Upon the Motion of the Mammalian Heart. 



565 



widening circle. The spread in the auricle differs in this respect only, that 

 it is confined to the muscle bands. The wave spreads up the superior cava 

 against the blood stream ; when it reaches the mouths of the inferior cava 

 and of the pulmonary veins, it similarly spreads up them ; it spreads from 

 the base to the apex of each auricular appendage ; it spreads down the 

 septum towards the ventricle. There are no special paths of conduction ; the 

 spread is uniform and from one muscle element to contiguous ones, and 

 involves the two auricles as though they composed a single sheet of muscle.* 

 The spread in the auricle is ordered upon a simple plan. 



Spread from Auricle to Ventricle. 

 Between the contraction of the auricle, and the ventricle there is con- 

 siderable delay ; the delay is very pronounced in the frog and tortoise. It 

 was Gaskell who showed, by placing a clamp upon the groove which 

 separates these two chambers and by gradually tightening it, that precisely 

 the same disturbance of sequence may be induced as when a clamp is 

 applied to an isolated strip of auricular muscle. By clamping, the passage 

 of the wave is at first hindered, later the ventricle fails to respond after 

 occasional auricular contractions, later still as pressure is increased the 

 responses of the ventricle become fewer until there is no response at all. 

 These experiments led Gaskell to the conclusion that the sequence of the 

 ventricular upon the auricular beat can be explained without the interven- 

 tion of any special nervous mechanism, that the passage of the impulse over 

 the groove is of the same nature as its passage over a bridge of tissue in the 

 incised auricle. He showed also that this sequence is not disturbed by 

 removal of Bidder's ganglia. In the frog the junction between auricle and 

 ventricle is formed by a ring of tissue, and composed in the main of muscle 

 fibres of relatively primitive type. This primitive muscle formed, in 

 Gaskell's view, the functional bridge between the two chambers. Following 

 Gaskell's studies came the early experiments of Tigerstedt (23), and of 

 Wooldridge (25) and MacWilliam (19) in this country. Their experiments 

 determined that conduction across the A- V groove in the mammal is subject 

 to the same disturbances as is conduction in the frog, and showed that the 

 impulse received by the ventricle is not the filling with blood by the auricle, 

 and that the impulse is not conveyed through the superficial cardiac nerves. 

 But at this time and for many years afterwards the auricle and ventricle of 

 the mammalian heart were not known to be joined by a bridge of muscle 



* In this conclusion we differ from Eyster and Meek (6), who believe that there is 

 specially rapid conduction to the A- V node. Our criticisms of their methods have been 

 published (16). 



VOL. LXXXIX. — B. 3 B 



