NERVES OF THE HEART. CARDIAC MUSCLE. 261 



vencB cavce and the pulmonary veins, and there each normal 

 beat commences, the contraction spreading rapidly over the 

 whole auricle and thence to the ventricle without the brief in- 

 termediate pause observable in the frog. In the mammal, also, 

 the ventricles if supplied with blood from the auricles go on 

 beating although all nerve and muscular continuity between 

 auricle and ventricle has been destroyed, by passing rigid tubes 

 through the auriculo-ventricular openings and then tying a 

 ligature tight on the outside of the heart along the auriculo- 

 ventricular groove, so as to crush the tissues between the 

 string and the tubes. If the ligatures be so placed as not to 

 impede the flow in the coronary vessels the ventricles beat 

 long and powerfully, but with a rhythm independent of that 

 of the auricles and usually slower. Also when the mammalian 

 heart is dying slowly, as in a suffocated animal, the auricles 

 usually continue to beat after the ventricle has ceased, the 

 small dog's-ear-shaped projection of the auricles (which it 

 may be noted has given its name to the whole auricle) usually 

 being the last portion to come to rest, especially that on the 

 right side, which was accordingly named ultima moriens by 

 the old physiologists. On the whole we are perhaps justified 

 in assuming that the myocardium of the mammal is automatic, 

 like that of the frog, and that in it also the presence and 

 influence of ganglion-cells favor the production of a beat, 

 but do not initiate it. 



The muscle of the frog's heart is, we have seen, co-ordi- 

 native: the isolated ventricular apex can perform a regular 

 beat. It is probable that this is not the case in the mammal. 

 When a dog's heart is injured the ventricles sometimes cease 

 to give true beats though the muscle bundles constituting 

 them go on contracting, but it is with no combined action 

 such as would empty the ventricle. Irregular and useless 

 contractions travel simultaneously over the myocardium in 

 various directions, so that the whole mass seems trembling. 

 Such a state (known as "fibrillar contraction") is especially 

 apt to follow wounds in the region of the main nerve-trunks 

 running down the ventricles alongside the larger branches of 

 the coronary arteries, and is probably due to the injury of some 

 nervous apparatus concerned in securing the proper co-ordi- 

 nated contractions of the normal beat. In many other 

 regions wounds may be inflicted on the ventricle with con- 

 siderable impunity. 



