CHAP, iv.] THE VASCULAR MECHANISM. 225 



the body-weight. Beyond that, a large and frequently a sudden 

 dangerous permanent depression is observed. 



The restoration of the pressure after the cessation of the 

 bleeding is too rapid to permit us to suppose that the quantity of 

 fluid in the blood-vessels is repaired by the withdrawal of lymph 

 from the extra-vascular elements of the tissues. In all probability 

 the result is gained by an increased action of the vaso-motor 

 nerves, increasing the peripheral resistance, the vaso-motor centres 

 being thrown into increased action by the diminution of their 

 blood-supply. When the loss of blood has gone beyond a certain 

 limit, this vaso-motor action is insufficient to compensate the 

 diminished quantity (possibly the vaso-motor centres in part 

 become exhausted), and a considerable depression takes place ; but 

 at this epoch the loss of blood frequently causes anaemic con- 

 vulsions. 



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 medulla oblongata 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 

 medullary vaso-motor centre, is very low, is permanently raised by 

 the injection of blood. At each injection the pressure rises, falls 

 somewhat afterwards, but eventually remains at a higher level than 

 before. This rise continues until the amount of blood in the 

 vessels above the normal quantity reaches from 2 to 3 per cent, 

 of the body-weight. Beyond this point there is no further rise of 

 pressure. 



These facts 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 vaso-dilator action of the 

 vaso-motor centres, so that the normal blood-pressure remains con- 

 stant. 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 con- 

 siderable 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 quan- 

 tity 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 of the 

 internal organs), which are abnormally distended to contain the 

 surplus. 



We learn from these facts the two practical lessons, first, that 

 blood-pressure cannot be lowered directly by bleeding, unless the 

 quantity removed be dangerously large, and secondly, that there is 



F. 15 



