56 THE HEART 



of conduction from auricle to ventricle must be nervous, as he 

 could find no muscular path. Since the discovery of the A-V bundle 

 this piece of negative evidence is of less value ; but the fact that 

 the bundle contains nerve fibres is insisted upon by the upholders 

 of the neurogenic theory. This point we shall deal with when 

 considering the evidence of muscular conduction by the A-V 

 bundle. 



(3) In the heart of Limulus Carlson has shown that section of 

 the nervous strand immediately abolishes the synchronism of the 

 different parts. The parts on either side of the cut continue to 

 pulsate, but with a different rhythm. This points to nervous 

 conduction of the master rhythm. Further, it is interesting to 

 note that in this heart, so long as the nerves are intact, section of 

 the muscle produces no inco-ordination whatever. 



In estimating the value of these experiments of Carlson it 

 must be pointed out that the muscle tissue of this invertebrate 

 heart differs very materially in its properties from that of the 

 mammalian heart, being in fact much more akin to mammalian 

 smooth muscle. It possesses no refractory period, gives sub- 

 maximal contractions, and is capable of tetanisation. 



It was in 1882 that the first serious criticism of the neurogenic 

 theory was advanced. Gaskell came to the conclusion, as the 

 result of his experiments upon the heart of the turtle, that in cold- 

 blooded animals the heart's excitatory wave arises in the heart 

 muscle itself and is conducted by it. He showed that if the 

 ventricle be warmed its beat was not quickened, whereas warm- 

 ing of the sinus caused a quickening of the whole rhythm of the 

 heart. This therefore proved that the stimulus is discontinuous 

 and not continuous, as was thought before. But were the nerve 

 cells concerned in the origin of the excitatory wave ? Gaskell 

 found that if the Stannius ligature be applied to the turtle's heart, 

 the effect at first is essentially the same as with the frog's heart. 

 Soon, however, the auricle starts beating with its own rhythm, 

 slower than that of the sinus, and the ventricle follows it. If 

 now the second ligature be applied, the ventricle stops for a 

 moment, but quickly resumes beating with its own rhythm, which 

 is slower than that of the auricle. Therefore in the same heart it 

 is possible to obtain the sinus, auricle, and ventricle beating with 

 different rhythms. It seemed that in such a case as this it would 

 be strange for nerves to set a different rhythm in different parts 



