l8o THE CIRCULATION OF THE BLOOD AND LVMPH 



nerve, as a rule, produces little or no change. Vaso-constrictor 

 fibres pass to the salivary glands from the cervical sympathetic 

 along the arteries, and stimulation of that nerve causes narrowing of 

 the vessels and diminution of the blood-flow, sometimes almost to 

 complete stoppage. 



The nervi erigentes are the nerves through which erection of the 

 penis is caused. When they are divided there is no effect, but 

 stimulation of the peripheral end causes dilatation of the vessels of 

 the erectile tissue of the organ, which becomes overfilled with 

 blood. During stimulation of these nerves, the quantity of blood 

 flowing from the cut dorsal vein of the penis may be fifteen times 

 greater than in the absence of stimulation. It spurts out in a strong 

 stream, and is bi filter than ordinary venous blood (Eckhard). 

 Stimulation of the peripheral end of the nervus pudendus causes 

 constriction of the vessels of the penis, so that it contains vaso- 

 constrictor fibres which are the antagonists of the nervi erigentes. 



Vaso-Motor Nerves of Veins. Like arteries, veins have plexuses 

 of nerve-fibres in their walls, and contract in response to various 

 stimuli. In some cases e.g., in the wing of the bat rhythmical 

 contractions of the veins are strikingly displayed, but they do not 

 depend on the central nervous system, as they persist after section 

 of the brachial nerves. The existence of vaso-constrictor fibres for 

 the venules given off in the liver by the portal vein is indicated by the 

 fact that adrenalin diminishes the blood flow through the organ even 

 when the hepatic artery has been tied (Burton-Opitz ; Macleod and 

 Pearce, etc.). Stimulation of the distal end of the hepatic plexus 

 causes similar effects. The fibres 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. The arterioles arising from the 

 hepatic artery have their own vaso-motor supply, which is more com- 

 plete than that of the portal vessels. When the liver is enclosed in 

 a plethysmograph, 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-pres- 

 sure rises in the portal vein (Francois-Franck). 



The vena portae 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 portions of the 

 superficial veins of the leg. 



