246 



THE CIRCULATION OF THE BLOOD 



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 



Fig. 75. Diagram showing the probable arrangements of the vasomotor reflexes. 



A. Muscle of arteriole. 



D. Vasodilator nerve fiber terminating on A and inhibiting its natural tonus, as indicated by - 

 sign. 



C. Vasoconstrictor fiber also ending in A, but exciting it ( + ). These two kinds of fiber arise 

 from the dilator center (DC) and the constrictor center (CC) respectively. 



F. Afferent depressor fiber, dividing into two branches, one of which (-) inhibits the con- 

 strictor center, while the other (+) excites the dilator center causing dilatation of the arteriole and 

 fall of blood pressure. 



R. Pressor fiber exciting CC and inhibiting DC, and therefore causing vasoconstriction and rise 

 of blood pressure. 



a, b, c, and d represent the synapses of the pressor and depressor branches with the efferent 

 neurons. (From Bayliss.) 



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. 



