CHAP, iv.] THE VASCULAR MECHANISM. 303 



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

 but each part just as it is entering 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 when a beat, natural or excited, occurs. 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. 

 Attempts have been made, by carefully observing the exact times 

 at which the several parts of the ventricle become negative to 

 determine whether the contraction begins at one part before 

 another, at the base for instance before the apex ; but the results 

 as yet obtained are not decisive. 



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 parts 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 we 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 founda- 

 tion 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, 

 when we apply a weak stimulus we get a small contraction ; within 

 certain limits (see 79) the contraction is proportional to the 

 stimulus. This is not the case with the quiescent ventricle or 

 heart. When we apply a strong induction-shock we get a beat of 

 a certain strength ; if we now apply a weak shock we get either no 

 beat at all or quite as strong a beat as with a stronger stimulus. 

 That is to say the magnitude of the beat depends on the condition 

 of the ventricle (or heart) and not on the magnitude of the stimulus. 

 If the stimulus can stir the ventricle up to beat at all, the beat is 

 the best which the ventricle can at the time accomplish ; the 

 stimulus produces either its maximum effect or none at all. It 

 would seem as if the stimulus does not produce a contraction in 

 the same way that it does when it is brought to bear on a skeletal 

 muscle, but rather stirs up the heart in such a way as to enable it 

 to execute a spontaneous beat which, without the extra stimulus, 

 it could not bring about. And we have reason to think that the 

 normal beat of the heart within the body is the maximum beat of 



