THE FORMATION OF LYMPH 349 



and lymph vessels, which act in the same way as the tension of the tissues 

 to favor the flow. Besides, throughout such passive tissues as tendons and 

 fascia stresses occur, in some cases regularly and rhythmically as the result 

 of voluntary movements, but more often quite accidentally, which favor the 

 movements of the lymph. 



In certain animals (rats and guinea pigs), the walls of the lymph vessels 

 execute rhythmical contractions, and in the Amphibia the flow of lymph is 

 aided materially by the so-called lymph hearts small contractile structures situ- 

 ated on both sides of the coccyx and beneath the scapula. The lymph is forced 

 by their contractions into the iliac and jugular veins respectively. 



The liquid flowing from the villi of the intestine through the lacteals is 

 forced along by contraction of the smooth muscle fibers of the villi. Finally, 

 the suction of the thorax must be taken into account, inasmuch as it affects the 

 flow of the lymph just as it does the flow of blood in the central veins. It is 

 evident at once that the valves of the lymph vessels are of great importance for 

 all of the above-named factors. 



The smooth muscles of the receptaculum chyli and of the thoracic duct 

 at least are under the influence of the central nervous system. The left 

 splanchnic contains dilating fibers, and, though in smaller numbers, con- 

 strictors also, for the receptaculum. The motor nerves for the thoracic duct 

 are in the thoracic sympathetic. Here also the dilator fibers are superior 

 to the constrictors in their control over the wall of the duct. The dilating 

 nerves can be excited reflexly by various afferent nerves (Gley and Camus). 



3. THE FORMATION OF LYMPH 



Since the blood pressure in the capillaries is higher than the tension of 

 lymph in the surrounding tissues, it was for a long time supposed that the 

 lymph is pressed out of the capillaries by this difference in pressure (filtra- 

 tion), and that the osmotic processes between the blood and lymph exercise 

 a more or less considerable influence on both the quantity and composition 

 of the latter. 



The most important experimental support of this view was the easily 

 confirmed fact that the lymph streams become swollen considerably after 

 tying off a vein, as a consequence of which the pressure in the capillaries is 

 increased (venous hyperaemia). On the other hand it was shown that an 

 increase in capillary pressure produced by dilatation of an artery (arterial 

 hyperaamia) often does not increase the formation of lymph in the least. 



If the cervical and brachial nerves of an animal be cut so that the vessels 

 of the arm are removed from the influence of the nervous system, and the cer- 

 vical spinal cord be then stimulated, the blood vessels all over the body, with the 

 exception only of the arm, contract, wherefore the blood flow to the arm, and 

 consequently the blood pressure in its capillaries are greatly increased. Not- 

 withstanding this, the quantity of lymph flowing from the lymph vessels of the 

 arm by the aid of passive movements is not increased in the least, but continues 

 to fall gradually as before (Ludwig and Paschutin). The same is true of the 

 submaxillary gland, when after section of the cervical sympathetic of an animal 

 22 



