564 CIRCULATION OF BLOOD AND LYMPH. 



elevator of the lip in a horse in which the muscles were exercised 

 normally by the act of eating. The blood-flow was increased as 

 much as five times over that observed during rest, and that this 

 increase was due in part at least to a local dilatation seems to be 

 proved by the fact that the blood-pressure in the artery supplying 

 the muscle fell, while that in the vein rose. While, therefore, our 

 experimental knowledge of the vasomotors of the muscles needs 

 further investigation, we may provisionally accept the view ad- 

 vocated by Gaskell, namely, that the vasomotor supply to the 

 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. 



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, however, 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 

 physiologically of an artery in regard to the liver. Roy and 

 Sherrington f give some evidence for the existence of venomotor 

 nerves to the large veins of the neck, and Thompson, as also Ban- 

 croft^ 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. The whole subject, however, of venomotor 

 nerves has been but little investigated, and at present little or no 

 use is made of this possible system in explaining the facts of the 

 circulation. 



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 veins the pressure is very low; 

 in the vein, in fact, it may be zero or even negative. 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 pres- 

 sure in the latter is lower than that in the lymphatic duct. At the other ex- 

 tremity of the system, in the tissue spaces to which the lymphatic capillaries 

 are distributed, the pressure, on the contrary, is high. Its exact amount is 

 not known, but, 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 

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



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



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



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



