630 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) metabohc products on the blood-vessels (p. 611). 



The Vasomotor Nerves to the Veins and Capillaries. — The 

 probabihty that the veins and capillaries have a motor nerve supply 

 has been referred to on p. 612. Some of the evidence in favor of 

 this view is as follows: Mall has given very clear proof that the 

 portal vein receives vasoconstrictor fibers from the splanchnic 

 nerve.* Roy and Sherrington f give some evidence for the exist- 

 ence of venomotor nerves to the large veins of the neck, and 

 Thompson, as also Bancroft,! 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. Hooker 

 has demonstrated that in the veins from the large intestine a rise 

 of pressure may be obtained by direct and reflex (asphyxia) 

 stimulation, indicating that these veins are supplied with veno- 

 motor fibers. Finally, it has been shown that solutions of epi- 

 nephrin cause contraction in rings of vein as they do in arterial 

 strips. On the accepted explanation of the way in which epi- 

 nephrin acts this fact implies that the muscle in the veins is sup- 

 plied by sympathetic autonomic nerve fibers. In regard to the 

 capillaries Steinach and Kahn in experiments upon the nictitating 

 membrane of the frog, and Hooker in the case of the cat's ear have 

 shown that constriction of these vessels may be obtained from 

 direct stimulation of nerve fibers supplying the area under ob- 

 servation.! Whether or not such a nervous control will be found 

 for the capillaries of the body in general can only be determined 

 by further experiments. 



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 

 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, 



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



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



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



§ For literature consult Hooker, "Physiological Reviews," 1, 112, 1921. 



