638 CIRCULATION OF BLOOD AND LYMPH. 



muscles consists essentially of dilator fibers and that these fibers 

 are brought into action reflexly whenever the muscles contract, 

 thus providing an increased blood-flow in proportion to the func- 

 tional activity. It should be added that the local dilatation in 

 the muscles during activity may be due also to the chemical action 

 of the (acid) metabolic products on the blood-vessels (p. 620) . 



The Vasomotor Nerves to the Veins. It is assumed in physi- 

 ology that the vasoconstrictors and vasodilators end in the muscula- 

 ture of the small arteries. The veins also have a muscular coat, 

 and it is possible that if this musculature were innervated from 

 the central nervous system we should have another efficient factor 

 in controlling the blood-flow. Mall has given very clear proof that 

 the portal vein receives vasoconstrictor fibers from the splanchnic 

 nerve,* but this supply may be exceptional, as the portal system 

 itself is unique. The portal vein, indeed, plays the role physiolog- 

 ically of an artery in regard to the liver. Roy and Sherringtonf 

 give some evidence for the existence of venomotor nerves to the 

 large veins of the neck, and Thompson, as also Bancroft,t reports 

 experiments in which it was found that stimulation of the sciatic 

 nerve caused a visible constriction of the superficial veins of the 

 hind limbs. Finally, it has been shown that solutions of epinephrin 

 cause contraction in rings of vein as they do in arterial strips. 

 On the accepted explanation of the way in which epinephrin acts 

 this fact implies that the muscle in the veins is supplied by sympa- 

 thetic autonomic nerve-fibers. If such a system exists it must 

 exert an important influence upon the supply of blood to the 

 heart. 



THE CIRCULATION OF THE LYMPH. 



The direction of flow of the lymph is from the tissues toward the large 

 lymphatic trunks, the thoracic and the right lymphatic duct. The flow is 

 maintained in this direction mainly by a difference in pressure at the two ends. 

 At the opening of the large trunks into the jugular veins the pressure is very 

 low; in the vein, in fact, it may be zero or even negative as compared with 

 the atmospheric pressure. The opening between the lymph vessel and 

 the vein is protected by a valve which opens toward the vein, and the lymph, 

 therefore, will flow into the vein as long as the pressure in the latter is lower 

 than that in the lymphatic duct. At the other extremity of the system, 

 in the tissue spaces to which the lymphatic capillaries are distributed, the 



Eressure, on the contrary, is high. Its exact amount is not known, 

 ut, since the pressure in the blood capillaries is equal to 40-60 mms. Hg, 

 the pressure in the liquid of the surrounding tissues must also be consider- 

 able. The tissues are, in fact, in a condition of turgidity owing to the 

 pressure of the lymph in the tissue-spaces. This difference in pressure 

 at the two ends of the lymphatic system is the main constant factor in mov- 

 ing the lymph. It is obvious that in the long run it is dependent upon the 

 pressure within the blood-vessels and therefore upon the force of the heart 



* Mall, "Archiv f. Physiologic," p. 409, 1892. 



t Roy and Sherrington, "Journal of Physiology," 11, 85, 1890. 



t Bancroft, "American Journal of Physiology," 1, 477, 1898. 



See Henderson, "American Journal of Physiology," 23, 345, 1909. 



