322 PHYSIOLOGY. 



there the vasomotor fibers emerge from the anterior roots to reach 

 the blood-vessels. 



When a vasoconstrictor nerve, as the sympathetic, is cut, the 

 blood-vessels of the rabbit's ear supplied by it dilate. This fact indi- 

 cates that the circulatory vessels have tonic impulses going to them 

 from the central nervous system through the vasoconstrictor nerves. 



This tonus of the vasoconstrictor nerves does not exist in all vaso- 

 motor nerves to the same degree. It is a variable factor may be 

 depressed or absolutely removed. To decide that a nerve is a vaso- 

 constrictor nerve, it becomes necessary to irritate the nerve with an 

 electrical current and then to see the blood-vessels supplied by it 

 contract. 



A B 



Fig. 106. Curves Obtained by Enclosing the Hind Limb of a Cat 

 in the Plethysmograph and Stimulating the Peripheral End of the Cut 

 Sciatic Nerve (Bowditch and Warren, 1886). (HowELL.) 



The curves read from right to left. In each case the vertical lines show 

 the duration of the stimulus, namely, fifteen induction shocks per second dur- 

 ing 20 seconds. Curve A shows the contraction of the vessels produced by an 

 equal excitation of the freshly divided nerve; curve Ji, the dilation produced 

 by an equal excitation of the nerve of the opposite side four days after section, 

 the vasoconstrictor nerves having degenerated more rapidly than the vasodilators. 



When tonus exists in a vasoconstrictor nerve and it is then cut, 

 there results an effect opposite to that of an irritation. That is, there 

 is a condition of dilatation in the arterioles and capillaries. By this 

 section of the vasoconstrictors the volume of the parts increase in 

 direct proportion to the increased blood-supply. If a cut be made 

 into the organ, the blood flows more rapidly than before there was 

 section of the nerve. The temperature of the organ increases and is 

 perceptibly higher than that of the opposite side. 



With increase in dilatation there is a concomitant fall in blood- 

 pressure. If a large vasoconstrictor nerve like the splanchnic be cut, 

 then the blood-pressure is marked by a most decided fall. 



If, now, the vasoconstrictor be irritated, preferably with elec- 



