THE CIRCULATION OF THE BLOOD. 285 



nary artery, provided it supply a sufficiently large territory, e. g., 

 the arteria circumflexa, is sufficient to cause arrest in at least 80 

 per cent, of animals (Porter). With the ligation of this vessel there 

 occurs a gradual diminution in the force and frequency of the systole, 

 As the power of coordinate contraction ceases the heart-muscle 

 frequently exhibits a series of independent contraction of individual 

 fibers and cells known as nbrillary contraction. All the results which 

 follow ligation are to be attributed in the light of experiment to the 

 sudden anemia which is thus established. The removal of the 

 ligature and the return of the blood will restore the nutrition and re- 

 establish coordinate contractions. The excised heart of the mammal 

 may be again made to beat by passing warm defibrinated blood 

 through the coronary vessels under a suitable pressure. 



In frogs and allied animals the heart is nourished by blood flow- 

 ing, during the diastole, from the interior of the heart into a system 

 of irregular channels which penetrate the walls in all directions. 

 With the systole the blood is returned to the cavities. The excised 

 heart of the mammal e. g., the cat may be partially nourished in 

 a similar manner through the foramina Thebesii. If the warm 

 defibrinated blood of the same animal be introduced into the ventricle 

 under a pressure of about 75 mm. of blood, the heart will recommence 

 and continue to beat for a period varying from one to several hours. 



The Causation of the Heart-beat. The beat of the heart, its 

 frequency and regularity, its continuance from the early stages of 

 fetal development till death, has long been an interesting subject 

 for physiologic investigation. Though related to the functional 

 activities of the body at large, the activity of the heart is in a sense 

 independent of them, for it will continue for a variable length of time 

 after they have ceased. The heart of the frog and the turtle will 

 continue to beat under appropriate conditions for some hours after 

 separation of all its anatomic connections and removal from the body. 

 The heart of the dog or cat will, however, beat but for a few minutes. 

 The human heart would in all probability act in the same way. 



The reason for the longer continuance of the beat of the excised 

 heart of the cold-blooded animal beyond that of the warm-blooded 

 animal lies probably in the difference in the rate of their respective 

 metabolisms. There is reason to believe that each cell of the heart- 

 muscle, in common with other tissue-cells, during life stores up and 

 holds in reserve a larger quantity of nutritive material than is necessary 

 for its immediate needs. When separated from the general blood- 

 supply, the cells at once begin to utilize this reserved material. With 

 its exhaustion the irritability declines and in a short time disappears. 

 As the metabolism is far more rapid in the warm-blooded than in the 

 cold-blooded animal, it is probable that the reserved nutritive material 

 is utilized much more quickly in the former than in the latter. So 



