CIRCULATION. 485 



out and opened the subrnaxillary vein. The blood contained in the vein was 

 dark. The nerve-branch coming to the gland from the sympathetic was now 

 ligated, whereupon the venous blood from the gland grew red and flowed more 

 abundantly ; no saliva was excreted. The sympathetic nerve was now stimu- 

 lated between the ligature and the gland. At this the blood in the vein became 

 dark again, flowed in less abundance and finally stopped entirely. On allow- 

 ing the animal to rest the venous blood grew red once more. The chorda 

 tympani nerve, coming from the lingual nerve, was now ligated, and the end 

 in connection with the gland stimulated. Then almost at once saliva streamed 

 into the duct, and large quantities of bright scarlet blood flowed from the vein 

 in jets, synchronous with the pulse. 



This experiment may be said to close the earlier history of the vaso-motor 

 nerves. 1 It was now established beyond question that the size of the blood- 

 vessels, and thus the quantity of blood carried by them to different parts of the 

 body, is controlled by nerves which when stimulated either narrow the blood 

 vessels (vaso-constrictor nerves) and thus diminish the quantity of blood that 

 flows through them, or dilate the vessels (vaso-dilator nerves) and increase the 

 flow. The section of vaso-constrictor nerves, for example those found in the 

 cervical sympathetic, causes the v&ssels previously constricted by them to dilate. 

 The section of a vaso-dilator nerve, for example the chorda tympani, running 

 from the lingual nerve to the submaxillary gland, does not, however, cause the 

 constriction of the vessels to which it is distributed. And finally, it was now 

 determined that vaso-motor fibres are found in the sympathetic system as 

 well as in the spinal cord and the cerebro-spinal nerves. 



It remained for a later day to show that vaso-motor nerves are present in 

 the veins as well as in the arteries. Mall 2 has found that when the aorta is 

 compressed below the left subclavian artery, the portal vein receives no more 

 blood from the arteries of the intestine, yet remains for a time moderately full, 

 because it cannot immediately empty its contents through the portal capil- 

 laries of the liver against the resistance which they offer. If the peripheral 

 end of the cut splanchnic nerve is now stimulated, the portal vein contracts 

 visibly and may be almost wholly emptied. Thompson 3 has extended the 

 discovery of Mall to the superficial veins of the extremities. He finds that 

 the stimulation of the peripheral end of the cut sciatic nerve, the crural artery 

 being tied, causes the constriction of the superficial veins of the hind limb. 

 The contraction begins soon after the commencement of the stimulation, and 

 usually goes so far as to obliterate the lumen of the vein. Often the contrac- 

 tion begins nearer the proximal portion of the vein and advances toward the 

 periphery. More commonly, however, it is limited to band-like constrictions 

 between which the vein is filled with blood. After stimulation ceases the 

 constrictions gradually disappear. A second and third stimulation produce 



1 Further information regarding the history of this subject is given by Vulpian, Lemons stir 

 1'appareil vaso-moteur, Paris, 1875; Longet, Traite de physiologic, Paris, 1869, t. ii. p. 199; and 

 Schiff, Untersuchungen zur Physiologic des Nervensystems, PYankfort-am-Main, 1855, Bd. i. p. 124. 



2 Mall, 1890, p. 57 ; 1892, p. 409. * Thompson, 1893, p. 104. 



