104 A MANUAL OF PHYSIOLOGY 



superposed upon much longer, and often, as registered in this 

 way, larger waves, caused by the movements of respiration. So 

 much having been said by way of definition, we have now to 

 consider the amount of the mean arterial pressure, the varia- 

 tions which it undergoes, and the factors on which its maintenance 

 depends. 



As to its amount, it will be sufficiently accurate to say that 

 in the systemic arteries of warm-blooded animals in general 

 (including birds), and of man in particular, the mean pressure 

 does not, under ordinary conditions, descend much below 100 mm. 

 of mercury, nor rise much above 200 mm. ; while in cold-blooded 

 animals it seldom exceeds 50 mm., and may fall as low as 20 mm. 



It does not seem possible, at least with our present data, to further 

 subdivide these two great groups ; nor do we know precisely whether 

 the distinction depends mainly on morphological or mainly on 

 physiological differences, whether, that is to say, the warm-blooded 

 animal has a higher blood-pressure than the cold-blooded chiefly 

 because its vascular system (and especially its heart) is anatomically 

 more perfect, or because its heart beats faster and works harder. It 

 may be that it is for both of these reasons that the birds, which in 

 certain other respects are more nearly related to the reptiles than 

 to the mammals, mount, as regards the pressure of the blood, into 

 the mammalian class, and that a manometer in the carotid of a goose 

 will rise as high, or almost as high, as in the carotid of a horse, a 

 sheep, or a dog, while the pressure in the aorta of a tortoise is no 

 higher than in the aorta of a frog. But we know that the mere 

 average rate of the heart has of itself comparatively little influence 

 on the blood-pressure within either group, for the heart of a rabbit 

 beats, on the average, very much faster than the heart of a dog, and 

 yet the arterial pressure in the dog is certainly at least as great as in 

 the rabbit. Nor does the size of the body seem to have any definite 

 relation to the mean pressure, even in animals of the same species ; 

 and there is no reason to suppose that the pressure is materially less 

 in the radial artery of a dwarf than in the radial artery of a giant. 



Measurement of the Blood-pressure in Man. In man the blood- 

 pressure has been estimated by adjusting over an artery an 

 instrument known as a sphygmomanometer or sphygmometer, 

 which, in its most modern form, consists essentially of a hollow 

 rubber pad or bag containing liquid or air, and connected with 

 a metallic pressure gauge or a mercurial manometer. 



The sphygmomanometer of Erlanger (Fig. 37) is arranged to obtain 

 graphic records of the pulse, from which both the maximum and 

 the miminum blood-pressures may be deduced. The mean pressure 

 cannot be directly measured, but must lie much nearer to the mini- 

 mum than to the maximum, since the line of mean pressure bisects 

 the area enclosed by the pulse-curve, and this area is broad at the 

 base and narrow at the apex. The rubber bag is applied in the 

 form of a cuff or armlet to the arm above the elbow over the brachial 

 artery. It communicates with a mercury manometer, which gives 

 the pressure exerted upon the arm. It is also connected with a 



