CIRCULATION OF THE BLOOD. 



549 



The frequency of the pulse in man and animals depends upon the 

 age, sex. state of nutrition, size, and various other conditions. 



In man the normal rate of the pulsations of the heart is about 72 

 per minute ; in the female, about 80, though great variations may be met 

 with in perfectly normal individuals. Up to fifty years of age the rate 

 of the pulse is in inverse ratio to the age, as is shown in the following 



table : 



Beats per Minute. 

 Newly born infant, 

 One year, . 

 Two years, 

 Three " 

 Four " 

 Five " 

 Ten '" 



The pulse-rate is increased by muscular exercise in creating a 

 greater demand for arterial blood, by increasing blood pressure, by in- 

 creased temperature (fever), in digestion, by various mental disturbances, 

 and in extreme debility. It is more frequent in the erect than in the 

 recumbent position, and varies inversely with the barometric pressure. 



FIG. 231. TRACING OBTAINED FROM THE FROG'S HEART ON STIMULATION 

 OF THE PNEUMOGASTRIC NERVE. (Foster.) 



The current entered the nerve at a, and was shut off at b. The latent period is indicated by the fact 

 that one pulsation occurred after the stimulation entered the nerve. It is further evident that the inhi- 

 bition persisted some time after the stimulation ceased. 



In addition to the intrinsic nervous system of the heart, the pulsa- 

 tions of this organ are also governed by impulses coming from the 

 central nervous system. These are of two different kinds, inhibitory 

 and accelerating. 



The Inhibitory Nerves of the Heart. It was discovered in 1848 

 that stimulation of the pneumogastric nerve or of its divided periph- 

 eral extremity had the effect, in the dog, of retarding the pulsations 

 of the heart, and, when the stimulation was sufficiently strong, of entirely 

 arresting the heart in diastole. This effect is not produced immedi- 

 ately on the application of the electric current to the nerve, but is pre- 

 ceded by a latent period amounting to about one-tenth of a second ; so, 

 also, the effect lasts a certain time after the shutting off of the current, 

 if the application has not been too prolonged (Fig. 231). On the other 



