RODDIE, I. C. 
places in the brain stem which discharge these fibres in anesthe- 
tized cats provoke the 'fight or flight' reaction in conscious cats. 
It seemed possible, therefore, that emotional stress might be the 
effective physiological stimulus. 
It has been known for a long time that stimuli such as mental 
arithmetic frequently cause an increase in forearm blood flow 
(Abramson and Ferris, 1940; Grant and Pearson, 1938). GoUehhoff en 
and Hildebrandt (1957), using Hensel's heated thermocouples to 
measure skin and muscle blood flow, found that the vasodilatation 
occurred mainly in muscle, but this response was usually attributed 
to epinephrine. Evidence that cholinergic vasodilator fibres con- 
tributed to vasodilatation in muscle during emotional stress was ob- 
tained in some experiments where we tried to provoke a 'fight or 
flight' reaction in our subjects (Blair, Glover, Greenfield, and 
Roddie, 1959). Medical students were frightened, worried, or em- 
barrassed by means judged likely to be most effective for each in- 
dividual (Figs. 4 and 5). In most experiments the subject had an 
indwelling needle in the brachial artery. The operators, by their 
whispered conversation and demeanour, led the subject to believe 
that they were alarmed because of blood loss at the arterial punc- 
ture site and the precarious state of the subject's health. About 
half of the subjects complained of pain in the arm and throbbing in 
the head. After a few minutes the real purpose of the hoax was ex- 
plained and anxiety was promptly relieved. Figure 6 shows the result 
of one such experiment. During the period of stress forearm blood 
flow rose to about 50 ml/100 ml/min, a level similar to that found 
immediately after severe exerciseof the forearm muscles and much 
greater than that usually achieved by full release of vasoconstrictor 
tone. The size and rate of recovery of the vasodilation were 
greater than that usually seen during epinephrine infusions. The 
changes in hand blood flow and arterial pressure also were not typ- 
ical of those seen with epinephrine infusions. It seemed unlikely 
that release of epinephrine from the suprarenal glands in response 
to stress could fully explain the response. The fact that forearm 
but not hand blood flow increased during stress suggested that 
muscle vessels might be responsible for the vasodilatation. This was 
supported by the finding that the oxygen saturation of muscle but 
not skin venous blood increased during stress (Fig.7). Sympathetic 
vasomotor fibres to muscle contributed to the vasodilatation since 
it was reduced by blocking the motor nerves to the forearm and by 
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