THE CIRCULATION. 247 



heart relaxes. This compression of the coronary vessels drives the 

 blood into the right auricle. The relaxation of the heart-walls does 

 not produce a noteworthy suction in the larger coronary vessels. 



At the beginning of systole the blood rushes into the coronary 

 arteries in the same fashion that it does into other arteries. How- 

 ever, later, during systole, the branches of the coronary arteries are 

 so squeezed by the strong ventricular contractions that the passage 

 of the blood is temporarily obstructed or even made to retrograde. 

 Before the blood can recede to any extent, systole has ended and the 

 blood then flows along as before. 



It has also been found that, during the beginning of a ventricu- 

 lar systole, a cut into the coronary artery of a living animal causes 

 a spurt of blood from the central end of the artery. 



A shortening of the diastolic period lessens the nutritive supply 

 to the heart. Diastolic distention of the left heart by "back pres- 

 sure" lessens the coronary flow. These facts are of much practical 

 import in diseases of the heart. 



Pratt found that the vessels of Thebesius open from the ven- 

 tricles and auricles into a * system of fine branches that communi- 

 cate with the coronary arteries and veins by means of capillaries 

 and with the veins, but not with the arteries, by passages of somewhat 

 large size. He also showed that with the coronary arteries absolutely 

 cut off the mammalian ventricles may be maintained under proper 

 conditions in rhythmic contraction for hours, by the blood through the 

 foramina of Thebesius. 



Ligation of both coronary arteries in the dog is followed by a 

 decreased force and .rate of the heart-beat, and often a series of inde- 

 pendent contractions of the individual muscle-fibers, which are called 

 fibrillary contractions, and finally complete arrest of the heart. Porter 

 found that the whole dog's heart can recover from the fibrillary con- 

 tractions by cooling the ventricles until all trace of fibrillation has 

 disappeared and then bringing the heart back to the normal tempera- 

 ture by circulating warmed defibrinated blood through the coronary 

 vessels. 



In those cases of degeneration where disease of the coronary vessel- 

 walls produces the condition known as atheroma, the symptoms of 

 ligaturing and of sudden death occur because of the sudden arrest of 

 the heart's action. 



Dr. Ida H. Hyde found that the quantity of blood flowing through 

 the heart is diminished by a greater internal pressure and a consequent 

 distension of the heart even when it is beating; 



