268 THE CIRCULATION OF THE BLOOD 



(2) whether this bloodflow can be altered by dilatation or constriction 

 of the vessels brought about through nerves. With regard to the pecu- 

 liarities of Hood flow, it may be stated that there are said to be two periods 

 in each cardiac cycle during which an increase takes place in the mass 

 movement of blood in the coronary vessels namely, at the beginning 

 of systole, and again at the beginning of diastole. Nevertheless the 

 pressure pulse has the same contour in the coronary as in the systemic 

 circulation. (W. T. Porter. 22 ) During systole the intramural branches 

 of the coronary artery are compressed and the blood pressed out of 

 them. This emptying of the vessels favors the flow of blood through 

 the heart walls. 



Regarding the presence of coronary vasomotor nerves, there is at pres- 

 ent a certain amount of doubt. When strips of the coronary artery are 

 suspended in a solution of epinephrine, they undergo relaxation instead 

 of contraction. On the assumption that the action of epinephrine on 

 blood vessels is the same as that of stimulation of the vasoconstrictor 

 fibers, this result has been taken as evidence of the absence of such 

 fibers and the possible presence of vasodilator fibers. A somewhat 

 similar type of experiment has been performed by injecting epineph- 

 rine into the fluid used to. perfuse the excised mammalian heart, 

 with the result that, when such injections are made into a heart that 

 is not beating, evidence of vasoconstriction is obtained, whereas when 

 injected into a beating heart, dilatation occurs. This latter result 

 may, however, be owing to the action of the epinephrine in stimulating 

 the cardiac contractions. Other observers, however, deny that the in- 

 jection of epinephrine into the coronary circulation has any influence 

 upon the outflow of the perfusion fluid. Taking the result of these 

 observations as a whole, we may at least conclude that epinephrine 

 does not produce the same marked vasoconstriction that it produces in 

 other blood vessels a fact, which, as already stated, may be taken 

 advantage of in bringing about the rise in coronary pressure that is 

 necessary for successful resuscitation of the heart. 



Attempts to demonstrate the presence of vasomotor fibers by electrical 

 stimulation of the vagus or sympathetic nerve have yielded results which 

 are quite inconclusive, although some observers assert that the vagus 

 nerve carries vasoconstrictor fibers to the coronary vessels, and that 

 the sympathetic carries vasodilator. 



Whatever may be the mechanism involved it is evident that adjust- 

 ment of bloodflow through the coronary arteries in order that this may 

 correspond to the greatly varying activities of the heart must be very 

 close. Evans and Starling, working on the heart-lung preparations 

 (page 163), have shown that changes in coronary bloodflow depend in- 



