THE CONTROL OF THE CIRCULATION 227 



pine; that is, they stimulate the postganglionic fibers and produce a 

 slowing and possibly an enfeebling of the beat. 



In the mammalian heart a large number of the fibers in the right 

 vagus nerve proceed directly to the sinoauricular node, where it can 

 be shown histologically that considerable masses of nervous tissue exist. 

 On the other hand, the great majority of the fibers in the left vagus 

 proceed to the auriculoventricular bundle, in which also nervous struc- 

 tures are abundant (page 184). As already indicated, the experimental 

 results which follow stimulation of either nerve can be explained by the 

 influence which 'the nerve exerts on the particular structure to which 

 the majority of its fibers proceed. In brief, stimulation of the right 

 vagus is likely to produce slowing and weakening of the beat, whereas 

 stimulation of the left vagus is more likely to institute a condition of 

 partial heart-block. 



On account of the different results which may be obtained by stimu- 

 lating the vagus, some authorities have assumed that the heart must 

 contain four kinds of fiber, more strictly, of vagus nerve endings, one for 

 each kind of influence which the vagus can develop. These four influ- 

 ences are, it will be remembered, on the strength, the rate and the 

 propagation of the heartbeat, and the excitability of the cardiac muscle. 

 It is, however, almost certainly unnecessary to make such an assump- 

 tion, for the results can be explained as merely dependent upon dif- 

 ferent degrees of stimulation of the same kind of fiber and upon the 

 exact part of the heart to which the fiber runs. Sometimes, for ex- 

 ample, when the right vagus nerve is stimulated very feebly, there may 

 be only a diminution in the force of the beats -without any change in 

 their rate, indicating that the effect has been upon the musculature of 

 the auricular walls and not on the sinoauricular node. If the stimulus 

 is increased a little, then both an enfeebling and a slowing of beat occur, 

 indicating that the stimulus has now passed both to the auricular mus- 

 culature directly and to tlie sinoauricular node. 



The Sympathetic Control 



The effect of the sympathetic nerve on the heart may be described as 

 being exactly opposite to that of the vagus. The pathway along which 

 the fibers of this nerve travel to the heart is more or less a devious one. 

 They arise in the mammal from nerve cells in the gray matter in the 

 upper thoracic portion of the spinal cord. The fibers leave by the cor- 

 responding spinal roots and pass by the white rami communicantes into 

 the sympathetic chain, up w r hich they travel to the stellate and inferior 

 cervical ganglia. Around the nerve cells of the stellate ganglion the 

 fibers end by synapsis, and the axons of the cells are then continued on 



