SECTION VIII 

 THE CAUSATION OF THE HEART BEAT 



IF the heart be cut out of the body of a cold-blooded animal, such as the 

 frog or tortoise, it will continue to beat with the normal sequence of its 

 different chambers for hours, or even days, provided that it be kept cool and 

 moist. In the case of a warm-blooded animal, the heart is similarly capable 

 of continuing its rhythmic contractions for some little time after excision. 

 The period of survival of the heart is less in warm-blooded than in cold- 

 blooded animals. The fact that in both cases the heart will continue to beat 

 after removal from all its connections with the central nervous system, and 

 when blood is no longer flowing .through it, shows that the causation of the 

 heart beat is to be sought in the walls of the heart itself. 



The heart wall consists of a muscular tissue resembling in many respects 

 voluntary muscle ; like this, it presents longitudinal and transverse striations ; 

 like this, it is capable of contracting in response to direct stimulation. 

 Normally voluntary muscle contracts only in response to impulses from the 

 central nervous system. When Remak described the existence of collections 

 of ganglion cells in the sinus venosus, it was natural that physiologists should 

 ascribe to these collections of nerve cells the same automatic rhythmic 

 functions that had been found by Flourens and others to be associated with 

 the grey matter of the medulla oblongata in connection with the maintenance 

 of the respiratory movements. 



ANATOMY OF THE FROG'S HEART 



The hearts of the frog and of the tortoise have figured so largely in the researches on 

 the causation of the heart beat that it may be profitable to mention briefly the main 

 points of their anatomy. 



The frog's heart consists of the' sinus venosus, which receives the anterior and 

 posterior venae cavae, two auricles, one ventricle, and the bulbus arteriosus, which opens 

 into the two aortae. The venous blood from the body flows into the sinus venosus by 

 the thn-c venae cavae, and thence into the right auricle, while the left auricle receives 

 the blood from the lungs. The ventricle thus receives mixed arterial and venous 

 blood, the arterial blood being directed by the spiral valve of the bulbus aorta 1 so 

 as to flow chiefly towards the head. 



The muscular fibres of the heart are less highly developed than those of the mamma- 

 lian heart. They a i v spi n< lie-shaped, and only dimly cross-striated. The cross -stri at ion 

 becomes more distinctly marked as we proceed from sinus to ventricle, the sinus muscle 

 fibre representing the most primitive condition. There is complete muscular con- 

 tinuity between nil the cavities of i lie heart. The eireiilar ring of muscle at the junction 

 of sinus with auricles and of auricles wit h ventricles presents only slight traces of cross- 

 si riation (Gaskell). 



The heart is well supplied \vith nerve fibres and ganglion cells. The two vagi enter 



