THE CIRCULATION OF THE bLOOD. 261 



tions prevailing at the time in the other parts of the vascular 

 system ? 



The matter is complex. The effect of vagus stimulation on 

 the blood-pressure is always very-marked, as would be supposed. 



As seen in the tracing, the beats, when the heart commences 

 its action again tell on the comparatively slack walls of the 

 arteries, distending them greatly, and this may be made evident 

 by the sphymograph as well as the manometer ; indeed, may be 

 evident to the finger, the pulse resembling in some features that 

 following excessive loss of blood. 



If the heart has been merely slowed, or its pulsation weak- 

 ened, the effects will of course be less marked. 



The Quantity of Blood.— The blood-pressure may also be 

 augmented, the cardiac frequency remaining the same, by the 

 quantity of blood ejected from the ventricles, which again 

 depends on the quantity entering them, a factor determined 

 by the condition of the vessels, and to this we shall presently 

 turn. 



In consequence of changes in different parts of the system by 

 way of compensation, results follow in an animal which might 

 not have been anticipated. 



Thus, bleeding, unless to a dangerous extreme, does not lower 

 the blood-pressure except temporarily. It is estimated that the 

 body can adapt itself to a loss of as much as 3 per cent of the 

 body-weight. 



The adaptation is probably not through absorption chiefly, 

 but through constriction of the vessels by the vaso- motor 

 nerves. 



Again, an injection of fluid into the blood does not cause an 

 appreciable rise of blood-pressure, so long as the nervous sys- 

 tem is intact ; but, if by section of the spinal cord the vaso- 

 motor influences are cut off, then a rise may take place to the 

 extent of 2 to 3 per cent of the body-weight, the extra quan- 

 tity of fluid seeming to be accommodated in the capillaries and 

 smaller veins. These facts are highly sig^niflcant in illustrat- 

 ing the adaptive power of the circulatory system (protective in 

 its nature), and ^ire of practical importance in the treatment of 

 disease. 



We think the benefit that sometimes follows bleeding has 

 not as yet received an adequate explanation, but we shall not 

 attempt to tackle the problem now. Changes in the circulation 

 depend on variations in the size of the blood-vessels. 



