222 THE CIRCULATION OF THE BLOOD 



increases when the continuity of the vagus nerve is broken either by 

 cutting or by freezing a portion of nerve (Fig. 26). The effect is usually 

 inconspicuous when one nerve only is cut, but in most mammals it be- 

 comes quite marked when both are cut. Change in the heart rate pro- 

 duced by muscular effort is much more gradual in animals with marked 

 vagus tone, such as hunting dogs, than in those with little vagus tone, as in 

 domestic rabbits. The degree of vagus tone therefore bears a relation- 

 ship to the staying power of the animal for prolonged muscular effort. 

 It is usually ill developed in cold-blooded animals. It is quite marked 

 in the case of man, as is evident on observing the heartbeat before and 

 after giving a sufficient dose of atropine to paralyze the termination of 

 the vagus in the heart. 



The exaet location of the nerve cells that form the center of discharg- 

 ing impulses along the vagus fibers to the heart can not be made out 

 with certainty, but they are no doubt part of the great motor nucleus 

 (ambiguus), from which arise the fibers not only of the vagus but of 

 the glossopharyngeal nerve. The tone of this vagus center is almost 

 without doubt dependent upon the constant transmission to it along the 

 sensory or afferent fibers of impulses coming from various portions of 

 the body. According to the strength or number of these impulses, the 

 tone may be increased or diminished, thus altering the rate of the heart. 

 It is possible of course that the tone can be maintained, independently 

 of the afferent impulses, by the action on the center of chemical meta- 

 bolic products or hormones produced in the cells or carried to them in 

 the blood. We know at least that, like the respiratory center, that of 

 the vagus is excitable by such hormones as the hydrogen-ion concen- 

 tration of the blood. The tonicity of the vagus center is, however, mainly 

 dependent upon the passage to it of afferent impulses, and as evidence 

 for this conclusion may be cited the observation that, after section of 

 most of the afferent nerves to the medulla (as by cutting the spinal cord 

 high up in the cervical region), subsequent section of the two vagi does 

 not produce anything like the usual degree of change in the heart rate. 



The Afferent Vagus Impulses. The afferent vagus impulses may come 

 from practically any part of the body, having been first discovered by 

 the simple experiment of tapping the abdomen of the frog with the han- 

 dle of a scalpel, when slowing of the heart rate is observed. Cutting the 

 vagi abolishes the reflex. Similar cardiac inhibition is produced by me- 

 chanical stimulation of the tail or gills of an eel. In mammals stimula- 

 tion of the central end of any sensory nerve usually slows the heart, 

 though sometimes the opposite effect occurs. The pulmonary branches 

 of the vagus are particularly sensitive in producing reflex inhibition, 

 and distinct results are usually obtained: by stimulation of the termina- 



