THE CIRCULATION OF THE BLOOD AND LYMPH 103 



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. 30). 

 As has been said, a tracing taken with a mercury manometer 

 gives approximately the mean blood-pressure. Each beat 

 of the heart is represented on it by a single elevation of no 

 great size, sometimes not amounting to more than one- 

 twentieth of the height of the curve above the line of zero 

 or atmospheric pressure. The small oscillations due to the 

 heart-beat are superposed upon much longer, and often, as 

 registered in this way, larger waves, caused by the move- 

 ments of respiration. The line of mean pressure intersects 

 the respiratory waves midway between crest and trough 

 (Fig. 30). 



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 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 



