i88 THE CIRCULATION OF THE BLOOD AND LYMPH 



of the nerves that regulate the work of the heart, that in animals 

 to which the upright position is normal (monkey) and in man the 

 influence of changes of posture on the circulation is almost com- 

 pletely compensated.* The pressure in the upper part of the 

 human brachial artery has been measured with a sphygmoman- 

 ometer, first in the horizontal and then immediately afterwards in 

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

 practically unchanged (Hill) . But if the person was overworked or 

 out of sorts, the compensation was less complete. It is well known 

 that in debilitated persons, especially if long confined to bed, the 

 sudden assumption of the upright position may cause vertigo, and 

 even syncope, the normal compensatory mechanism being deranged. 

 In such animals as the rabbit this compensation 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, and 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 anaemia (Salathe, 

 Hill). The head-down position has no ill-effects. In wild rabbits, 

 whose abdominal wall is more tense and elastic, these fatal symp- 

 toms 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 circulation 

 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 particularly 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 horizontal axis passing under the neck, the blood-pressure in the 

 carotid artery falls greatly when the animal is made to assume the 

 vertical position with the head up, and either rises a little or remains 

 practically unchanged when the head is made to hang down. So 

 great may the fall of pressure be in the former position that death 

 may occur if it be long maintained (Practical Exercises, p. 212). 



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

 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 hydro- 

 dynamic 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 mean 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 it 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, by the change of position, this differ- 

 ence would be equal to the pressure of a column of blood twice as high as the 

 straight-line distance between the cannula and the point of the arterial system 

 at which the pressure is the same with head up as with head down (indifferent 

 point). 



