originating in the system.
SYNONYM.—Angioleucitis.
DEFINITION.—Lymphangitis may be either simple or septic. As a rule, the disease is localized, but may, especially when induced by some septic poison, be widely diffused, implicating extensive areas of lymphatic tissue and extending to contiguous structures.
Simple lymphangitis may be either reticular or tubular. In the former the fine capillary network or plexus is involved; in the latter the trunkal vessels are inflamed. Very frequently both forms exist at the same time.
ETIOLOGY.—Simple lymphangitis may be either idiopathic or traumatic. It is, however, rarely spontaneous. External irritation, such as solar rays, pressure, and friction, may set up a superficial inflammation, though usually there is some form of injury—a wound, scratch, sprain, contusion, abrasion, prick, or sting of an insect. The graver forms are caused by neighboring inflammation, suppuration, and ulceration. The products of these morbid changes are absorbed and conveyed along the vessels. The inflammation may be continuous along the course of the vessel, or separated from the origin of the morbid product by an area of intervening healthy tissue. Absorption of the secretions and parenchymatous fluids of inflamed parts is an active and frequent agency in the causation of lymphangitis. It may also be caused by contiguous inflamed tissue and by obstruction of the current of the lymph. Lymph-thrombosis, from whatever cause produced, may excite inflammation at the locality of formation, which is usually in the immediate vicinity of a valve, or the thrombi may disintegrate or undergo puriform liquefaction, and thus extend and diffuse the inflammation.
Slight pricks, scratches, and abrasions, which in themselves are so trifling as not to attract attention, may admit irritating substances from without. This is a frequent cause among medical men, whose hands and fingers are constantly exposed to irritating and ichorous discharges.