THE CIRCULATION. 179 



The Depressor Nerves. 



It was pointed out previously that along with these in- 

 hibitory nerves in the pneumogastric there reach the heart 

 along the same avenue some sensory or afferent nerves. 

 These instead of going to the ganglia are probably distrib- 

 uted between the muscle fibres themselves, and carry to the 

 medulla sensations from the heart. Of course these sensa- 

 tions very seldom reach consciousness and so we are not aware 

 of them, but in the medulla these afferent impulses from 

 the heart affect the centers and bring about important phy- 

 siological results. In a normal heart about the only sensa- 

 tion that might be expected to come from the heart would 

 be the sensation due to too great a pressure of blood in that 

 organ. Such afferent impulses brought by this nerve to the 

 medulla cause immediate dilatation of arteries all over the 

 body, a condition of things which at once serves to reduce 

 the blood pressure. Artificial experiments confirm this as- 

 sertion, for if this nerve be cut and its central end, that is 

 the end connected with the medulla, stimulated, a dilata- 

 tion of the blood vessels over the body occurs and the 

 blood pressure sinks at once. On account of this action 

 this afferent nerve was called the depressor nerve, because 

 the result of its impulses is to depress; that is, to lower the 

 pressure in the arteries. In the rabbit this depressor nerve 

 runs as a separate and distinct nerve to the heart, while in 

 most animals and in man, as already pointed out, it runs 

 along the common pneumogastric trunk. 



Cardio- accelerator or Sympathetic Nerve. 



In addition to these two extrinsic nerves a third one 

 reaches the heart. This is a nerve which increases the rate 

 of heart beat and is called the cardio-accelerator. This 

 cardio-accelerator has its origin in the medulla and spinal 

 cord, also, but runs from this through the communicating 

 branches to the sympathetic ganglia lying in the lower 

 cervical and upper dorsal region, and from these ganglia 

 reaches the heart. Passing as it does from these sympa- 



