THE CIRCULATION OF THE BLOOD. 239 
Some find it difficult to understand how the heart after sys- 
tole may regain its original form apart from the assistance of 
diastolic muscles, which are assumed to act so as to antagonize 
those causing systole. 
Others think the elasticity of the heart’s muscle sufficient of 
itself to account for the organ’s return to its original form. 
But there is surely a misconception involved in both of 
these views. 
If small portions of the heart of the frog, tortoise, or other 
cold-blooded animal, just removed from the body, be observed 
under a microscope it will be seen that they alternately con- 
tract and relax. Now, it is only necessary to suppose that the 
relaxation of the heart is complete after each systole, to under- 
stand how even an empty heart regains its diastolic form. 
That there should be a negative pressure in, say, the left 
ventricle, follows naturally enough from the fact that not only 
are the contents of the ventricle expelled with great sudden- 
ness, but that its walls remain (see Figs, 210 and 214) pressed 
together for a considerable portion of the time occupied by the 
whole systole; so that in relaxation it follows that there must 
Fig. 218.—Diagram pho wing te relative height of the blood-pressure in different parts of the 
vascular aie (after Yeo). h, heart; a, arterioles; v, small veins ; 4, arteries ; c, cap- 
illaries ; V, large veins: H. V, representing the zero-line, i. e., atmospheric pressure ; the 
blood-pressure is indicated by the height of the curve. The numbers on the left give the 
pressure, approximately, in mm. of mercury. 
be an empty cavity to fill, or that there must be an aspiratory 
effect toward the ventricle; hence also one factor in the closure 
of the semilunar valves. 
