14 BIOLOGY AND SOCIAL PROBLEMS 



a stage in the evolution of neuromuscular 

 mechanisms at which no reflex can occur, the 

 adult organ shows, in certain relations at least, 

 a simple form of true reflex. The heart of the 

 rabbit is invaded by a small nerve which arises 

 as a branch from the vagus and which has 

 been called the depressor nerve of the heart. 

 If this nerve is cut and its peripheral end is 

 stimulated, no result follows, thus showing 

 that the nerve has no motor relations with 

 the heart. If, on the other hand, the central 

 end is stimulated, a general fall in blood pres- 

 sure follows, demonstrating that this nerve 

 is an afferent nerve whose activity induces 

 through the central nervous system an en- 

 largement of the blood vessels of the body. 

 The receptor endings for this nerve are in 

 large part in the heart and are stimulated by 

 each stroke of that organ. When, therefore, 

 the action of the heart is vigorous the effect 

 on the endings of the depressor nerve is such 

 as to call forth a general enlargement of the 

 blood vessels of the body, thus providing the 

 heart, in a simple reflex way, with an ample 

 outlet for the blood. Since this adjustment 

 goes on quite without our knowledge, it is 

 correctly described as a reflex unassociated 



