CIRCULATION, RESPIRATION AND EXCRETION 131 



In the lymphatics there is a continuous slow movement of the 

 lymph from the tissues towards the main trunks, the lymphatics, 

 like the veins, being provided at intervals with valves prevent- 

 ing a backward flow. This lymph 

 flow is sustained in part by a 

 slightly greater pressure in the 

 lymphatic spaces but largely by 

 the rhythmic motions of breath- 

 ing, and is aided by muscular 

 activity. Thus, in addition to the 

 exchange of substances between 

 the lymph and the blood through 

 the walls of the capillaries, there 

 is a general movement of the 

 lymph itself over the surface of 

 the cells which tends to facilitate 

 the exchanges between it and the 

 protoplasm. 



187. Adjustment of circula- 

 tion. The activity of the various 

 tissues varies at different times. 

 A muscle, for example, is some- 

 times at rest and sometimes 

 actively contracting. Conse- 

 quently, a greater or less supply 

 of food material and of oxygen will 

 suffice according to circumstances, 

 and the blood supply needs to be 

 regulated accordingly. 



This regulation is effected in 

 substantially two ways. First, 

 when the cells of any particular 

 tissue increase their activity they 



consume more oxygen and give 

 J ? FIG. 20. Mam lymphatic trunks 



off more waste products than be- (in white) . (Hough and sedgwick, 

 fore, tending to produce a relative The Human Mechanism.) 

 deficiency of the one and an ex- 

 cess of the other in the lymph and blood. These conditions 

 bring about an increase in the heart action (194), probably 

 by means of a nerve stimulus, so that the amount of blood 



