958 PHYSIOLOGY 



On account of the refractory period which ensues on the commencement 

 of the contractile process on heart muscle, it is impossible to throw the 

 muscle into a tetanus, since all the stimuli which fall during systole are 

 entirely ineffective. By using very strong stimuli it is possible to intercalate 

 extra contractions before the heart has returned to the base line, i.e. before 

 diastole is complete. So that in this way one may obtain almost a continuous 

 contraction, presenting, however, waves on its summit, which differs from 

 the tetanus of skeletal muscle in the fact that its height is no greater than 

 the height of a single contraction. 



Only when the functional continuity of the heart-muscle is impaired 

 by the * block ' effect of vagal stimulation or the administration of muscarine 

 is it possible to obtain phenomena even superficially analogous to the summa- 

 tion of contractions in skeletal muscle.* 



FACTORS MODIFYING THE ACTIVITY OF CARDIAC MUSCLE 



INFLUENCE OF TENSION AND DISTENSION 



When we examine the behaviour of a heart isolated from the central nervous 

 system and from the rest of the body, as, for instance, in the heart-lung 

 preparation (vide p. 911) we find that it has a marvellous power of adaptation, 

 i.e. of regulating its activity according to the mechanical demands which are 

 made upon it. Thus while we may maintain the venous inflow constant so 

 that the heart is sending out a litre of blood per minute, it makes no difference 

 to the output of the heart whether the average arterial pressure, and there- 

 fore the resistance to the outflow of blood, be maintained at 80 or 160 mm. 

 Hg., although in the latter case the heart must do exactly twice as much 

 work in order to maintain the outflow at the same level. In the same way 

 we may maintain the arterial pressure constant and alter the venous inflow 

 and we find that within very wide limits the heart is able to expel against the 

 arterial resistance the whole of the blood which flows into it from the veins. 

 In this way we can alter the output of a small heart of 50 gms. from 300 to 

 3000 c.c. per minute. As we should expect, this variation in the work 

 done by the heart is associated with corresponding variations in the chemical 

 changes which occur at each heart-beat. Evans has shown that the 

 respiratory exchanges of the heart increase pari passu with the work it is said 

 to do. Careful investigation of the volume and pressure changes of the 

 heart under varying conditions of arterial resistance and venous filling 

 enables us to throw some light on the mechanism of this power of adaptation. 

 Let us take first the changes in volume as recorded by the cardiometer. 

 A heart is contracting 100 times per minute and forcing out at each beat 

 10 c.c. of blood into the aorta against an average pressure of 80 mm. Hg., 

 with systolic and diastolic pressures respectively of 100 and 60 mm. Hg. In 

 order that the left ventricle may force 10 c.c. of blood against this resistance, 



* According to Mines the effect of vagus excitation in enabling the production of 

 summation is due to the shortening of the refractory period which results from vagal 

 stimulation. 



