58 PRACTICAL PHYSIOLOGY 



cooling the sinus and warming the ventricle the sequence of the heart 

 can be reversed, for the excitability of the ventricle is by these means 

 raised, while that of the sinus is lowered. 



By gently clamping a strip of tortoise auricle muscle between two 

 little bits of cork an artificial block can be created, and the piece of 

 auricle below the clamp then beats in sequence to the piece above the 

 clamp. The natural delay in conductivity at the auriculo-ventricular 

 junction is thus imitated (Gaskell). The conductivity is decreased by 

 the clamp (see Advanced Coursejtpage 346). 



The nerve cells of the heart are placed in the least differentiated 

 parts : in the sinus (Remak's ganglion), in the inter-auricular septum 

 (v. Bezold's ganglion), and in the auriculo-ventricular groove 

 (Bidder's ganglion). The nerve cells are the cell stations of the 

 vagus nerve. The nervous system regulates, but does not initiate 

 either the rhythm or sequence of the heart. The maintenance of the 

 rhythm depends on the blood, and there is evidence to show that 

 it especially depends on the oxygen, and on the mineral salts which are 

 in solution in the blood. 



The chief mineral salts, chlorides and phosphates of sodium, potassium, 

 and calcium, are dissolved in the blood in minute traces, and are in a 

 state of ionisation. The presence of these ions seems to be absolutely 

 necessary for the production of the excitatory state. During diastole 

 the production of synthetic compounds is, it is supposed, pushed beyond 

 the limit of stability until there results in systole an explosive liberation 

 of energy. As the mineral salts in the serum, with a due supply of 

 oxygen and water, are sufficient to maintain the frog's heart in rhythmic 

 activity for hours, it is clear that the heart muscle contains a large 

 supply of explosive material in its sarcoplasm. 



CHAPTER XIII. 

 METHODS OF RECORDING THE HEART. 



The Suspension Method of Recording the Heart-beat. The frog is 

 placed on a cork plate which is fixed to the stand beneath the crank 

 myograph lever. A fine pin is bent into the shape of a hook and passed 

 through the tip of the apex of the ventricle. A thread is attached to 

 this hook and to the lever. A sufficient weight is hung over the pulley 

 on the axis of the lever so as to slightly stretch the heart. The tissues 

 round the base of the heart are pinned down to the cork plate on which 

 the frog rests. 



