I 3 6 A MANUAL OF PHYSIOLOGY 



strength of beat, or by both. When the temperature of the 

 heart is low, increased frequency; when it is high, increased 

 strength, is generally seen during this period of secondary 

 augmentation.* The cause of this secondary augmentation, 

 and of the primary augmentation sometimes seen in fresh 

 preparations and often in hearts that have been long 

 exposed (Fig. 49), excited much speculation before it was 

 known that sympathetic fibres existed in the vagus. There 

 is no longer any doubt that it is due to the stimulation of 

 these accelerator or, as it is better to call them (since mere 

 acceleration is not the only consequence of their stimula- 



FIG. 47. FROG'S HEART. VAGUS STIMULATED. 



Temperature of heart 8 C. , 78 mm. between the coils. Diminution in force of auricle 

 and ventricle, but not complete standstill. Time tracing shows two-second intervals. 



tion), augmentor fibres in the mixed nerve. For (i) excita- 

 tion of the roots of the vagus proper within the skull, and 

 therefore above the junction of the sympathetic fibres, 

 causes no secondary augmentation, or very little, and the 

 inhibition lasts far longer than when the mixed trunk is 

 stimulated. 



(2) Excitation of the upper or cephalic end of the sym- 

 pathetic cord before it has joined the vagus causes, after a 



* Augmentation is termed 'secondary' when it is preceded by inhibi- 

 tion, 'primary' when it is not so preceded. 



