THE VASOMOTOR NERVES. 



617 



and in this case we may suppose that the sensory impulses carried 

 by the heat nerves inhibit the tone of the vasoconstrictor center 

 and cause dilatation or flushing of the skin. So far as man is 

 concerned, experiments made with the plethysmograph show very 

 clearly that the vasoconstrictor center is easily affected in a pressor 

 or depressor manner by psychical states or activities. Mental 

 work, especially mental interest, however aroused, is followed by 

 a constriction of the blood-vessels of the skin, a pressor effect (see 

 Fig. 254) ; and we may find an explanation of the value of the reflex 

 in the supposition that the rise of arterial pressure thus produced 



Fig. 255. Effect of stimulating the central end of the depressor ner^e of the heart m 

 a rabbit. The time record marks seconds. On and off mark the beginning and end I of 

 the stimulation. The blood-pressure rises slowly after the removal of the stimulus and 

 eventually reaches the normal level. This complete recovery is not shown in the portion 



of the record reproduced. (Dawson.) 



forces more blood through the brain (p. 638). On the other hand, 

 feelings of embarrassment or shame may be associated with a de- 

 pressor effect, a dilatation in the vessels of the skin manifested, for 

 example, in the act of blushing. In both cases we must assume 

 intracentral nerve paths between the cortex and the center in the 



