988 PHYSIOLOGY 



nerves without interfering in any way with the normal sequence of contrac- 

 tions. Moreover the pause observed between the contractions of auricles 

 and ventricles has been shown by Gaskell to be due to the retardation of the 

 excitatory wave which occurs in its propagation through the muscular 

 tissue in the auriculo-ventricular junction. A similar retardation of the 

 wave can be produced at any point either in auricles or ventricle by diminish- 

 ing the conducting muscular tissue to a sufficiently small extent. Thus, 

 if the auricle of the tortoise be divided as -in the diagram (Fig. 434), it will 

 be noticed that the sinus first contracts, then the auricular half As: a 

 distinct pause then occurs while the contractile process is passing over the 

 ' bridge,' and finally Av contracts, followed by the 

 ventricle. The apparent pause between the contrac- 

 tion of the auricles and ventricle is due therefore to 

 a partial ' block ' at the auriculo-ventricular junc- 

 tion. If the block be increased in the experiment 

 just quoted, as, for instance, by allowing the bridge 

 of tissue to dry or by making it still narrower, it may 

 be found that only one out of every two contractions 

 passes across the bridge (Fig. 435), and the slightest 



FIG. 435. Contraction i ncre ase in the resistance to the propagation of the 

 of auricles and ventn- 11111 



cle of tortoise heart, wave may lead to the block becoming complete. On 



iar 6 a roove\7s nt teen moistenin g tne brid g e a g ain ever 7 contraction may 

 . clamped so as to pro- be seen to pass. 



allo^g *a? ev^ B y the methylene-blue method it is possible to 



second contraction to demonstrate a close network of non-medullated fibres 



surrounding all the muscle cells of the heart. It is 



obvious that the experiment just quoted would not 



exclude the possibility of propagation occurring through such a nerve 

 network. The properties of the network would have to differ from those 

 of any of the nerve tissues with which we are acquainted ; whereas we know 

 that under certain circumstances impulses may be transmitted from fibi 

 fibre, even in striated muscle, and such a mode of propagation is the most 

 ol.vious explanation of the phenomena observed in the heart. 



If the auricles be soaked for some time in distilled water, they enter into 

 a condition of what is known as water-rigor (Wasserstarre). In this eon- 

 dit ion they are incapable of contracting, but can still propagate the wave from 

 sinus to ventricle. This experiment has been regarded as a demonstration 

 of the part taken by nerve fibres in the propagation of the wave, but such 

 ;m explanation is not necessary, since a similar condition of water-rigor in a 

 voluntary muscle fibre has Wn shown to allow the passage of an excitatory 

 wave through the affected part to tin* normal portion of the muscle, which 

 then responds by a contraction. 



A series of interesting researches by Carlson on the mechanism of the heart beat in 

 the king-crab Limulus has been thought to throw light on the vexed question of the 



automatism of the vertebrate heart. 



In Limulus the heart forms a segmented tube of ordinary striated muscular fibres. 

 In laiLv .-]>( unens the tube may b<- from 10 to 15 cin. long and 2 to 2 cm. broad. 



