r26 THE CELL AND MAMMALIAN HISTOLOGY 



indefinitely unless there was at some point a return. This takes 

 place through the lymphatic system. All over the body is a tree- 

 like arrangement of small tubes ; their ends are closed, but the 

 major trunks open into the veins at a number of points. In these 

 vessels runs lymph, which is effectively blood minus red corpuscles 

 and many of the white, and it presumably gets into them by the 

 same means as that by which it escaped from the capillaries. Its 

 return flow is directed by valves, but these are absent from the frog 

 and other cold-blooded vertebrates, which possess instead con- 

 tractile lymph hearts (p. 373)- Most of the lymph passes into the 

 left innominate or internal jugular vein, but there are subsidiary 

 openings into the veins of the right side, and in many mammals 

 there are others as well. The lymph vessels are hardly to be seen 

 in dissection, as even in man the largest of them, the thoracic 

 duct, is only two or three milHmetres in diameter, and their walls 

 are extremely thin. 



Before opening into the main ducts all lymphatic vessels pass 

 into structures generally called lymph glands, although lymph 

 node is a better term since they are not known to have any 

 glandular function. They are numerous in the inguinal and 

 axillary regions and in the neck, where their swelling in bacterial 

 infection is known to most people. Each is a capsule containing 

 large numbers of white corpuscles of a type called the lympho- 

 cyte, with strands of connective tissue running amongst them. 

 Also present, and making a network, are cells similar to the 

 histiocytes or macrophages of areolar tissue. The whole material 

 of a lymph node, on account of its fine netted structure, is some- 

 times called reticular tissue and classified as a subdivision of 

 connective tissue. As, however, most of its cells are amoebocytes, 

 the alternative name of lymphoid tissue is preferable. Other 

 parts of the body, with a structure somewhat similar to that of 

 lymph nodes, are the spleen and bone marrow. The chief function 

 of all these organs is the formation of blood corpuscles. The 

 lymph nodes make lymphocytes, and pass them into the circu- 

 lation ; the bone marrow, which occupies the centres of hollow 

 long bones and the spaces of cancellous bone, makes red 

 corpuscles. It is possible that lymphocytes are their raw material, 

 and certainly the nuclei of the cells are lost before they are passed 

 into the blood. The marrow also makes white corpuscles of many 

 types, and other sources of them are the liver and spleen. 



