THE CIRCULATION 



309 



tion here, for the influence on the lymphatics is identical with that 

 exerted on the veins. It probably is even more complete, however, owing 

 to the greater abundance of valves in the lymphatics than in the veins. 

 The last force concerned in the production of the lymph-flow which 

 we need consider is at present more or less problematical. It may be 

 mentioned, however, as the slight constriction of the lymphatics them- 

 selves by contraction of the circular muscular fibers in their walls. The 

 relative importance of this as a motive force it is not easy to estimate, 

 but it undoubtedly exists. When, owing to abnormal conditions, the 

 pressure in the lymphatics tends to rise too high, it may become an 



FIG. 171 



The vessels and lymphatics of the anterior face of the mammary glands. (Sappey.) 



important means of restoring the onward flow. In many animals of 

 simpler structure than man, frogs, for example, there are distinct pulsatile 

 lymph-hearts (see Appendix, page 508), while in others (e.g., guinea-pigs) 

 the lymph-vessels themselves pulsate. Whether or not such action is of 

 much account normally in man remains to be investigated. Perhaps it 

 is, especially in the larger vessels. 



Under the combined influence of these forces blood-plasma oozes 

 out of the capillaries, soaks in between the cells composing the body, 

 collects by osmosis gradually in a myriad minute channels, and makes 

 its way more or less rapidly into the subclavian veins at the base of the 



