Fig. 9.—Central (superficial) lymphatic glands of the axilla. (After Leaf.)
The [lymphatic glands] are small oval glandular bodies and occur here and there along the course of the lymphatics. Before entering one of them the vessel breaks up into several afferent vessels which form a plexus within and then emerge again as several efferent vessels which soon unite to form one trunk. These glands occur chiefly in the mesentery, along the great vessels, and in the mediastinum, [axilla], neck, elbow, groin, and popliteal space.
The lymph varies in character with the locality, being a little thicker and more opalescent in the lacteals, as the lymphatics of the small intestine are called, especially during digestion, when fat is present. Here it is called chyle. Otherwise it is generally a clear, transparent and slightly opalescent fluid, which, owing to the presence of fibrin, clots when drawn from the body and allowed to stand. In fact, it resembles blood plasma very closely in composition and, as it also contains a certain number of corpuscles or leucocytes that just correspond to the white corpuscles of the blood, it is practically blood without the red corpuscles. These leucocytes have considerable power of amœboid movement and are thought by some to play an important part in the absorption of food.
Owing to intracapillary pressure, the lymph transudes into the lymph spaces and bathes the tissues, being carried away again by the lymphatics. The amount of transudation is determined by the blood pressure—the greater the pressure, the greater the amount of transudation—and is increased by some organic action of the cells in the walls of the vessels. In the process of transudation a certain amount of solid matter goes through the wall of the vessel and it is probable that certain protein elements can be carried thus from the blood-vessels to the lymphatics, though they do not pass through the capillary wall as readily as other substances. Some lymph is also probably formed by the action of the tissues themselves, though the process is not understood.
All muscular movements, active or passive, including the respiratory movements, tend to drive the lymph on its way by pressure, the valves of the vessels keeping it from flowing back. Moreover, its flow is from the capillaries to the veins or from a region of high pressure to one of less pressure. There is probably also some contraction in the walls of the vessels themselves, and the continual formation of lymph helps to drive it along. If an obstruction to the circulation occurs, however, back-pressure results and causes too great transudation. In that event a limb becomes swollen, pale, and generally cool. It pits on pressure, the pressure driving the lymph out and there being no circulation to bring it back. This condition is called œdema and occurs in liver, kidney, and heart troubles, being generally first observed at the ankles. In ascites, hydrothorax, hydrocephalus, and pericardial and pleural effusions the fluid corresponds to lymph in its composition and the large amount is due to excessive formation of the fluid, which is normally present in small quantities.
Lymph gives the tissues substances from the blood that they need and carries off those they do not, whether waste or substances of use to other tissues. Because they thus absorb certain materials not needed by the tissues and convey them to the circulation, the lymphatics have also been called absorbents. Indeed, lymph may be spoken of as the middleman between the blood and the tissues.
Another function of the lymph is to lubricate. Thus, the synovial fluid of the joints is lymph and the pleuræ and the pericardium contain lymph or serum to reduce the friction between the adjoining surfaces as much as possible. The brain and spinal cord do not quite fill the cavities of the cranium and the spinal column but float on a cushion of lymph, the cerebro-spinal fluid. When the brain, which is subject to increase and diminution in size, increases in size, it drives the lymph out, and when it diminishes, the lymph returns.
The lymph glands serve as a protection to adjacent parts and when it leaves the gland the lymph is purer and richer in leucocytes than when it entered. In fact, they filter harmful matter from the lymph and apparently also form white corpuscles. Normally they can with difficulty be felt, but in disease, if the leucocytes are unable to destroy or carry off the poison, the lymph carries it along to the glands, which swell and become tender. If the infection is not severe the swelling goes down and the tenderness passes after a short time, but if it is severe, there may be suppuration and abscess formation and the gland even perhaps be destroyed, giving its life for the health of the part. Thus a wound in the foot, if infected, may cause irritation and enlargement of the glands at the knee and in the groin.
The lymphatic glands are frequently the seat of tubercular infection, especially in the neck, and are enlarged in scarlet fever, tonsillitis, and diphtheria. In syphilis there is general glandular enlargement, and the glands in the groin become enlarged in all diseases of the genital organs. In malignant growths, such as cancer, the extension of the disease is often along the lines of the lymphatics.
Glands.—Of glands in general a word might now be spoken. They are of two kinds, excreting and secreting, and, when simple, are formed by the folding in of a free surface, as in the case of the salivary, gastric, and sebaceous glands, the cells at the gland becoming so modified as to be able to perform the function of excreting or secreting. In racemose glands the gland is broken up into many pockets. Excreting glands take from an organ or from a part substances which have outlived their usefulness and are to be cast out of the body, while the secreting glands form from the blood substances that did not exist in it before, but which are of use to the body, as the ptyalin of the saliva. A strict line cannot, however, be drawn between the two kinds of glands, most glands partaking more or less of both functions, though the sebaceous and sweat glands are probably purely excreting glands and the salivary glands are almost purely secreting. The glands, moreover, are more or less interchangeable in their functions, that is, they have vicarious function, and one gland can take up and do for another what that other is for some reason unable to do. In jaundice, where there is stoppage of the bile duct, the kidneys help out the liver by excreting the bile. If one [kidney] is removed the other does work for both, and the glands of the skin may help out the kidneys or vice versa. Hemorrhage from the lungs sometimes occurs in suppression of the menses.