702 THE LYMPHATIC SYSTEM 



have also been described accompanying the blood-vessels in the Haversian canals in bones 

 (Rauber, Schwalbe, Budge). Nothing is known concerning the lymphatics of the bone marrow. 

 Cartilage lacks both blood and lymphatic capillaries. 



The capsular membranes of joints are richly provided with lymphatic capillaries (Tillmanns). 

 They are arranged in two layers^ — an inner layer made up of a rich plexus of wide capillaries, 

 lying just outside the subendothelial blood-capillaries, and an outer layer, consisting of a rich 

 plexus in the subsynovial tissue. The lymphatic capillaries have no open connection with the 

 joint cavity. 



The membranes surrounding the pleural, pericardial and peritoneal cavities are richly sup- 

 plied with lymphatic capillaries, which form here thick plexuses outside the endothehum. These 

 plexuses are usually described with the underlying organ, as the subserous lymphatic capillaries 

 of the intestine, etc. In the central tendon of the diaphragm the subperitoneal lymphatics are 

 extremely rich. They widen out here to form very large endothehal-lined cavities which, in the 

 spaces between the connective-tissue bundles, lie directly in contact with the peritoneal 

 epithelium. The existence of open connections between these capillaries and the peritoneal and 

 pleural surfaces (the "stomata" of von Recklinghausen) has recently been disproven. The 

 capillaries on the two surfaces of the central tendon communicate freely with one another. 



2. THE LYMPHATIC VESSELS 



The lymph which enters the lymphatic capillaries passes over into collecting 

 vessels (ducts), which carry it through the lymph-glands (nodes) to the large veins 

 at the base of the neck. The lymph-vessels course in the loose subcutaneous 

 tissues, in the connective tissues between muscles and organs, often accompanying 

 the arteries and veins, sometimes forming networks around them. An idea of 

 their arrangement can be best obtained by glancing at the illustrations of the 

 lymphatics of special regions. In general they are made up of numerous long, 

 narrow vessels, rarely more than half or three-fourths of a millimetre in diameter, 

 which occasionally communicate with one another, and which radiate toward 

 groups of lymph-glands placed in certain definite regions. In the lymph-glands 

 the afferent lymph-vessels break up into capillaries, which again collect into 

 efferent vessels. Several of these efferents from each lymph-gland may pass to a 

 second lymph-gland, where they undergo a second widening into capillaries. In 

 this way the lymph, passing through one, two, three or more lymph-nodes in 

 succession, eventually reaches the thoracic duct, or one of the short ducts, all of 

 which empty into the large veins at the base of the neck. The thoracic duct, 

 which receives, at its lower end, the lymph from the lower half of the body, is the 

 only lymphatic vessel which attains any considerable size (four to six millimetres 

 in diameter) and is usually the only one large enough to be seen readily without 

 injection. 



In structure the lymphatic vessels much resemble the veins. They possess an intima, a 

 media and an adventitia, although the Une of demarcation between the different layers is not 

 sharp. In the thoracic duct, the endothelium of the intima is succeeded by a dehcate layer of 

 fibres, mainly elastic; outside of this is the media, made up mainly of circular smooth muscle- 

 ceUs, interspersed with elastic and connective-tissue fibres; then follows a layer of coarse elastic 

 and connective-tissue fibres, which is succeeded by the adventitia, containing longitudinal and 

 transverse bundles of smooth muscle-ceUs, as well as blood-vessels and nerves. The other lym- 

 phatic vessels possess the three layers, which, however, toward the capillaries, grow thinner, and 

 eventually reach a stage in which, outside the endothehum, there are found only single muscle- 

 cells, or muscle-cells in groups of two or three. 



The lymphatic vessels are characterised by their great richness in valves, which are present 

 throughout their entire course, from their beginnings in the capillary region to their openings 

 into the veins of the neck. The valves are bi- or tri-cuspid, and are always arranged so as to 

 prevent the flow of lymph back to the capillaries. They thus aid indirectly in the movement of 

 the lymph, in that any external pressure on the vessels must always force the lymph onward. 



Nerves of lymphatic vessels. — That the thoracic duct and the smaller lymphatic vessels are 

 provided with nerves has been shown by several observers. According to Kytmanoff (in dogs) 

 the nerves to the lymphatics are mainly non-meduUated, and are both motor and sensory 

 They form four sets of plexuses — adventitial, supranuiscular, intermuscular and subendothelial. 

 Sen.sory nerve-endings (fig. .550) are found in adventitia and media, in the form of free-ending 

 threads, and bush-lii<^e endings. Motor endings are present in connection with the smooth 

 mu.scle cells of the media. In tlic intima there is a plexus of extremely fine varicose threads. 

 The physiological action of the nerves Kup[)Iying the receptaculum chyli has been tested by 

 Camus and Gley who found in dogs a dilatation of the receptaculum as the result of electrical 

 stimulation of the splanchnic nerve. 



Movement of the lymph. — It has been estimated (Ludwig) that the amount of lymph which 

 pa.sses through the lymphatic ducts of a dog aggregates, during the twenty-four hours, one-third 

 the bofly-wfiight. In the Mioracic, duct the lymph is under n sufficient pressure to burst the duct 

 behind a ligature. In the absence of any especial propulsive organ, such as the heart for the 

 blood-circulation, what are the forces whicli move the lymph? Tliore must be recognised pri- 

 mary and accessory forces. As accessory forces there are the movement of the muscles and the 



