CIRCULATION. 65 



quently, the quantity of blood in a limb, and the velocity 

 of the venous blood-stream, are much influenced by posi- 

 tion ; but this does not affect the quantity of blood .trans- 

 mitted through the part in a given time. A slight dimi- 

 nution of the pressure in the veins nearest to the heart 

 accompanies each pulsation : this is due to the diminution 

 of the volume of the heart, which occurs at the moment of 

 ventricular systole. When, from disease, the tricuspid 

 valve is incompetent, the opposite effect is produced, and 

 is called the venous pulse. 



Veins are contractile, but there is no proof that their 

 contractility is of physiological importance in man. In 

 certain animals, the veins contract rhythmically. 



The Lymph- Sir earn. The progressive motion of the 

 lymph is dependent on the difference between the pressure 

 under which liquid exudes from the capillaries into the 

 tissue interstices from which the lymphatics spring, and 

 the pressure which exists in the lymphatic trunks. In all 

 muscular parts it is promoted by the alternate tension and 

 relaxation of the tendons and aponeuroses. In the visceral 

 cavities it is similarly aided by the respiratory variations 

 of external pressure to which the trunks are subjected, as 

 well as by the circumstance that the mean pressure in the 

 abdomen is greater than in the thorax. Solid particles, 

 if of sufficient minuteness, whether introduced into the 

 blood-stream or into the tissues, find their way into the 

 lymphatics, which can usually be " injected " by the intro- 

 duction of any particulate liquid into living tissue. It is 

 probable that such particles are for the most part arrested 

 in the lymph glands. The particulate constituents of 

 chyme are forwarded from the intestinal cavity into that 

 of the lacteals by the agency of the living protoplasm of 

 the epithelium and mucosa. The further progress of the 

 chyle in the mesentery is promoted by muscular action. 



