EXPERIMENTS ON THE HEART. 79 



(6) After every contraction of the heart there is a short period of "diminished 

 excitability" or Marey's "refractory period," during which the heart is less 

 susceptible to further stimulation. 



(7) The non-ganglionic apex of the heart, when it is not stimulated, no longer 

 beats spontaneously, but it responds each time by a single contraction to a single 

 direct stimulus. If, however, a continuous stimulus, e.g., a continuous current of 

 electricity, be applied to it, it executes a series of beats. Such continuous stimuli 

 are obtained through a continuous pressure of fluid, exerted on the interior of the 

 heart or by moistening the heart with chemical substances. 



(8) The auricular centres seem to be more excitable than those of the ventricle ; 

 hence, in a heart left to itself the auricles pulsate longest. 



(9) The heart may be excited (reflexly) from its inner surface. Weak stimuli 

 applied to the inner surface of the heart greatly accelerate the heart's action, the 

 stimulus required being much feebler than that applied to the external surface of 

 the heart. Strong stimuli, which bring the heart to rest, also act more easily when 

 applied to the inner surface than when they are applied to its outer surface. The 

 ventricle is always the first part to be paralysed. 



(10) In order that the heart may continue to contract, it is necessary that it be 

 supplied with a fluid which, in addition to 0, must contain the necessary nutritive 

 materials. The most perfect fluid, of course, is blood. Hence the heart after a 

 time ceases to beat in an indifferent fluid (0-6 per cent, sodium chloride), but its 

 activity may be revived by supplying it with a proper nutritive fluid. 



Cardiac Nutritive Fluids. These nutritive fluids are such as contain serum-albumin, e.g., 

 blood, serum, or lymph. Serum retains its nutritive properties even after it has been subjected 

 to diffusion {Martins and Kronecker). Milk and whey (v. Ott), normal saline solution mixed with 

 blood, albumin, or peptone, and 0'3 per cent, sodium carbonate {Kronecker, Merunowicz and 

 StUnon), a trace of caustic soda (Gaule), or a solution of the salts of serum, are suitable. Alka- 

 line solution of soda revives a feebly beating heart by neutralising the acid formed in the cardiac 

 muscle, or normal saline containing calcic phosphate and potassic chloride {S. Pdnger). 



(11) The independent pulsations of parts of the heart which are devoid of 

 ganglia, show that the presence of ganglia is not absolutely necessary in order to 

 have rhythmical pulsation. Direct stimulation of the heart may cause these move- 

 ments. But the ganglia are more excitable than the heart muscle itself, and they 

 conduct the impulses which lead to the regular alternating action of the various 

 parts of the heart, so that, under normal circumstances, we must assume that the 

 action of the heart is governed by the ganglia. 



(12) If a heart be cut into pieces, so that the individual pieces still remain 

 connected with each other, the regular peristaltic or wave-like movements proceeding 

 from the auricles to the ventricle may continue for a long time (Bonders, 

 Engelmann). If the heart, however, be completely divided into two distinct pieces 

 (auricle and ventricle), the movements of both parts continue, but not in the same 

 sequence they beat at different rates. 



The chief experiments upon "which the above statements are based are as 

 follows : 



I. Experiments by cutting and ligaturing the heart. These experiments have 

 been made chiefly upon the heart of the frog. The ligature experiments are per- 

 formed by tightening and then relaxing a ligature placed around the heart, so that 

 the physiological connection is destroyed, while the anatomical or mechanical con- 

 nections (continuity of the cardiac wall, intact condition of its cavities) still exist. 

 The most important of these experiments are 



(1) Stannius's Experiment. If the sinus venosus of a frog's heart be separated 

 from the auricles, either by an incision or by a ligature, the auricles and ventricle 

 stand still in diastole, whilst the veins and the remainder of the sinus continue to 

 beat (fig. 58, 1). If a second incision be made at the auriculo- ventricular groove,. 



