260 PHYSIOLOGY. 



CARDIAC NERVES. 



Engelmann has divided the action of the vagi and accelerator 

 nerves on the heart into four different heads: (1) The inotropic 

 influence, positive or negative, affecting the force of the contraction. 

 (2) The chronotropic influence, positive and negative, affecting the 

 rate of contraction, positive chronotropic actions producing an acceler- 

 ation. (3) Bathmotropic influence, affecting the irritability of the 

 muscular tissue and may be positive or negative. (4) Dromotropic 



SM - 



GP-Y-; 



Fig. 78. Course of Vagus Nerve in Frog. (STIRLING.) 



SM, Submentalis. LU, Lung. V, Vagus. GP, Glosso-pharyngeal. US, Hypo- 

 glossal. L, Laryngeal. PH, SH, OH, OH, Petro-, sterno-, genio-, and omo- 

 hyoid. EG, Hypoglossus. H, Heart. BR, Brachial plexus. 



influence, positive or negative, affecting the conductivity of the tissue. 

 Both cardiac nerves, when irritated, are slow in acting; that is, have 

 a long latent period. Both cardiac nerves can be fatigued, and if in 

 a cold-blooded animal one vagus has been irritated until exhausted, 

 then irritation of the peripheral end is without effect. The vagus, 

 according to Gaskell, is an anabolic nerve, as it retards chemical 

 changes in the cardiac muscle. The accelerator, according to him, is a 

 catabolic nerve, for it increases the chemical changes in the muscle. 



The change of rate and rhythm of the heart in disease may be in 

 the muscle or the nervous supply of the heart. Thus the propagation 



