THE XERVOUS REGULATION OF THE BLOODVESSELS 191 



Finally, it is in virtue of the amaxing 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 of the blood has been withdrawn that there is any notable effect 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 stoweH 



1 n 1 1 1 1 1 1 n i 1 1 1 1 1 1 1 



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

 Therespiratory centre beingparalyzed, 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 oxgyen thus taken in was enough to 

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

 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 

 from the body, and yet death may occur from haemorrhage into the 

 pleural or the abdominal cavity, into the stomach or intestines. 

 Not only so, but a man may bleed to death into his own blood- 



* 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 : zi"j. i : 22-7. i : 20-6, i : 18-6. i : 16, i : 13-5. In the last case, the 

 blood clotte I with abnormal slo .vnes ;. and the animal died in a few minutes. 



