206 CIRCULATION OF THE BLOOD 



stagnates in the veins, and its flow to the heart is lessened considerably. The 

 fall in blood pressure, therefore, is occasioned not only by the diminished 

 resistance but also by the deficient flow to the heart. 



The resistance in the vessels is increased either by constriction in a large 

 vascular area, or by compression of a large arterial stem, for example the 



FIG. 78. Blood pressure curve, showing a pronounced fall due to slowing of the heart, as the 

 result of vagus stimulation of medium intensity. To be read from right to left. The 

 time of stimulation is indicated by the two vertical lines. | | = ten seconds. 



abdominal aorta. In the first case the blood flow to the heart is at the same 

 time augmented, since the blood contained in the vessels is driven forward 

 by their contraction. It is possible therefore that a larger quantity of blood 

 will be expelled by the heart under these circumstances. 



It might be supposed that by compression of the vessels the pressure could 

 be forced up indefinitely. But this is not the case. The reflex mediated by 

 the depressor nerve comes into play, so that either the vessels are dilated or 

 the heart beats are retarded. But even if this reflex fails, there is an upper 

 limit to the blood pressure beyond which it cannot pass. The activity of the 

 heart is not unlimited, and we have good reason, based on many observations, 

 for asserting that with a high resistance in the vessels the quantity of blood 

 expelled in a given time actually diminishes. 



3. Quantity of Blood. Investigations on the influence of the blood volume 

 go to show that when the vessels are overfilled the blood pressure does not 

 exceed the normal physiological limits, and that when the vessels contain less 

 than the normal quantity mechanisms are at hand for the purpose of main- 

 taining the blood pressure at its normal height. 



In what follows we shall consider first the results of adding to the normal 

 quantity of fluid in the vessels. Let us suppose that blood or some other harm- 



