THE CIRCULATION OF THE BLOOD AND LYMPH 155 



The glands flush red; more blood is evidently passing 

 through their vessels. Allowed to escape from a divided 

 vein, the blood is seen to be of bright arterial colour and 

 shows a distinct pulse. The small arteries have been dilated 

 by the action of the vaso-motor fibres in the nerve. The 

 resistance being thus reduced, the blood passes in a fuller 

 and more rapid stream through the capillaries into the 

 veins, and on the way there is not time for it to become 

 completely venous. These vaso-dilator fibres are apparently 

 not in constant action, for section of the 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 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 seem to depend on the nervous 

 system, as they persist after section of the brachial nerves. 

 But up to a very recent date there was no clear proof of 

 the existence of vaso-motor nerves for veins. In 1892, 

 however, Mall 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 



