238 PHYSIOLOGY OF THE HEART [CH. XXI. 



death ; the heart, deprived of its normal blood-supply, beats irregu- 

 larly, goes into fibrillary twitchings, and then ceases to contract 

 altogether. 



In fatty degeneration of the heart in man, sudden death is by 

 no means infrequent. This is in many cases due to a growth in 

 thickness of the walls of the coronary arteries called atheroma, which 

 progresses until the lumen of these arteries is obliterated, and the 

 man dies almost as if they had been ligatured. 



Self-steering Action of the Heart. This expression was originated by Briicke. 

 He supposed that the serailunar valves closed the orifices of the coronary arteries 

 during the systole of the heart. Unlike all the other arteries of the body, the 

 coronary arteries would therefore fill only during diastole, and this increased fulness 

 of the vessels in the heart walls during diastole would assist the ventricle to dilate. 

 This, however, is incorrect ; the valves do not cover the mouths of the arteries ; and 

 at the beginning of systole the velocity and pressure in the coronary arteries 

 increase ; but later on during systole the ventricular wall is so strongly contracted 

 that the muscular tension becomes greater than the coronary pressure, and so the 

 coronary arteries and their branches are compressed, and the blood driven back 

 into the aorta; the coronary arteries are then again filled with the commencing 

 diastole. Self-steering action of the heart therefore exists, but it is brought about in 

 a different way from what Briicke supposed. 



Cardiographs. 



A cardiograph is an instrument for obtaining a graphic record 

 of the heart's movements. In animals the heart may be exposed, 

 and levers connected to its various parts may be employed to 

 write on a revolving blackened surface. 



A simple instrument for the frog's heart is the following : 



\F 



FIG. 215. Simple Cardiograph for frog's heart. 



The sternum of the frog having been removed, the pericardium 

 opened, and the fraenum (a small band from the back of the heart 

 to the pericardium) divided, the heart is pulled through the open- 

 ing, a minute hook placed in its apex, and this is fixed by a silk 

 thread to a lever pivoted at F as in the figure. The cardiac wave 

 of contraction starts at the sinus, this is followed by the auricular 

 systole, and that by the ventricular systole and pause. This is 



