342 EFFECTS OF LOSS AND INCREASE OF BLOOD. [BOOK i. 



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 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. 



These facts seem 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 vaso-dilator action of 

 the vaso-motor centres, 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 

 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 no necessary connection between a high blood-pressure and 

 fulness of blood or plethora, since an enormous quantity of blood 

 may be driven into the vessels without any marked rise of 

 pressure. 



