256 



COMPARATIVE PHYSIOLOGY. 



There is usually a certain order in which the heart recom- 

 mences after inhibition (viz., sinus, auricles, ventricles); but 

 there are variations in this, also, for different animals. It is 

 also a fact that in most of the cold-blooded animals the right 



K. Vagus. 



Heart. 



Brain above MednHa. 



Cardio-inhibitory Cen- 

 ter in Medulla Ob- 

 longata. 



■Afferent Nerve. 



Outlying Area with its 

 Nerves. 



Fia. 316. — Diagram of the inhibitory mechanism of the heart. The arrows indicate 

 in all cases the path the nervous impulses talce. I. Path of afferent impulses 

 from the heart itself. II. Path from parts of the brain above (or anterior to) the 

 vaso-motor center. A similar one might, of course, be mapped out along the 

 spinal cord. III. Path from some peripheral region. The downward arrows in- 

 dicate the course of efferent impulses, which probably usually pass by both vagi. 



vagus is more efficient than the left, owing, we think, not to 

 the nerves themselves so much as to their manner of distribu- 

 tion in the heart — the greater portion of the driving part of the 



