lsj EXPERIMENTAL PHYSIOLOGY 



marked if the dose be at all large that the mercury may be driven out 

 of the manometer. In the figure reproduced it is seen that the 

 pressure rose gradually from a mean pressure of 84 mm. Hg to one 

 of 136 mm., and from that point it gradually sank, though some 

 minutes later it was still distinctly higher than at the start. At the 

 commencement of the tracing there are some irregularities which were 

 due to alterations in the respiratory rhythm. This rise is due to a 

 marked constriction of the arterioles. A similar result, though not 

 to so marked an extent, is produced by an injection of a watery extract 

 of the medullary portion of the supra-renal gland. On the other hand 

 an extract of the cortex produces practically no effect upon the blood 

 pressure. 



7. The effect of asphyxia. — The experiment may be conveniently 

 concluded by killing the animal by asphyxia. This is effected by com- 

 pressing the trachea by clip forceps or a ligature. If the animal be 

 under the influence of curare, stopping the artificial respiration is 

 sufficient. 



Death by asphyxia is described as occurring in three stages, pro- 

 ducing typical results in the blood-pressure tracing. The first stage, 

 lasting from a to b (pi. 2), is the stage of dyspnoea. The blood pressure 

 gradually rises and the animal makes deep inspiratory efforts, each 

 being sustained for a time, and expiration is rapidly followed by a fresh 

 forcible inspiration. The blood pressure shows variations correspond- 

 ing to these respirations. This stage is followed after a varying time 

 by the second — the stage of convulsions. (From b to c.) 



In this stage the animal passes into a rapid series of convulsive 

 struggles, as seen upon the respiratory tracing ; and with each struggle 

 the blood pressure rises considerably. The mean blood pressure reaches 

 its greatest height during this stage. The third and last stage is 

 characterised by a gradual weakening and slowing of the heart and 

 a fall in blood pressure. The animal becomes quiet, and only a few 

 respirations are attempted. No other muscular efforts are made. 

 The heart gradually ceases to beat and the animal dies. 



Note that at death the blood pressure has not reached the zero 

 abscissa, but lies 8 mm. above it. If the aorta be cut into, and the 

 blood allowed to escape, the manometer falls to zero pressure. The 

 pressure at death is not zero pressure, because the blood is contained 

 in a closed system of tubes, which it overdistends, with a uniform 

 pressure throughout, and this pressure is spoken of as the mean general 

 pressure. During life the blood is very differently distributed, the 

 arteries being overfilled, and therefore the arterial pressure is 

 much above the mean general pressure. The large veins, on the 

 other hand, are not distended to the full amount, and the pressure 

 is below that of the mean general pressure. 



