RODDIE, I. C. 
sections of digital skin (Hurley and Mescon, 1956; Armin et al, 
1953). Recently, AUwood et al (1960) have found an increase in hand 
blood flow in subjects exposed to a mental arithmetic test. Quite 
large vasodilations were seen in some patients who suffered from 
hyperhydrosis. Emotional stress is known to cause sweating in the 
hand, and it is possible that sweat gland activity, by leading to the 
formation of bradykinin-forming enzyme, may contribute to this 
type of vasodilation. It does not seem however, that cholinergic 
vasodilator fibres play an important part in the reflex changes in 
hand blood flow seen in normal individuals. 
Though vasoconstrictor fibres to the hand obviously play a large 
part in temperature regulatory reflexes, their activity is greatly in- 
fluenced by trivial and often inappropriate stimuli. Thehigh degree 
of reactivity which hand blood vessels normally exhibit has made the 
study of vascular reflexes very difficult. If a subject sees the door of 
the laboratory being opened, his hand blood flow frequently falls to 
zero. Inflating a pneumatic cuff on one arm (Roddie, 1951) or taking 
a deep breath (Bolton, Carmichael, andSturup, 1936) causes a highly 
significant vasoconstriction in the hand. This is especially true in the 
untrained subject. It is therefore practically impossible to tell 
whether the vasoconstriction which often occurs in the hand during, 
say, carbon dioxide inhalation (GelDiorn and Steck, 1938), exercise 
(Blair et al, 1961a; Muthetal, 1958), or positive pressure breathing 
(Fenn and Chadwick, 1947) is a specific response to a particular 
stimulus, or merely the normal sequence of the psychic disturbance 
which the stimulus unavoidably causes. Nevertheless, careful studies 
would suggest that these fibres are not involved in postural reflexes 
(Beaconsfield and Ginsburg, 1955) nor those associated with intra 
thoracic pressure changes (Roddie et al, 1958). 
To summarize, the bloodvesselsof the extremities of the limbs 
are normally subjected to a hi^ degree of vasoconstrictor tone even 
though the subject is comfortably warm. The very large increase in 
hand blood flow during body heatingseems due entirely to release of 
vasoconstrictor tone, and there is no evidence that vasodilator fibres 
contribute to this increase. Inthe forearm skin the cutaneous vessels 
are not subjected to appreciable vasomotor influence when the sub- 
ject is comfortably warm. Cooling the subject causes flow to fall 
due to vasoconstrictor fibre activity. 
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