354 YASO-MOTOR NERVES OF THE VEINS. [BOOK i. 



arrangements governing the veins is at present very limited. The 

 portal vein, the walls of which are conspicuously muscular, the 

 muscular fibres being arranged both as a circular and as a longi- 

 tudinal coat, is like the veins just mentioned subject to rhythmic 

 variations of calibre; these might be due to active rhythmic 

 contractions of the portal vein itself or might be of a passive 

 nature, due to a rhythmic rise and fall in the quantity of blood 

 discharged into it from the vessels of the viscera. The former 

 view is supported by the observation that after the aorta has been 

 obstructed, so that no blood can pass into the portal vein from the 

 mesenteric and other arteries, contractions of the portal vein may 

 be obtained by stimulating the splanchnic nerves. The great 

 distension of the venous system with blood which occurs in the 

 frog when the brain and spinal cord are destroyed, and which 

 renders the heart almost bloodless, the greater part of the blood 

 being lodged in the veins, has also been supposed to point to some 

 normal tone of the veins dependent on the central nervous 

 system. 



