,6 4 A MANUAL OF PHYSIOLOGY 



is normal (monkey) and in man the influence of changes of 

 posture on the circulation is almost completely compensated.* 

 The pressure in the upper part of the human brachial 

 artery has been measured by Hill with the sphygmometer, 

 first in the horizontal and then immediately afterward in 

 the standing posture, and in health it has been found to 

 remain practically unchanged. But if the person was 

 over-worked or out of sorts, the compensation was less 

 complete. In such animals as the rabbit this compen- 

 sation is totally inefficient When a domesticated rabbit, 

 which has been kept in a hutch, is suspended vertically 

 with the feet down, the blood drains into the abdominal 

 vessels, syncope speedily ensues, an.d in a period that 

 ranges from less than a quarter to three-quarters of an 

 hour the animal dies in the convulsions of acute cerebral 

 anemia (Salathe", Hill). The head-down position has no 

 ill effects. In wild rabbits, whose abdominal wall is more 

 tense and elastic, these fatal symptoms are not easily 

 produced, and the same is true of cats and dogs. But in 

 all animals, when the compensation is destroyed, as in 

 paralysis of the vaso-motor centre by chloroform, the cir- 

 culation may be profoundly influenced by the position of the 

 body : elevation of the head may lead to cerebral anaemia, 

 syncope, and even death; elevation of the legs, and par- 

 ticularly the abdomen, may restore the sinking pulse by 

 filling the heart and the vessels of the brain. If a chloralized 

 dog be fastened on a board which can be rotated about a 



* Two factors may be distinguished in the blood-pressure, the hydro- 

 static and the hydrodynamic elements. The hydrostatic portion of the 

 pressure is due to the weight of the column of blood acting on the vessel ; 

 the hydrodynamic portion of the pressure is due to the work of the heart. 

 If a dog be securely fastened to a holder arranged in such a way that the 

 animal can be placed vertically, with the head up or down, and the mea 

 blood-pressure in the crural artery be measured in the two positions, there 

 will be a considerable difference. For when the legs are uppermost the 

 heart has to overcome the weight of the column of blood rising above 

 to the crural artery ; when the head is uppermost the action of the heart 

 is reinforced by the weight of the blood. And if no change were produced 

 in the action of the heart, or in the general resistance of the vascular path, 

 bv the change of position, this difference would be equal to the pressure 

 of a column of blood twice as high as the straight-lme distance between 

 e cannula and the point of the arterial system at which the pressure is 

 the same with head up as with head down (indifferent point). 



