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 con- 

 nections 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 only contracts 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 cavse, 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 three vense 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 aortae so as to flow chiefly towards the head. 



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

 mammalian heart. They are spindle-shaped, and only dimly cross-striated. The 

 cross-striation becomes more distinctly marked as we proceed from sinus to 



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