1140 PHYSIOLOGY 



and veins and the increased capillary pressure, combined with the 

 watery condition of the blood, induces a great transudation of lymph, 

 especially in the abdominal organs (Fig. 478). The lymph is more 

 watery because the blood also is diluted. That the action of these bodies 

 is purely mechanical is shown by the fact that, if the rise of capillary 

 pressure be prevented by bleeding the animal immediately before the 

 injection, the increase in the lymph-flow is also prevented (Fig. 478, B), 

 although the concentration of the sugar or salt in the blood 

 is still greater than in the experiments in which bleeding was not 

 performed. 



MOVEMENT OF LYMPH 



In the frog the circulation of lymph is maintained by rhythmically 



contracting muscular sacs, which are placed in the course of the main 



ymph- channels, and pump the lymph into the veins. In the higher 



animals and in man the onward flow of lymph is effected partly by 



FIG. 479. A lymphatic vessel laid open to show arrangement of the 

 valves. (TESTUT.) 



the pressure at which it is secreted from the capillaries into the inter- 

 stices of the tissues, but also to a large extent by the contractions of 

 the skeletal muscles. In the smaller lymph-radicles the pressure of 

 lymph may attain 8 to 10 mm. soda solution. In the thoracic duct, 

 at the point where it opens into the great veins of the neck, the pres- 

 sure is obviously the same as in these veins, that is to say, from 4 to 

 mm. Hg, the negative pressure being occasioned by the aspiration of 

 the thorax. This difference of pressure is sufficient to cause a certain 

 amount of flow. It must be remembered, however, that under normal 

 circumstances no lymph at all flows from a resting limb. The only 

 part of the body which gives a continuous stream of lymph during rest 

 is the alimentary canal, the lymph in which is poured out into the 

 lacteals, and thence makes its way through the thoracic duct. Move- 

 ment, active or passive, of the limbs at once causes a flow of lymph from 

 them. Since the lymphatics are all provided with valves (Fig. 479), 

 the effect of external pressure on them is to cause the lymph to flow 

 in one direction only, i.e. towards the thoracic duct and great veins. 

 Hence we may look upon muscular exercise as the greatest factor 

 in the circulation of lymph. The flow of lymph from the commence- 

 ment of the thoracic duct in the abdominal cavity to the main part of 



