THE VASOMOTOR MECHANISM 337 



The Vasoconstrictor Nerves. If the car of a white rabbit be 

 held up against the light while the animal is kept quiet and not 

 alarmed, the red central artery can be seen coursing along the 

 translucent organ, giving off branches which by subdivision be- 

 come too small to be separately visible, and the whole ear has a 

 pink color and is warm from the abundant blood flowing through 

 it. Attentive observation will show also that the caliber of the 

 main artery is not constant; at somewhat irregular periods of a 

 minute or more it dilates and contracts a little. 



If the sympathetic trunk have been previously divided on the 

 other side of the neck of the animal, the ear on that side will pre- 

 sent a very different appearance. Its arteries will be much dilated 

 and the whole ear fuller of blood, redder, and distinctly warmer; 

 the slow alternating variations in arterial diameter also have 

 disappeared. We get thus evidence that the normal mean caliber 

 of the artery is maintained by influences reaching its muscular 

 coat through the cervical sympathetic. Stimulation of the upper 

 end of the cut nerve confirms this opinion. It is then seen that 

 the arteries of the corresponding ear gradually contract until 

 even the main vessel can hardly be seen, and in consequence the 

 whole ear becomes pale and coid. After the stimulation is stopped 

 the arteries again slowly dilate until they have regained their 

 full paralytic size. 



Quite similar phenomena can be observed in transparent parts 

 of other living animals, as in the web of a frog's foot, the arteries 

 of which dilate after section of the sciatic nerve and constrict 

 when the peripheral end of the nerve is stimulated. In the case 

 of other parts changes in temperature may be used to detect 

 alterations in the flow of blood. In a dog or cat, for example, a 

 sensitive thermometer placed between the toes indicates a rise 

 of temperature, owing to increased flow of warm blood through 

 the skin, after section of the chief nerve of the limb, and a fall of 

 temperature (usually) during stimulation of the peripheral end 

 of the divided nerve. 



When the vasoconstrictor nerves cut are those controlling a 

 large number of arteries, the dilatation of the latter so much 

 diminishes peripheral resistance to the blood-flow as to lead to a 

 marked fall of general arterial pressure; and, due care being taken 

 to avoid or to allow for concomitant variations in the rate or 



