Anatomy and Embalming / A Treatise on the Science and Art of Embalming, the Latest and Most Successful Methods of Treatment and the General Anatomy Relating to this Subject

by Albert John Nunnamaker; Charles Otto Dhonau

Fig. 45—Collateral anastomosis of veins (Poirier and Charpy)

The Lymphatic Circulation.

—The lymphatic system is a system of vessels which occurs abundantly in almost all portions of the body and converge and anastomose to form two or more main trunks, which open into the subclavian veins just before they are joined by the internal jugular. The vessels contain a fluid termed lymph, usually colorless and containing numerous white blood corpuscles known as lymphocytes.

In those vessels which have their origin in the wall of the small intestines, the contained fluid has, especially during digestion, a more or less milky appearance, owing to the lymphocytes being loaded with particles of fat which they have taken up from the intestinal contents. On this account, these vessels are usually spoken of as lacteals, although it must be recognized that they are merely portions of the general lymphatic system.

In certain respects the vessels of the system strongly resemble the veins. They arise from a capillary network, their walls have a structure closely resembling that of the veins, they are abundantly supplied with valves, and it may be said that the fluid which they contain flows from the tissues towards the subclavian veins. With these similarities there are combined marked differences. One of the most important of these consists in the fact that the capillaries of the lymphatics are closed and do not communicate with any other set of vessels as the venous capillaries do with the arterial; and another important difference is to be found in the frequent occurrence upon the lymphatic vessels of characteristic enlargements, the so-called lymphatic nodes or glands, quite different from anything occurring in connection with the veins.

Throughout the body spaces of varying size are found, containing a clear, more or less watery fluid, which are called lymph spaces. These spaces do not communicate with the capillaries of the lymphatics, but are in such close relationship with them that the fluid easily finds its way into the lymph capillaries by osmosis, absorption, lymphocytes going out into these spaces and returning filled with the lymph fluid.

The lymphatic capillaries, which are arranged in the form of networks of very different degrees of fineness and complexity, closely resemble in structure the blood capillaries, their walls consisting of a single layer of endothelial cells. They differ from those of the blood vascular system not only in their ultimate branches being closed, but also in their general appearance. They are of greater caliber.

The lymph vessels, which issue from the capillary networks and convey the lymph ultimately to the subclavian veins, have the arrangement closely resembling that of the veins; the larger ones are usually situated alongside and accompany the course of the blood vessels. Just as the veins unite to form larger trunks as they pass from the capillaries toward their termination, so, too, the lymphatics, but the lymphatics present two peculiarities which distinguish them from the veins. They do not anastomose as abundantly as veins and there is not the same proportional increase in the size of the lymphatic vessel. The left trunk or thoracic duct is much larger than the right, beginning in the abdominal region and traversing the entire length of the thorax to reach its destination. It receives all the lymph returned from the lower limbs, the pelvic walls and viscera, the abdominal walls and viscera, the lower part of the right half and the whole of the left half of the thoracic viscera, the left side of the neck and head, and the left arm. The other trunk, the right lymphatic duct, is very short and sometimes wanting. It receives the lymph from the upper part of the right side of the thoracic wall, from the right half of the thoracic viscera and the upper surface of the liver, the right side of the neck and head, and from the right arm. The structure of the larger lymphatic vessels is similar to the veins, but, as a rule, their walls are thinner than those of the veins of corresponding caliber and their valves are more numerous. The walls of the most robust trunks, particularly those of the thoracic duct, consist of three coats. From within outward these are: (a) the intima, composed of the endothelial lining and the fibro-subendothelial layer; (b) the media, made up of involuntary muscle interspersed with fibro-elastic tissue; and (c) the adventitia, consisting of fibro-elastic tissue and longitudinal bundles of involuntary muscle.