THE ARTERIAL PRESSURE 193 



pressure above what can be obtained with a normal amount of blood. 

 Similarly, bleeding to the extent of 2 to 3 per cent, of the body weight 

 causes little or no fall of arterial pressure. 



The taking of alcohol lowers the arterial pressure. During chloro- 

 form anaesthesia the pressure falls (Fig. 104). During deep sleep the 

 pressure is lower, but not lower than in the waking state when the 

 body is recumbent and at rest. Immersion in a hot bath lowers, in 

 a cold bath raises, the pressure. 



The arterial pressure is considerably higher in warm than in cold 

 blooded animals at least three times greater. The pressure is inde- 

 pendent of the size of the animal, and thus may be as great in the cat 

 as in the horse. 



It will thus be seen that the maintenance of a mean arterial pressure 

 of constant height is the function of the circulatory mechanism. On 

 the one hand, we are convinced that this object is attained during life ; 

 on the other hand, we know that countless and ceaseless variations 

 are occurring in all parts of the circulatory system. The whole system 

 is, therefore, so craftily biiilt and so delicately balanced that every 

 variation in one part is compensated by a simultaneous and contrary 

 variation in another part, and thus, throughout the wear and tear of 

 life, the aortic pressure is kept at a constant mean height. 





