88 HUMAN PHYSIOLOGY 



diffusion through the walls of the vessels play a part. It 

 is still a question whether the physical processes alone cause 

 the lymph formation, or whether, besides them, a special 

 activity of the capillary endothelium aids in this formation, 

 whereby the lymph is secreted into the tissues (just as the 

 gland epithelium secretes the gland secretion). 



TJic movement of lympJi. The movement of the lymph is 

 maintained by the force which the ever following lymph 

 forming in the tissues exerts upon that previously formed. 

 The movement is aided by the compression of lymph vessels 

 by the skeletal muscles. The backward movement of the 

 lymph is prevented by the valves. Aspiration, by means 

 of the negative pressure in the thorax, also aids the move- 

 ment of the lymph. 



Many animals are provided with lymph hearts which aid in the 

 circulation of the lymph. 



The serous cavities (pleural, pericardial and peritoneal cavities) 

 may be regarded as very large lymph spaces; they generally con- 

 tain small quantities of serous fluid, corresponding to the lymph 

 in composition. Soluble substances injected into these serous 

 cavities are absorbed from the cavities partly by the blood capil- 

 laries, partly by the lymph vessels. Concerning the force \vhich 

 causes this absorption, the opinions of authors differ. But it is 

 certain that this absorption is aided by respiration. By means of 

 the alternate dilation and constriction of the lymph spaces of the 

 diaphragm and pleura, the lymph is now sucked from the serous 

 cavities into the lymph spaces and now forced from the lymph 

 spaces into the lymph vessels. Also finely divided solid substances 

 (e.g. fat, pigments) can be absorbed from the serous cavities by 

 the lymph vessels. 



2. THE LYMPH GLANDS 



These are composed of reticular connective tissue in 

 whose meshes are found groups of cells. Here the leucocytes 

 originate and are passed into the lymph which enters the 

 meshes by the afferent vessel and leaves by the efferent 

 vessel. 



The lymph glands also filter the lymph and retain worn- 

 out lymph cells, aljo injurious substances, e.g. Bacteria, 



