CHAP, iv.] THE VASCULAR MECHANISM. 297 



neck is suddenly cut off from the brain and so from the spinal 

 bulb, a rise of blood pressure is observed. When the loss of blood 

 has gone beyond a certain limit, this vaso-constrictor action is 

 insufficient to compensate the diminished quantity (possibly the 

 vaso-niotor centre in part becomes exhausted), and a considerable 

 depression takes place ; but at this epoch the loss of blood 

 frequently causes ansernic convulsions. 



Similarly, when an additional quantity of blood is injected into 

 the vessels, no marked increase of blood pressure is observed so 

 long as the vaso-motor centre in the spinal bulb is intact. If, 

 however, the cervical spinal cord be divided previous to the in- 

 jection, the pressure, which, on account of the removal of the 

 bulbar vaso-motor centre, is very low, is permanently raised by the 

 injection of blood. At each injection the pressure rises ; it falls 

 somewhat afterwards, but eventually remains at a higher level than 

 before. This rise is stated to continue until the amount of blood 

 in the vessels above the normal quantity reaches from 2 to 3 

 per cent, of the body-weight, beyond which point it is said no 

 further rise of pressure occurs. The absence of any marked rise 

 of blood pressure, so long as the bulbar vaso-motor centre is intact, 

 shews that the addition of the extra quantity of blood stimulates 

 that centre to increased activity. But while a diminution of blood 

 supply seems to affect the centre directly, an increase of blood 

 supply probably acts in an indirect manner. When the arteries 

 in the neck are ligatured, the rise of blood pressure is much more 

 marked if the depressor nerves be divided ; so long as these 

 nerves are intact impulses passing along them from the heart 

 withstand the stimulating effects on the vaso-motor centre of the 

 loss of blood. And we may perhaps infer that when an extra 

 quantity of blood is injected, the greater fulness stimulates 

 the endings of the depressor nerves in the heart, and so by 

 developing depressor impulses lessens the activity of the vaso- 

 motor centre. 



The facts stated seem, then, to shew, in the first place, that when 

 the volume of the blood is increased, compensation is effected by 

 a lessening of the peripheral resistance by means of a diminished 

 action of the vaso-motor centre, so that the normal blood pressure 

 remains constant. They further shew that a much greater quantity 

 of blood can be lodged in the blood vessels than is normally present 

 in them. That the additional quantity injected does remain in 

 the vessels is proved by the absence of extravasations, and of any 

 considerable increase of the extra-vascular lymphatic fluids. It 

 has already been insisted that, in health, the veins and capillaries 

 must be regarded as being far from filled ; for were they to receive 

 all the blood which they can, even at a low pressure, hold, the 

 whole quantity of blood in the body would be lodged in them 

 alone. In these cases of large addition of blood, the extra quantity 

 appears to be lodged in the small veins and capillaries (especially 



