MOVEMENT OF THE LYMPH. 213 



them have their origin in that organ as well as in the intestinal 

 follicular tissue and in the red marrow of the bones. Although 

 their number is relatively small, lymphatic cells occur in the 

 lymph channels that are unconnected with a lymphatic gland, 

 and these cells no doubt come from the blood, which, as we shall 

 see, contains many cell elements, which are actually the lymph cells 

 poured into it from the lymphatic duct. These cells, when they 

 arrive at the minute blood vessels, sometimes leave the vessels and 

 creep by amoeboid movements into the interstices of the tissue, along 

 with the irrigation stream. They may permanently abide in the tis- 

 sue, or they may be washed back into the larger lymph channels 

 with the stream of surplus lymph. When the abnormal increase 

 of activity of a tissue known as inflammation occurs, this escape of 

 the white cells from the blood takes place with great rapidity, and 

 the stages in the process can be watched under the microscope. 



Still another source of the lymph cells may be from prolifera- 

 tion of the cells which lie in the tissues. The fixed tissue cells 

 are said to be capable of producing cells identical with lymph 

 cells, and, by division, possibly multiply and produce their like, 

 which may be carried along by the lymph stream as lymph cells. 



The enormous number of cells which accumulate as pus when 

 an abscess forms, are structurally identical with lymph cells, and 

 probably arise from these combined sources, viz., escape from the 

 blood vessels and proliferation of the tissue cells. 



The lymph cells, therefore, whether they have their origin in 

 a lymph gland, the spleen, or in connective tissue, perform a 

 kind of circuit, going with the lymph into the blood, and are 

 distributed with the latter to the tissues, whence they may be 

 once more carried into the lymph stream. 



MOVEMENT OF THE LYMPH. 



In some of the lower animals small muscular sacks occur in 

 the course of the main lymph channels which pump the lymph 

 into the great veins by contracting rhythmically, much in the 

 same way as the heart. 



In man and the higher animals, no such lymph hearts have 

 been found ; the onward movement of the fluid depends chiefly 



