When .the heart begins beating again, the pressure-curve rises, not 

 by a continuous ascent, but by successive leaps, each corresponding 

 to a beat of the heart, and each overtopping its predecessor, till the 

 old line of minimum or of mean pressure is again reached. 



The mean arterial blood-pressure is the permanent pressure plus 

 one-half of the average pulsatory oscillation. In a blood-pressure 

 tracing the line of permanent pressure joins all the minima; the line 

 of maximum pressure joins all the maxima ; the line of mean pressure 

 is drawn between them in such a way that, of the area included 

 between it and the blood-pressure curve, as much lies above as below 

 it (Fig. 43). As has been said, a tracing taken with a mercury man- 

 ometer gives approximately the mean blood-pressure. Each beat of 

 the heart is represented on it by a single elevation of variable size, 

 sometimes not amounting to more than one-twentieth of the height 

 of the curve above the line of zero or atmospheric pressure, but some- 

 times much larger. The oscillations due to the heart-beat are 

 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 variations which it under- 

 goes, 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 physio- 

 logical 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 com- 

 paratively 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. 



