THE CIRCULATION OF THE BLOOD 151 



external pressure on lymphatics. Familiar examples are the 

 movements of arms and legs in locomotion, of the diaphragm 

 in breathing, and of the lungs in respiration. 



It has also been supposed that a third cause of the 

 lymph flow is the passage of waves of constriction (peri- 

 stalsis, cf. p. 125) over the larger lymphatics. This, however, 

 probably plays only a minor part. 



Finally, in the formation of lymph from the blood, more 

 water generally passes from the capillaries to the lymph 

 spaces than from the lymph spaces into the capillaries. 

 Under these circumstances, at least at certain times, the 

 lymph spaces become distended and a certain low pressure 

 obtains in them. This we may speak of as the " active 

 force " of lymph formation, and it constitutes a fourth factor 

 in causing the lymph flow. 



We have already pointed out the importance of the lymph 

 flow in maintaining the lymph currents about the living 

 cells ; we are now able to appreciate the importance of those 

 agents which secure this flow. As enumerated above, they 

 are four in number: (1) suction action of the breathing 

 movements ; (2) pumping action of muscular or passive 

 movements ; (3) active force of lymph formation ; (4) peri- 

 staltic contractions of the large lymphatics. 



Of these the fourth is at least doubtful and in no case of 

 great importance ; the other three may therefore be regarded 

 as the chief causes of the lymph flow, and of these the first 

 arid second are brought into most effective action by mus- 

 cular activity; this deepens the breathing movements and 

 so increases their suction action on the lymph, while the 

 movements of the body exert on the lymphatics a pumping 

 action which is largely lacking during complete inactivity. 

 The great practical importance of this aspect of the subject 

 will be discussed beyond in those chapters which deal with 

 the hygiene of muscular activity (Part II). 



