THE HEART 63 



of the bundle in different hearts has, in a measure at least, to 

 do with the rate of conduction required for that particular heart. 

 In the dog's heart Stassen finds the time taken is '08 to 10 of a 

 second. 



To summarise, the chief points usually adduced in favour 

 of muscular conduction in the vertsbrate and especially in the 

 mammalian heart are : 



(1) The zig-zag experiment of Engelmann and the bridge 

 experiment of Fredericq. 



(2) There is muscular connection between auricle and ventricle. 

 Disturbance of conduction follows cutting or compression of this 

 connection. 



(3) There is no effect upon stimulating or cutting the nerves 

 from auricle to ventricle. 



(4) The conduction of the excitatory wave may occur after the 

 nerves have degenerated. 



(5) The rate of conduction is more in accordance with muscular 

 conduction. 



(6) Conduction takes place from the point of stimulation in all 

 directions. 



(7) Reverse conduction occurs from ventricle to auricle. 



The balance of evidence lies in favour of the myogenic theory. 

 Certain objections, however, may be urged against it. For 

 example, the evidence brought from the invertebrate kingdom in 

 its favour is no more applicable than that obtained upon the heart 

 of Limulus. Against the zig-zag experiment may be brought the 

 histological evidence of certain observers. If every fibre is sur- 

 rounded by a nervous network, then that network may manifestly 

 be the conducting medium. As to the rate of conduction, it has 

 been shown by Nikolai and Garten that non-medullated nerves 

 conduct at a rate compatible with that of the excitatory wave of 

 the heart. On the other hand, Bokelmann has found that the non- 

 medullated terminals in the cornea of a dog are not appreciably 

 slower in conduction than ordinary nerve. More recently Bethe 

 has made experiments on the warm-blooded heart, and concludes 

 that the rate of conduction of nerve in the dog is 130 to 225 cm. 

 per second a result quite in accordance with that required for this 

 heart. Bethe also states that it is difficult to induce degeneration 

 of nerve in the manner usually employed, namely, by placing a 

 ligature round the heart. He himself has been surprised to hnd 



