THE LYMPHATIC SYSTEM 



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into a capillary network in the node. Numerous small vessels emerge from the 

 node, which unite into one lymphatic vessel, which joins a larger lymphatic 

 trunk, which empties into a tributary of the superior vena cava. The deep 

 lymphatics, fewer in number and larger than the superficial, accompany the deep 

 bloodvessels. Their mode of origin is probably similar to that of the superficial 

 vessels. The lymphatics of any part or organ exceed the veins in number and in 

 capacity, but in size they are much smaller. Their anastomoses also, especially 

 those of the large trunks, are more frequent, and are effected by vessels equal in 

 calibre to those which they connect, the continuous trunks retaining the same 

 diameter throughout. 



The lymph nodes, or lymphatic glands (lymphoglandulae) , are small, solid, gland- 

 ular bodies situated in the course of the lymphatic and lacteal vessels. They 

 vary from microscopic dimensions to the size of an olive, and their color, on section, 

 is of a pinkish-gray tint, excepting the bronchial 

 nodes, which in the adult are mottled with black, 

 the hepatic nodes, which are yellow, and the splenic 

 nodes, which are brown. Each node has a layer or 

 capsule of cellular tissue investing it, from which 

 prolongations dip into its substance, forming parti- 

 tions. The lymphatic and lacteal vessels traverse 

 these nodes in their passage to the lymphatic ducts. 

 Before entering a node a lymphatic or lacteal vessel 

 divides into several small branches, which are named 

 afferent vessels (vasa afferenlia). As they enter, the 

 external coat becomes continuous with the capsule of 

 the node, and the vessels becoming much thinned, 

 and consisting only of their internal or endothelial 

 coat, pass into the node, and branch out upon and 

 in the tissue of the capsule, these branches opening 

 into the lymph sinuses of the node. There is an ex- 

 tensive sinus beneath the capsule; from this subcap- 

 sular sinus numerous channels run inward to a central 

 sinus. From both sinuses fine branches proceed to 

 form a plexus, the vessels of which unite to form a 

 single efferent vessel (vas efferens), which, on emerg- 

 ing from the node, is again invested with an external 

 coat from the gland capsule. The lymph nodes are 

 filters or traps through which lymph and chyle flow, 

 and also have a cytogenic function. In the nodes are masses of newly formed 

 lymphocytes which attack any bacteria in the lymph or chyle. 



Like the lymphatics, the lymph nodes are arranged in superficial and deep sets; 

 they are usually embedded in fat and are distinctly movable. Occasionally a 

 node exists alone, but, as a rule, from eight to twelve, or even more, are assembled 

 in communities or chains, and are usually arranged around bloodvessels. The 

 nodes have a plentiful blood supply, and contain not only vasomotor nerves, but 

 definite nerve plexuses. Besides the nodes, the body contains numerous lymphoid 

 areas, which, in structure and function, are allied to lymph nodes (tonsils, Peyer's 

 patches, etc.). 



Hemolymph nodes exist in various regions, but are mest common in the abdomen 

 in front of the vertebrae. They are like ordinary lymph nodes in form and also 

 in size, but differ from them in being deep red instead of light pink. Their func- 

 tion is to destroy red blood cells and to form lymphocytes, phagocytes, and eosino- 

 philes. Hemolymph nodes develop like the ordinary lymphatic nodes except that 

 the sinuses are blood channels. 



FIG. 550. A lymph node with its 

 afferent and efferent vessels. (Tes- 

 tut.) 



