ACTION OF HEAT ON THE HEART. 



105 



This is an important fact in connection with any theory of the cardiac 

 beat.] 



(7.) If the bulbus aortse (frog) be ligatured, it still pulsates, provided 

 the internal pressure be moderate. Should it cease to beat, a single 

 stimulus makes it respond by a series of contractions. Increase of 

 temperature to 35C., and increase of the pressure within it increase 

 the number of pulsations (Engelmann). 



(II.) Direct Stimulation of the Heart. All direct cardiac stimuli act 

 more energetically on the inner than on the outer surface of the heart. 

 If strong stimuli are applied for too long a time, the ventricle is the 

 part first paralysed. 



(.) Thermal Stimuli. [Heat affects the number or frequency and the 

 amplitude of the pulsations, as well as the duration of the systole and diastole and 

 the excitability of the heart.] Descartes (1614) observed that heat increased the 

 number of pulsations of an eel's heart. A. v. Humboldt found that when a frog's 

 heart was placed in lukewarm water, the number of beats increased from 12 to 40 

 per minute. As the temperature increases, the number of beats is at first con- 

 siderably increased, but afterwards the beats again become fewer, and if the 

 temperature is raised above a certain limit the heart stands still, the myosin of 

 which its fibres consist is coagulated, and "heat-rigor" occurs. Even before 

 this stage is reached, however, the heart may stand still, the muscular fibres 



Fig. 35. 

 Fig. a, Contractions of a frog's heart at 19C.j b, at 34C. ; c, at 3C. 



