CONTROL OF PERIPHERAL CIRCULATION 
cervical sympathectomy (Fig. 8). It was not affected by blocking 
only the cutaneous nerves to the forearm. The following observa- 
tions suggested that the vasomotor contribution was mediated through 
activity of cholinergic vasodilator fibres rather than release of 
vasoconstrictor tone. In some cases emotional stress produced a 
greater vasodilation in the normal forearm than in the contralateral 
nerve-blocked forearm. The vasodilation was reduced and oc- 
casionally abolished by atropinizing the test forearm (Fig. 9). 
Bretylium tosylate, in doses which abolished reflex vascular re- 
actions usually attributed to alterations in vasoconstrictor tone in 
muscle, did not affect the vasodilation during stress. It was con- 
cluded that cholinergic vasodilator fibres are distributed to human 
skeletal muscle and that they are activated during emotional stress. 
It has been suggested that generalized vasodilator activity at 
the beginning of exercise might help to adapt the circulation to the 
circulatory needs of exercise rapidly (Uvnas, 1954). In an attempt 
to determine the part played by vasomotor fibres in the limbs in the 
general circulatory response to exercise, blood flow was measured 
in the hands andforearms of recumbent subjects during leg exercise 
on a bicycle ergometer (Blair, Glover, and Roddie, 1961). Exercise 
resulted in a considerable increase in vasoconstrictor tone in 
muscle butthere was no evidence that discharge of vasodilator fibres 
was an integral part of thegeneral vasomotor response to exercise. 
In inexperienced subjects vasodilator discharge was occasionally 
seen during exercise but it is likely that this discharge was due to 
associated emotional stress. Emotional stress may also contribute 
to the muscle vasodilation described during fainting (Barcroft and 
Edholm, 1945). 
Vasomotor Nerves to Skin 
Because of the relative ease with which estimation of blood flow 
can be made in the hand or foot, the vascular innervation of the 
extremities has been extensively studied. However, it should be 
stressed that the vasculature of the hand and foot are very hi^ly 
specialized and adapted to serve temperature regulation. Many false 
121 
