262 TEE HUMAN BODY. 



The Heart-beat is not a Tetanic Contraction. We have 

 seen that it is possible by rapidly succeeding stimuli to throw 

 the skeletal muscles into a prolonged and apparently contin- 

 uous contraction, and that there is good reason, afforded by 

 the phenomena of " secondary tetanus," for the belief that 

 all normal contractions of the voluntary muscles are compound 

 or tetanic contractions. This is not the case with the heart. 

 It is possible by repeated stimuli to hurry the beat of a frog's 

 heart, but not to fuse two or more beats into a single longer 

 uninterrupted contraction. And as regards the normal beat 

 of the heart, experiments as to secondary tetanus prove the 

 same thing. If the heart of an anaesthetized dog or other 

 mammal be carefully laid bare and the nerve of a nerve- 

 muscle preparation be laid on it, we get for each beat a single 

 twitch of the signal muscle, and not a short tetanus lasting 

 as long as the ventricular contraction, such as must arise 

 were this contraction tetanic. 



The Ventricular Contraction is always Maximal. It 

 has been pointed out with reference to the skeletal muscles 

 that within limits the extent of a contraction varies with the 

 stimulus used: a feeble stimulus giving a small contraction, 

 a stronger a greater. This is not the case with cardiac mus- 

 cle. A quiescent ventricle or strip of ventricle taken from 

 the heart of a frog or turtle can often be made to contract by 

 stimulation; but provided the stimulus is powerful enough 

 to cause a beat at all, it always causes the fullest contraction 

 the piece of heart is capable of at the time.f Increase of 

 stimulus causes no increase of contraction. There is good 

 reason to believe that in the physiological working of the 

 ventricles of the mammalian heart each completely expels 

 during its contraction all the blood contained in it: the 

 papillary muscles pulling down the flaps of the auriculo- 

 ventricular valves so that they finally form a cone on which 

 the rest of the ventricular boundaries can fit closely so as to 

 obliterate the cavity they enclose. This being so, the quantity 

 of blood driven into the arteries by each contraction of the 

 ventricles depends on the amount in the latter when their 

 beat commences. This amount depends partly upon the 

 quantity of blood returned from the great veins during the 

 preceding diastole and partly upon the force with which 

 the auricles contract, for they, although each contraction is 

 probably maximal for their 'condition at the time being, do 



