io8 COMPARATIVE PHYSIOLOGY 



However, the origin and conduction of the heart-rhythm, 

 in Limulus at all events, is quite different from what is generally 

 accepted to be the case in vertebrates. In Limulus each 

 contraction starts at the posterior end of the heart, and travels 

 forward to the region from which the main arteries have 

 their origin. When the heart is removed from the body, 



12 



11 



10 9 S"7 



Fig. 26. — Heart and nerves of Limulus (Carlson). 

 7-8 Cardiac nerve from brain ; 9-1 1 cardiac nerve from abdominal 



ganglia 



cardiac ganglion ; p.n.c. lateral nerves. 



so that all its connexions with the central nervous system are 

 severed, it still continues to beat with its normal rhythm of 

 about twenty to thirty a minute. From this it might be thought 

 that the heart-rhythm is an inherent property of the cardiac 

 muscle, as in vertebrates. From two lines of experimental 

 evidence, however, Carlson has shown conclusively that this 

 is not the case. He first investigated the effect of stripping 

 off the ganglion referred to above. When the cardiac ganglion 



iliiii__MAiJLLi^ 



Fig. 27.- 



-Inhibition of heart beat of Limulus by electrical stimulation 

 of brain with weak current (Carlson). 



is removed the heart ceases to beat. The normal isolated 

 heart will only beat in plasma or sea water if the ganglion 

 is left intact ; after removal of the ganglion the heart may be 

 made to contract rhythmically in isotonic sodium chloride 

 after immersion for half an hour ; in this respect it agrees, 

 however, with vertebrate striped muscle, which acquires a 

 regular contractile rhythm in the absence of calcium salts. 

 If, in the isolated heart, the ganglion is divided into four 



