THE NERVOUS REGULATION OF THE BLOODVESSELS 189 



Finally, it is in virtue of the amazing power of accommodation 

 possessed by the vascular system, as controlled by the vaso-motor 

 and cardiac nerves, that so long as these are not disabled the total 

 quantity of blood may be greatly diminished or greatly increased, 

 without endangering life, or even causing more than a transient 

 alteration in the arterial pressure. It is not until at least a quarter 

 of the blood has been withdrawn that there is any notable effect ' 

 on the pressure, for the loss is quickly compensated by an increase 

 in the activity of the heart and a constriction of the small arteries. 

 An animal may recover after losing considerably more than half its 

 blood.* Conversely, the volume of the circulating liquid may be 

 doubled by the injection of blood or physiological salt solution 

 without causing death, and increased by 50 per cent, without any 

 marked increase in the pressure. The excess is promptly stowed 



Fig. 85. Blood- Pressure Tracing from a Dog poisoned with Alcohol. 

 The respiratory centre being paralyzed, respiration stopped, and 

 the typical rise of blood-pressure in asphyxia took place. The 

 pressure had again fallen, and total paralysis of the vaso-motor 

 centre was near at hand, when at A the animal made a single 

 respiratory, movement. The quantity of oxygen thus taken in was enough 

 to restore the vaso-motor centre, and the blood-pressure again rose. This 

 was repeated five or six times. (Three-fourths original size.) 



away in the dilated vessels, especially those of the splanchnic area; 

 the water passes rapidly into the lymph, and is then more gradually 

 eliminated by the kidneys. 



From these facts we can deduce the practical lesson, that blood- 

 letting, unless fairly copious, is useless as a means of lowering the 

 general arterial pressure, while it need not be feared that transfusion 

 of a considerable, quantity of blood, or of salt solution, in cases of 

 severe haemorrhage will dangerously increase the pressure. And 

 from the physiological point of view the term ' haemorrhage ' includes 

 more than it does in its ordinary sense. For as dirt to the -sani- 

 tarian is ' matter in the wrong place/ haemorrhage to the physiolo- 

 gist is blood in the wrong place. Not a drop of blood may be lost 



* It is not usually possible to obtain quite two-thirds of the total blood by 

 bleeding a dog from a large artery. In seven dogs bled from the carotid, 

 the ratio of the -weight of the blood obtained to the body-weight was 

 i : 24-7, i : 21-7, i : 20-7, i : 20-6, i : 18-6, i : 16, i : 13-5 In the last case, 

 the blood clotted with abnormal slowness, and the animal died in a few 

 minutes. 



