178 THE CIRCULATION OF THE BLOOD AND LYMPH 



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 

 th^ 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 (Francois-Franck). 



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 portions of the 

 superficial veins of the leg. 



