METHODS OF RECORDING ARTERIAL PRESSURE. 81 



in small animals, because the ventricular output increases with the 

 size of the heart. The work per unit weight of heart muscle in 

 different animals has never been estimated. The mean arterial 

 pressure, when the animal is in the horizontal position, is, within certain 

 limits, nearly the same in arteries of different calibre, and at varying 

 distances from the heart. Poiseuille l found that the mercury in the 

 hgemodynamometer rose to nearly equal heights when the instrument 

 was connected in turn with the iliac, carotid, radial, facial, and other 

 arteries. This is so, on account of the slight amount of resistance 

 in the larger arteries compared to that in the smaller arteries and 

 arterioles. 



If the measurements of mean pressure are repeated in different 

 arteries when the animal is placed, not in the horizontal, but in the 

 vertical feet- down posture, then the arterial pressure varies owing 

 to the effect of gravity (see p. 90). 



Although, in the horizontal posture, the mean pressure in different arteries 

 varies but little, records of the systolic pressure show considerable differences. 

 With his spring manometer Hiirthle has obtained a series of comparative 

 measurements, from which the following table of diastolic and systolic pressures 

 is taken : 



The two most striking results to be observed in this series are, first, 

 the near agreement of the diastolic pressures ; and, second, the fact that in 

 the femoral, the systolic pressure is higher than in the carotid artery. This 

 last result has been explained by supposing that the first centripetal positive 

 pressure wave, which is reflected from the periphery, is of such a period that it 

 adds itself in this artery to the primary centrifugal positive wave before 

 the latter has passed away. It is, however, possible that the result is due to 

 instrumental error. The rise of systolic pressure lasts longer in the femoral 

 than in the carotid artery, and may thus be more correctly recorded. The 

 experiments require repetition with the use of a maximal manometer. 



The maintenance of a mean arterial pressure of constant height is the object 

 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 must therefore be so craftily built and so delicately 

 balanced, that every variation in one part is compensated by a simultaneous 



1 "Rech. sur la force du eoeur aortique," These, Paris, 1828. 

 VOL. II. 6 



