CIRCULATION. 195 



It remained for a later day to show that vasomotor nerves arc present in 

 the veins as well as in the arteries. Mall 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 ' 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 arterv 

 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 

 much less constriction. The superficial veins of the rabbit's abdomen are 

 constricted by the stimulation of the cervical spinal cord at the second ver- 

 tebra. 



The observations of Bernard and his contemporaries led to a very great 

 number of researches on the general properties and the distribution of the 

 vaso-motor nerves, in the course of which a variety of ingenious methods of 

 observation have been devised. 



Methods of Observation. — One fruitful method of research has been 

 already incidentally mentioned, namely, the direct inspection of the vessel, or 

 region, the vaso-motor nerves of which are being studied. 



A second method consists in accurately measuring the outflow from the 

 vein. If the blood-vessels of the area drained by the vein are constricted by 

 the stimulation of a vaso-motor nerve, the quantity escaping from the vein in 

 a given period previous to constriction will be greater than that escaping in an 

 equal period during constriction. This well-known method is especially avail- 

 able where an artificial circulation is kept up through the organ studied, as 

 the blood drained from the vein does not then weaken the animal and thus 

 disturb the accuracy of the observations. 2 



A third method is founded on the principle in hydraulics that the lateral 

 pressure at any point in a tube through which a liquid How- depends, othei 

 things being equal, on the resistance to be overcome below the poinl at which 

 the pressure is measured. In the animal body the resistance to be overcome 

 by the blood-stream varies with the state of contraction of the smaller vessels, 

 and thus the variations in the lateral pressure of a given artery may. under 

 certain restrictions, be used to determine variations in the size of the smaller 



1 Thompson: Archivfur Physiologic, isii.",, p. KM; Bancroft: American Jou mI of Physiology, 

 1898, i. p. 177. 



'Cavazzani ami Manca: Archives italiennes de Biologie, 1895, xxiv. p. '■'*'.'>. 



