THE HEARt 49 



the heart from base to apex is provided for by the A.V. bundle 

 of conducting muscular tissue described at p. 12, which links 

 up auricle and ventricle, while the base of the heart is provided 

 with muscle more pronouncedly automatic than that found in 

 the ventricles, which thus insures the normal sequence of events 

 from base to apex. 



Within the right auricle, below and to the right of the coronary 

 sinus, lies the auriculo-ventricular node (Fig. 12), the com- 

 mencing portion of the A.V. bundle ; if this connection be cut 

 or compressed, disturbance of conduction follows known as heart- 

 block. 



In a work of this kind it is not necessary to enter deeply into 

 vexed questions such as the one we are here considering, but 

 two more points in favour of the muscular theory of contraction 

 may be briefly mentioned. 



The normal direction of the wave of contraction of the heart 

 muscle from auricle to ventricle may, under certain conditions, 

 be reversed — viz., from ventricle to auricle. For example, if a 

 ligature be passed around the heart of the frog between the 

 sinus and the auricle (Stannius's experiment), the auricle and 

 ventricle cease to beat ; if now the ventricle be stimulated to 

 contract, the auricle follows. On the theory of muscle con- 

 duction this experiment can be explained, but not on that of 

 nerve conduction. 



The balance of evidence is in favour of Gaskell's myogenic 

 path as the conducting medium of automatic action, but this 

 still leaves the question of the nature of heart automaticity 

 untouched. 



Heart Automaticity. — A heart, even one which has been 

 apparently dead for some time, may be revived by placing it in 

 an atmosphere of oxygen and transfusing through its vessels a 

 solution containing sodium, calcium, and potassium chloride, 

 sodium carbonate, and grape-sugar. It was Ringer who was the 

 first to show that a fluid containing sodium, potassium, and 

 calcium chloride of a definite strength would keep the frog's 

 heart beating for days ; but it is only recently that the mam- 

 malian heart, and one even that has been apparently dead some 

 days, has been shown to be capable of resuscitation. Sodium, 

 potassium, and calcium chloride are not only required of a 

 definite strength, but practically no substitute is efficient. The 

 dextrose and sodium carbonate are added to increase the effec- 

 tiveness of the work, but they are not essential ; on a diet of 

 inorganic salts the heart is capable of beating rhythmically for 

 days. 



We have at p. 6 referred to the action of a 0*9 per cent, 

 solution of sodium chloride as a physiological solution. It is 



4 



