730 



A MANUAL OF PHYSIOLOGY 



by the action-current of the muscle, and causes its own muscle 

 to contract. A secondary tetanus can be obtained in this way 

 by dropping a nerve on an artificially tetanized muscle. The 

 beat of the heart causes usually only a single secondary con- 

 traction when the sciatic nerve of a frog is allowed to fall on it 

 (p. 188). But when the diphasic variation is well marked, as 

 it is in an uninjured heart, there may be a secondary contraction 

 for each phase i.e., two for each heart-beat. Excitation of one 

 muscle may in the same way cause secondary contraction of 

 another with which it is in close contact. 

 The electromotive phenomena of the heart and of the central 



___ nervous system are natu- 



f\- f\ A rally included under those 

 of muscle and nerve. 



Heart. Records of the 

 electrical changes obtained 

 with the capillary electro- 

 meter from the exposed 

 ventricles vary in certain 

 details with the position of 

 the two contacts. They 

 indicate that in the mam- 

 malian ventricles the rela- 

 tive negativity correspond- 

 ing to the contraction 

 begins at the base, then 

 develops in the region of 



Fir, 280.- ELECTROMETER RECORD FROM the a ? ex > and finall y in ~ 



Ivj 



RABBIT'S HEART (GoTCH). 



The heart was exposed and beating in situ. 

 Contacts, one on base of right ventricle, the 

 other on right apex. The commencement of 

 the beat is on the left-hand edge of the dark 

 line V. The length of the dark line shows the 

 duration of the beat. Upward movement 

 signifies relative negativity (activity) of the 

 part at or near the base contact. Time-trace 

 at top, one-fifth seconds. 



volves the portion of the 

 ventricles near the aorta 

 and pulmonary artery, pos- 

 sibly extending even into 

 the roots of these vessels. 

 When one contact is on 

 the base of the ventricles 

 (in the rabbit) near the 



auriculo- ventricular groove, 



and the other on the apex, the record is quadriphasic i.e., 

 the sign of the electrical disturbance (as shown by the change 

 of direction of movement of the mercury) changes four times 

 (Fig. 280) . For each beat of the ventricle the electrometer record 

 shows (i) a sharp rise, indicating relative negativity (activity) 

 of the base ; (2) an equally sharp fall, indicating relative 

 negativity at the apex ; (3) a slower but marked rise, indi- 

 cating an increase or a fresh development of relative negativity 

 at the base ; (4) a more rapid fall, which returns first slowly, 



