CONTROL OF PERIPHERAL CIRCULATION 
Effron, 1943). With this degree of suppression forearm flow did not 
increase during bocfy heating. It was clear, therefore that vasocon- 
strictor fibres to muscle were not involved in the peripheral vaso- 
dilatation during body heating. Blair, Glover, and Roddie (1960) 
found that the fall in forearm blood flow during body cooling could 
be prevented by blocking the cutaneous nerves in the forearm, pro- 
viding further evidence that vasoconstrictor fibres to muscle do not 
take part in thermoregulatory reflexes. The normal function of 
these fibres remained obscure. 
Over the last few years, the reflex effects of passively raising 
the legs of a recumbent subject on peripheral blood vessels have 
been studied in considerable detail (Roddie and Shepherd, 1956; 
Roddie, Shepherd, and Whelan, 1957). This stimulus caused reflex 
vasodilatation in the forearm but there was no comparable change 
in the hand (Fig. 2). The oxygen saturation of muscle venous blood 
was increased but that of skin was not affected. These findings sug- 
gested that muscle rather than skin vessels were responsible 
for the vasodilatation. The changes were mediated through sympa- 
thetic vasomotor nerves since they were abolished by acute nerve- 
block or by cervical sympathectomy. Release of vasoconstrictor 
tone rather than vasodilator fibre activity was never greater than 
could be accounted forby full release of vasoconstrictor tone; it was 
not reduced by atropinizing the forearm but was abolished by intra 
arterial infusion of the sympatholytic agent bretylium tosylate. It 
was concluded that alterations in vasoconstrictor tone in muscle 
were responsible for the changes in forearm blood flow with change 
in posture. 
A wide variety of stimuli are now thought to affect the level of 
vasoconstrictor tone in muscle. Negative pressure breathing (Blair, 
Glover, and Kidd, 1959), squatting (Sharpey-Shafer, 1956) and intra- 
thoracic pressure transients (Sharpey- Shafer, 1953; Roddie, Shep- 
herd, and Whelan, 1958) are thought to cause reflex vasodilatation. 
Tilting a subject into the vertical position (Bridgen, Howarth, and 
Sharpey-Shafer, 1950), positive pressure breathing (Blair, Glover, 
and Kidd, 1959), the Valsalva manoeurve (Roddie, Shepherd, and 
Whelan, 1958; Sharpey-Shafer, 1955), exercise (Blair, Glover, and 
Roddie, 1961), radial acceleration (Howard and Garrow, 1958), and 
hypercapnia (McArdle, Roddie, Shepherd, and Whelan, 1957) are 
115 
