286 FEATUEES OF CARDIAC CONTRACTION. [BOOK i. 



cardiac fibres, and gives, in an indirect manner only, the extent of 

 the contraction of the fibres themselves ; and the same is the case 

 with the other methods of recording the movements of the whole 

 ventricle. We may however study in a more direct way the 

 contraction of a few fibres by taking a slip of the ventricle (and for 

 this purpose the tortoise is preferable to the frog) and suspending 

 it to a lever after the fashion of a muscle nerve preparation. 

 We then get a curve of contraction, characterised by a long latent 

 period, a slow long-continued rise, and a slow long-continued 

 fall, a contraction in fact more like that of a plain muscular 

 fibre than of a skeletal muscular fibre. In the tortoise the con- 

 traction is particularly long, the contraction of even the skeletal 

 muscles being long in that animal ; it is less long but still long in 

 the frog, shorter still, but yet long as compared with the skeletal 

 muscles, in "the mammal. 



The beat of the ventricle then is a single but relatively slow 

 prolonged contraction wave sweeping over the peculiar cardiac 

 muscle-cell, passing through the cement substance from cell to cell 

 along the fibre, from fibre to fibre along the bundle and from 

 bundle to bundle over the labyrinth of the ventricular walls. 



Like the case of the skeletal muscle, this single contraction is 

 accompanied by an electric change, a current of action. The 

 intact ventricle at rest is as we have already said ( 66) isoelectric, 

 but each part just as it is about to enter into a state of contraction 

 becomes negative towards the rest. Hence when the electrodes of a 

 galvanometer are placed on two points A, B of the surface of the 

 ventricle a diphasic variation of the galvanometer needle is seen 

 just as a beat, natural or excited, is about to occur. Supposing 

 that the wave of contraction reaches A first, this will become 

 negative towards the rest of the ventricle, including B, but when 

 the wave sometime afterwards reaches B, B will become negative 

 towards the rest of the ventricle, including A. Compare 67. 



The beat of the auricles, that of the sinus venosus, and that of 

 the bulbus arteriosus are similar in their main features to that of 

 the ventricle, so that the whole beat may be considered to be a wave 

 of contraction sweeping through the heart from sinus to bulbus ; but 

 the arrangement of fibres is such that this beat is cut up into 

 sections in such a way that the sinus, the auricles, the ventricle, 

 and the bulbus have each a beat so to speak to themselves. In 

 a normal state of things these several parts of the whole beat 

 follow each other in the sequence Ave have described, but under 

 abnormal conditions the sequence may be reversed, or one section 

 may beat while the others are at rest, or the several sections may 

 beat out of time with each other. 



So far the description of the contraction which is the foundation 

 of the beat differs from that of a skeletal muscle in degree only ; 

 but now comes an important difference. When we stimulate a 

 skeletal muscle with a strong stimulus we get a large contraction, 



