RECORD OF BLOOD PRESSURE. 301 



the arteries the pressure though gradually falling is everywhere 

 high, while just before the blood reaches the capillaries, a sudden 

 fall occurs. The variation after this is merely a gentle descent 

 until the large venous trunks are reached, where the pressure is 

 below zero. From a purely physical point of view, then, the ven- 

 tricle may be regarded as pumping the blood up to an elevated 

 high-pressure reservoir of small capacity (the arteries), from which 

 it rapidly falls by numerous outlets into an expansive low-lying 

 irrigation basin (the wide capillaries), while it slowly trickles 

 back to the well (the auricle) under the pump, which lies below 

 the surface pressure. 



From this diagram the following points can be gathered : 



1. The great difference between the pressure on the arterial 



and venous sides of the circulation. 



2. The comparatively slight difference in pressure in the dif- 



ferent parts of the arterial or of the venous systems re- 

 spectively. 



3. The suddenness of the fall in the pressure between the small 



arteries and the capillaries, where the great resistance to 

 the outflow is met with. 



4. In the large veins the pressure of the blood is habitually 



below that of the atmosphere, only becoming positive 

 during forced expirations. 



VARIATIONS IN THE BLOOD PRESSURE. 



If the blood pressure be recorded with Ludwig's Kymograph, a 

 tracing will be obtained which shows that the pressure undergoes 

 periodic elevations and depressions of two different kinds. The 

 smaller oscillations are found to correspond with the heart beat, 

 the larger waves have the same rhythm as the respiratory move- 

 ments, and the average elevation of the mercurial column is 

 spoken of as the mean pressure. In the large arteries of the warm - 

 blooded animals this mean pressure varies with the size of the 

 animal from 90 mm., mercury, to more than 200 mm. In cold- 

 blooded animals it is comparatively low, from 22 mm. (frog, 

 Volkmann) to 84 mm. (large fi.sh). 



The general mean pressure in the arteries is increased by : (1), 



