240 THE CIRCULATION OF THE BLOOD - 



abdominal viscus becomes diminished evidence of general vasoconstric- 

 tion. But when the sensory nerve is stimulated with extremely weak 

 faradic shocks, an entirely different result is likely to be obtained; 

 namely, a fall of blood pressure and an increase in volume of the limb 

 or viscus, indicating that in this manner we have stimulated depressor 

 fibers. By careful experimentation with quantitatively graduated elec- 

 trical stimuli, it has been found by Martin and others 17 that on stimu- 

 lating an afferent nerve with weak shocks, a fall in blood pressure is 

 the first effect to be observed, and that this becomes more and more 

 marked as the strength of the stimuli is increased, until a certain opti- 

 mum is reached, after which the fall in blood pressure becomes less evi- 

 dent. When a certain strength of stimulation is exceeded, a rise instead 

 of a fall occurs. After this point additional increase in stimulation causes 

 more and more marked elevation of blood pressure through a very long 

 range of stimuli. 



Stimulation of two afferent nerves at the same time usually produces 

 a greater reflex vasomotor change than the stimulation with an equiva- 

 lent strength of current of either nerve alone. That is to say, the effect 

 produced by stimulating the central end of both sciatics simultaneously 

 will be greater than that produced by stimulating either alone with double 

 the strength of stimulus. 



As has been stated above, the reflex change in blood pressure is often 

 quite transitory in nature, although the stimulation of the pressor nerve is 

 maintained. When this decline has occurred, the pressor reaction can 

 often be renewed by shifting the stimulation to a second nerve. These 

 facts concerning the greater efficacy of combined stimulation of several 

 nerves are of considerable importance in connection with the general 

 question of reflex changes in blood pressure. For instance, many of the 

 pressor fibers found in the sciatic nerve are connected with the receptors 

 that mediate the sensations of the skin. When these receptors are 

 stimulated, as by heat or cold, reflex changes in blood pressure occur 

 (pressor reaction), (Fig. 74), and it is important to remember that 

 localized stimulation of the skin is less efficient in bringing about such 

 vascular changes than stimulation applied over large areas, even when 

 the local stimulus is intense and the general stimulus mild in character. 

 Jumping into a moderately cold bath will cause a much greater rise in 

 arterial blood pressure than plunging the hand into ice cold water. 



Mechanism of Action of Pressor and Depressor Impulses. When we 

 consider the exact mechanism by which these afferent impulses operate, 

 we have to bear in mind four possibilities: the reflex fall produced by 

 stimulation of a depressor afferent fiber may be due either to a stimula- 

 tion of the vasodilator part of the center or to an inhibition of the tone 



