70 ORGANISM AND ENVIRONMENT 
know, on variations in the calibre of the small arteries, 
caused by variations in the degree of contraction of 
the circular muscular coat with which they are pro- 
vided. It was discovered by Bernard that the muscu- 
lar coat is under the control of the nervous system 
through the vaso-motor nerves supplying the arteries. 
It is apparently, therefore, by these nerves that the 
rate of blood flow is controlled, though it may be that 
there is also some non-nervous means of control, due 
to the direct local action of chemical stimuli. 
But how are the vaso-motor nerves themselves 
excited? It is known that there is a centre in the 
medulla oblongata in connection with afferent nerves 
by the excitation of which a widespread reflex aug- 
mentation or inhibition of the impulses which are con- 
stantly passing from the centre to the arteries is 
brought about. When this centre is destroyed or its 
connections severed there is also a great general fall 
in arterial blood pressure owing to dilatation of the 
arterioles. But the action of this centre does not 
explain the local regulation of blood flow in different 
organs in accordance with local requirements. That 
such local regulation occurs is known from observa- 
tions of the local blood flow; it is known, also, that 
there are subordinate nerve centres controlling local 
blood supply, the response of these centres being to 
afferent impulses passing to the centres along locally 
distributed nerve-fibres. The afferent nerve-endings 
are apparently excited by excessive accumulation of 
products of metabolism or by deficiency of the sub- 
stances used up. It may be that the products of 
