THE CIRCULATION OP THE BLOOD. 231 



force of the heart-beat ; consequently it would be expected, and 

 it is actually found, that the pressure varies for di£ferent ani- 

 mals, size having, of course, in most instances a relation to 

 the result. It has been estimated that in the carotid of the horse 

 the arterial pressure is 150 to 200 mm. of mercury, of the dog 

 100 to 175, of the rabbit 50 to 90. Man's blood-pressure is not 

 known, but is probably high, we may suppose not less than 

 150 to 200 mm. 



After the fact that there is a certain considerable blood- 

 pressure, the other most important one to notice is that this 

 blood-pressure is constantly varying during the experiment, 

 and, as we shall give reason to believe, in the normal animal ; 

 and to these variations and their causes we shall presently turn 

 our attention. 



THE HEART. 



The heart, being one of the great centers of life, to speak 

 figuratively, it demands an unusually close study, 



THE CARDIAC MOVEMENTS. 



There is no special difficulty in ascertaining the outlines of 

 the heart by means of percussion on either the dead or the 

 living subject. Quite otherwise is it with the changes in form 

 which accompany cardiac action. Attempts have been made 

 to ascertain the alterations in position of the heart with respect 

 to other parts, and especially its own alterations in shape dur- 

 ing a systole, the chest being unopened, by the use of needles 

 thrust into its substance through the thoracic walls ; but the re- 

 sults have proved fallacious. Again, casts have been made of 

 the heart after death, in a condition of moderate extension, prior 

 to rigor mortis ; and also when contracted by a hardening fluid. 

 These methods, like all others as yet employed, are open to seri- 

 ous objections. 



ToUowing the rapidly beating heart of the mammal with 

 the eye produces uncertainty and confusion of mind. 



It may be very confidently said that the mode of contrac- 

 tion of the hearts of different groups of vertebrates is variable, 

 though it seems highly probable that the divergences in mam- 

 mals are slight. The most that can be certainly affirmed of the 

 mammalian heart is, that during contraction of the ventricles 



