254 



COMPARATIVE PHYSIOLOGY. 



4. In most animils the sinus venosus and auricles are af- 

 fected before the ventricles, and the vagus may influence these 

 parts when it is powerless, over the ventricle. 



5. After vagus inhibition, the action of the heart is (almost 

 unexceptionally) different, the precise result being variable, but 

 generally the beat is .both accelerated and increased in force. 

 We may say that the werking capacity of the heart is tem- 

 porarily increased. 



6. The improvement in the efficiency of the heart is in pro- 

 portion to its previous working power, and in cases when the 



Sti/mulatinn Vagiis. 



Fig. 215.— Effects of vagus stimulation, illustrated by a form of sphygmographic curve 

 derived from the carotid of a rabbit (Foster). 



action is feeble and irregular (abnormal) it might be said to be 

 in proportion to its needs. This is a very important law that 

 deserves to receive a general recognition. 



7. Section of both vagi nerves results in histological altera- 

 tions in the heart's structure, chiefly fatty degeneration, which 

 must, of course, impair its working capacity and expose it 

 to rupture or other accidents under the frequently recurring 

 strains of life. 



8. In the cold-blooded animals the heart may be kept at a 

 standstill by vagus stimulation till it dies, a period of hours 

 (one case of six hours reported for the sea-turtle). 



9. Certain drugs (as atropine), applied directly to the heart, 

 or injected into the blood, prevent the usual action of the vagus. 



10. During vagus arrest the heart substance undergoes a 

 change, resulting in an unusual dilatation of the organ. This 

 may be witnessed whether the heart contains blood or not. 



