ISO THE CIRCULATION. 



and the second sound of the heart ; and both it and the 

 progressive contractions of all the more distant parts main- 

 tain, as already said, that pressure on the blood during 

 the inaction of the ventricle, by which the stream of the 

 arterial blood is sustained between the jets, and is finally 

 equalized by the time it reaches the capillaries. 



It may seem an objection to this theory, that it would 

 probably require a larger quantity of blood to dilate all 

 the arteries than can be discharged by the ventricle at each 

 contraction. But the quantity necessary for such a pur- 

 pose is less than might be supposed. Injections of the 

 arteries prove that, including all down to those of about 

 one-eighth of a line in diameter, they do not contain on 

 an average more than one and a half pints of fluid, even 

 when distended. There can be no doubt, therefore* that 

 the three or four ounces which the ventricle is supposed 

 to discharge at each contraction, being added to that 

 which already fills the arteries, would be sufficient to 

 distend them all. 



A distinction must be carefullymade between the passage 

 of the wave along the arteries, and the velocity of the stream 

 (p. 155) of blood. Both wave and current are present; but 

 the rates at which they travel are very different, that of the 

 wave being twenty or thirty times as great as that of the 

 current. 



Returning now to the consideration of the pulse -tracings 

 (p. 147), it may be remarked that, in each, the up-stroke 

 corresponds with the period during which the ventricle is 

 contracting ; the down- stroke, with the interval between 

 its contractions, or in other words with the recoil, after 

 distension, of the elastic arteries. In the large arteries, 

 when at least there is much loss of tone, the up-stroke is 

 double, the almost instantaneous propagation of the force 

 of contraction of the left ventricle along the column of 

 blood in the arteries, or the percussion-impulse, as it 

 is termed by Dr. Sanderson, being sufficiently strong to 



