1 64 A MANUAL OF PHYSIOLOGY 



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 brighter 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 first clear proof of 

 the existence of vaso-motor nerves for veins was furnished by 

 Mall, who showed that vaso-constrictor fibres for the portal vein 

 exist in the splanchnic nerves. When these were stimulated, 

 after the disturbing effect of changes in the 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 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. 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-pressure rises 

 in the portal vein (Frangois-Franck). 



The vena portae and its branches are in the physiological 

 sense arteries rather than veins, since they break up into capil- 

 laries, 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 con- 

 traction of short portions of the superficial veins of the leg. 



