158 THE CIRCULATION OF THE BLOOD AND LYMPH 



while they are readily stopped by stronger stimulation. In other 

 cases the strongest stimulation may not produce complete standstill. 

 Again, the inhibitory effect produced on a heated heart by a given 

 strength of stimulation of the vagus may be greater than that caused 

 in a heart at the ordinary temperature or a cooled heart. This is 

 especially, evident on the auricular tracings when these are recorded 

 separately from those of the ventricle. Even on the verge of heat 

 standstill of the heart inhibition is easily obtained (Fig 71). Some 

 writers have assumed that the different inhibitory effects produced 

 by the vagus are due to th^existence in it of separate groups of fibres, 

 some affecting only the rate of the contraction, others its strength, 



Fig. 70. Tracing from Frog's Heart. A, auricular, V, ventricular tracing. Sinus 

 stimulated (primary coil 70 mm. from secondary). Heart at temperature 

 11-2 C. Complete standstill. The time-tracing between the curves marks 

 intervals of two seconds. 



others still the conductivity of the muscular tissue and its excita- 

 bility. This theory has enriched the vocabulary of physiology 

 with a number of sonorous terms derived from the Greek, but has 

 not otherwise established itself, although it has been useful in 

 emphasizing the fact that the inhibitory nerves can influence the 

 heart-beat in several distinct ways. 



But there are other points of importance to be noted in regard to 

 this inhibition : (i) It does not begin for a little time after stimulation 

 has begun. In other words, there is a distinct latent period; and 

 the length of this latent period is related to the phase of the heart's 



