xiv THE LYMPH 519 



lacunar system ; but it is inadequate, in so far as it holds the lymph 

 to be merely a product of simple filtration. The pumping of the 

 heart promotes the flow of lymph, not merely by favouring 

 filtration through the capillaries, but by another simpler mechanism. 

 At each systolic efflux the whole arterial tree is dilated by the 

 passage of the pulse wave, in consequence of which the whole of 

 the perivascular lymphatics immediately receive an impulse to 

 centripetal evacuation of the lymph which they contain. Since 

 it is shown from plethysmographic observations that the total 

 volume of the body is increased at each beat transmitted from the 

 heart, it may logically be admitted that the lymphatics which run 

 separate from, and independent of, the blood-vessels must, at each 

 pulsation of the arteries, be sensible of a pressor effect which 

 favours the movement of the lymph. 



More important, however, and certainly better demonstrated, 

 is the influence exerted on the lymph circulation by the active 

 <ind passive movements of the skeletal muscles. If a cannula is 

 introduced into the principal lymphatic vessel from the lower 

 extremity of a large dog, no flow of lymph will be perceived so 

 long as the muscles of the limb are relaxed and motionless. As 

 soon, however, as active movements are excited in the limb, or 

 alternate passive movements of flexion and extension are performed 

 on it, the flow of lymph through the cannula becomes suddenly 

 active. This fact shows that the muscular movements compress 

 the lymphatics and empty them in the centripetal direction, 

 because the valves prevent movement of the lymph, as of venous 

 blood, in a centrifugal direction. The rise of the lymph in the 

 lower limbs is principally effected by this mechanism. 



On the other hand, the respiratory mechanism exerts a 

 preponderating influence on the movements of the lymph in the 

 visceral lymphatics. The lymphatic, like the venous, current is 

 continuously affected by the normally negative pressure of the 

 thorax, by which the lymph is aspirated, like the venous blood, 

 from the extra thoracic to the intrathoracic vessels. This negative 

 thoracic pressure increases during inspiration, and the positive 

 abdominal pressure increases during active expiration. These 

 two factors accelerate the flow of lymph, particularly in the 

 visceral lymphatics and thoracic duct, and propel it to the 

 mouths of the two subclavian veins, where it mixes with the venous 

 blood. 



IV. The exact determination of the mechanism which effects 

 the formation of lymph is one of the most complex problems 

 in physiology, and has been much discussed of late years since 

 Heidenhain (1891) opposed to the mechanical theory of filtration 

 (a relic of the ancient doctrine of Bartholin and Mascagni, to 

 which Ludwig and his School endeavoured to give an experimental 

 basis) his secretory theory, in which he asserts that the formation 



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