156 A MANUAL OF PHYSIOLOGY 



circulation through the intestines had been eliminated by 

 compression of the aorta in the thorax, an actual shrinking 

 of the vein could be observed. The fibres appear to issue 

 from the spinal cord by the anterior roots of the third to 

 the eleventh dorsal nerves, but chiefly in the fifth to the 

 ninth dorsal (Bayliss and Starling). When the liver is 

 enclosed in a plethysmograph of special construction, and 

 the central end of an ordinary sensory nerve, like the sciatic, 

 excited, reflex vaso-constriction takes place in the portal 

 area, the volume of the organ diminishes, and the blood- 

 pressure rises in the portal vein. 



The vena portse and its branches are in the physiological 

 sense arteries rather than veins, since they break up into 

 capillaries, and it was to be expected that the regulation of 

 the blood-flow in them would be carried out in the same 

 way as in ordinary arteries, namely, by means of vaso-motor 

 nerves. But we must not, without special proof, extend the 

 results obtained in the portal system to ordinary veins. A 

 certain amount of evidence, however, exists that even such 

 veins as those of the extremities are supplied, though scantily, 

 with vaso-constrictor (veno-motor) fibres. After ligation of 

 the crural artery or aorta, stimulation of the peripheral end 

 of the sciatic has been seen to cause contraction of short por- 

 tions of the superficial veins of the leg (Thompson, Bancroft). 



Course of the Vaso-motor Nerves. In the dog the vaso-con- 

 strictors pass out as fine medullated fibres (r8 to 3-6 /JL in 

 diameter) in the anterior roots of the second dorsal to about 

 the second lumbar nerves (Gaskell). They proceed by the 

 white rami communicantes to the lateral sympathetic 

 ganglia, where, or in more distal ganglia such as the inferior 

 mesenteric, they lose their medulla, and their axis-cylinder 

 processes (Chap. XII.) break up into fibrils that come into 

 close relation with the nerve-cells of the ganglia. These 

 ganglion cells in their turn send off axis-cylinder processes, 

 which, acquiring a neurilemma, become non-medullated nerve 

 fibres, and now pass by various routes to their final destina- 

 tion, the unstriped muscular fibres of the bloodvessels. 

 Their course to the head has been already described. To 

 the limbs they are distributed in the great nerves (brachial 



