

NERVE MECHANISM OF HEART. 277 



vagus bear impulses of a checking or inhibitory nature to the 

 intrinsic nerves of the heart. 2. That these influences are con- 

 stantly in operation, or in other words, the vagi exert a tonic 

 inhibitory influence on the rapidity of the heart-beat. 3. The 

 tonic action of one vagus bears inhibitory influence sufficient to 

 regulate the heart's action. This tonicity of the vagus inhibition 

 is moreover more marked in man than in dogs and rabbits, and 

 is reduced to a minimum in frogs, where section of the vagi pro- 

 duces very little effect on the rate of the beat. 



Vagus inhibition is increased by the following circumstances: 

 (a) certain psychical phenomena, such as terror, which is said 

 to produce a temporary standstill ; (b) deficiency of arterial blood 

 in the medulla oblongata ; (c) increase of the blood pressure 

 within the cranium; and (d) reflexly by the stimulation of many 

 afferent nerves, particularly the sympathetic and those bearing 

 impulses from the abdominal viscera to the medulla, as well as 

 the ordinary sensory nerves, or by the afferent fibres of the oppo- 

 site vagus. (e~) Muscarine produces disastolic standstill of the 

 heart by exciting the local inhibitory ganglia or vagus terminals. 



Atropin causes quickening of the heart's action by paralyzing 

 the endings of the vagus, and also those intrinsic mechanisms 

 which are supposed to have an inhibitory effect. Nicotine pro- 

 duces at first a slowing of the heart by stimulating the inhibitory 

 tone of the vagus. This is soon followed by exhaustion of the 

 terminal fibres and a consequent quickening of the heart beat. 

 Large doses of curare paralyze the inhibitory fibres. Digitalis 

 excites the vagus centre in the medulla, and thereby reduces the 

 rapidity of the heart's beat. 



THE ACCELERATOR NERVES. 



After the possibility of increase of blood pressure has been re- 

 moved by section of the splanchnic nerves, and the tonic inhibi- 

 tion of the vagi has been cut off", it has been found that stimula- 

 tion of the cervical portion of the spinal cord causes quickening 

 of the heart beat. In the cervical portion of the spinal cord nerve 

 channels must then exist which are capable of stimulating the 

 muscle fibres of the heart, so as to cause it to beat more quickly. 



