428 VETERINARY PHYSIOLOGY 



to the galvanometer, it is possible to get an electrocardiogram, 

 the left foot being the pole connected with the apex, the 

 right hand that connected with the base. Such tracings 

 not only throw important light upon the nature and course 

 of contraction, but have proved of considerable im- 

 portance in the diagnosis of abnormal conditions of the 

 heart. The string galvanometer is generally used for this 

 purpose (p. 214). 



The base of the heart shows first (a) an electro-positive 

 phase (fig. 173) due to auricular contraction. This may 

 be followed by (6) an electro-negative variation just at the 

 beginning of ventricular contraction, which has been ascribed 

 to the early contraction of the papillary muscles at the apex, 

 making that part of the heart electro-positive to the base, 

 (c) This is immediately followed by the most marked electro- 

 positive variation, due to the contraction of the ventricle 

 starting at the base, (d) At the very end of ventricular 

 contraction another small electro-positive variation occurs, 

 and this has been ascribed to the contraction ending in the 

 infundibulum of the right ventricle. 



Heart muscle resembles visceral muscle in that the minimum 

 stimulus is also a maximum stimulus — i.e. the smallest 

 stimulus which will make the muscle contract makes it 

 contract to the utmost. This seems to be due to the fact 

 that, while in skeletal muscle a small stimulus calls into play 

 a few fibres a more powerful stimulus calls into play a greater 

 number, whereas in heart muscle all are stimulated at once 

 by the minimum effective stimulus, because of the continuity 

 of the fibres in the syncytial network. The general law of 

 the "all or nothing" in contraction applies to heart muscle. 

 But while this is the case, the strength of stimulus necessary 

 to call forth a contraction varies at different periods of the 

 cardiac cycle as indicated above. 



In cardiac muscle, perhaps more than in any other, a 

 staircase increase in the extent of contraction with a series of 

 stimuli is manifested. 



Tone of Heart Muscle. — As already indicated, tonus is a 



