376 SELF-REGULATION. [BOOK i. 



blood pressure which is too high for its powers, the condition of the 

 heart starts impulses which, passing along the depressor fibres up 

 to the spinal bulb, temper so to speak the blood pressure to suit 

 the cardiac strength. 



We have, moreover, reason to think that not only does the 

 heart thus regulate the blood pressure by means of the depressor 

 fibres, but also that the blood pressure, acting as it were in the 

 reverse direction, regulates the heart beat ; a too high pressure, by 

 acting on the cardio-inhibitory centre in the spinal bulb (either 

 directly, that is as the result of the vascular condition of the bulb 

 itself, or indirectly by impulses reaching the medulla along afferent 

 nerves from various parts of the body), may send inhibitory impulses 

 down the vagus, and so slacken or tone down the heart beats. 



In the following sections of this work we shall see repeated 

 instances, similar to or even more striking than the above, of the 

 management of the vascular mechanism by means of the nervous 

 system, and we therefore need dwell no longer on the subject. 



We may simply repeat that at the centre lies the cardiac 

 muscular fibre, and at the periphery the plain muscular fibre of the 

 minute artery. On these two elements the central nervous system, 

 directed by this or that impulse reaching it along afferent nerve 

 fibres, or affected directly by this or that influence, is during life 

 continually playing, now augmenting, now inhibiting, now the one, 

 now the other, and so, by help of the elasticity of the arteries and 

 the mechanism of the valves, directing the blood flow according to 

 the needs of the body. 



