RHYTHMIC RESPONSE IN PLANT AND ANIMAL 347 



shocks. An automatically moving Desmodium leaflet is also 

 incapable of being thrown into a state of tetanus (fig. 145). 

 Rapidly succeeding shocks do not produce tetanic contraction, 

 though some irregularity may occur in the pulsation ; exces- 

 sively strong shocks kill the plant, and the pulsation is then 

 permanently arrested. 



Theories regarding the causation of heart-beat— 

 Having thus seen how similar are the phenomena of rhyth- 

 micity in cardiac muscle and in plants, we may proceed to 

 inquire into the theories which have been proposed to 

 account for the automatic pulsation of the heart. It has been 

 suggested : 



(1) That discrete impulses are sent out from certain 

 motor nerve-centres in the heart to the muscular tissue, thus 

 causing the periodic heart-beat. Assuming the correctness 

 of this theory, however, the difficulty is merely transferred, 

 for we have still to account for the rhythmic excitation of the 

 nerve. But that the rhythmic heart-beat is not fundament- 

 ally due to rhythmic impulses from nerve-centres, has been 

 proved from facts discovered by various observers : (a) that 

 the isolated ganglion-free apex of the frog's heart may be 

 thrown into rhythmic activity by stimulus ; it has also been 

 shown by Gaskell (b) that the apex of the tortoise-heart, 

 which is free from nerve-cells, is capable of rhythmic move- 

 ments ; and (c) it is found that even in the embryo, before 

 any connection with the central nervous system has been 

 established, there is a rhythmic heart-pulsation. 



(2) That cardiac muscle may have the inherent property 

 of rhythmicity. This explanation, however, by itself, is incom- 

 plete, for it takes no account of the stimulus which must 

 exist, in order to give rise to rhythmic expression. 



(3) That the pulsation of the heart is maintained by some 

 ' inner stimuli,' its rhythmicity being brought about by the 

 long refractory period peculiar to cardiac muscle. 



Independent light, however, may be expected to be 

 thrown on the question of the causation of spontaneous 



