CHAP, iv.] THE VASCULAR MECHANISM. 333 



180. Vaso-motor nerves of the Veins. Although the veins are 

 provided with muscular fibres and are distinctly contractile, and 

 although rhythmic variations of calibre due to contractions may 

 be seen in the great veins opening into the heart, in the veins of 

 the bat's wing, and elsewhere, and similar rhythmic variations, also 

 possibly due to active rhythmic contractions, but possibly also of 

 an entirely passive nature, have been observed in the portal veins, 

 very little is known of any nervous arrangements governing the 

 veins. When in the frog the brain and spinal cord are destroyed, 

 very little blood comes back to the heart as compared with the 

 normal supply, and the heart in consequence appears almost blood- 

 less and beats feebly. This has been by some regarded as more 

 than can be accounted for by mere loss of arterial tone, and accord- 

 ingly interpreted as indicating the existence of a normal tone 

 in the veins dependent on the central nervous system. When 

 the latter is destroyed, the veins become abnormally distended 

 and a large quantity of blood becomes lodged and hidden as it 

 were in them. 



