230 THE CIRCULATION OF THE BLOOD 



(page 915). It is not the nerve fibers themselves that are responsible 

 for the inhibitory effect, for it has been found that if the peripheral 

 end of a cut vagus nerve is connected with the central end of one of 

 the anterior roots of the cervical portion of the spinal cord, the axons 

 of the latter when they grow down into the vagus trunk during the 

 regeneration which follows, stimulation of the regenerated fibers will 

 still produce inhibition of the heart. The nature of the fibers can not 

 therefore be the factor upon which the inhibiting influence of the vagus 

 is dependent. This leaves the terminal apparatus of the vagus fibers in 

 the heart as the structures in which the stimulus conveyed to them is 

 rendered inhibitory in nature. 



There has been considerable speculation as to what kind of change 

 must be occurring in the heart in order to cause the inhibition, but 

 practically nothing that is definite is known. One significant fact, how- 

 ever, is that the electrical current led off through nonpolarizable elec- 

 trodes from two portions of the auricle one of which is injured, does not 

 take the same direction when the vagus nerve is stimulated as that which 

 it takes when the motor nerve of a similarly observed muscle is stimu- 

 lated. A positive, instead of a negative variation is observed. Now, 

 since a negative variation is always accompanied by active chemical 

 breakdown changes occurring in the muscle to supply its energy of 

 contraction, it is assumed that the positive variation accompanying stim- 

 ulation of the vagus must indicate that, instead of a katabolic process, 

 a building up, or anabolic process, is being excited. This conclusion 

 would fit in perfectly with the well-known fact that, after the heart has 

 been held in standstill for some time by vagus stimulation, the beats are 

 stronger after the inhibition has passed off than they were before. The 

 vagus seems to have a conserving influence on the heart. During the 

 inhibition produced by it energy material is apparently stored up in the 

 heart, so that when the beat is reestablished it is stronger than before. 



The Manner of Termination of the Vagus Fibers in the Heart. This 

 subject is of considerable pharmacological and therefore therapeutic in- 

 terest. In approaching the problem it must be remembered that the 

 vagus fibers belong to the so-called bulbar autonomic system of nerves 

 (see page 894). They are therefore fibers which have cell stations situ- 

 ated near their peripheral termination cell stations, that is to say, in 

 which ganglionic medullated fibers, by forming synapses around nerve 

 cells, become connected with postganglionic nonmedullated fibers. The 

 existence of ganglia in the heart, particularly of the frog, has been 

 known for a long time. These ganglia are located at the sinoauricular 

 junction, at the interauricular septum, and in the ventricle near the 



