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 



