224 THE CIRCULATION OF THE BLOOD 



Several possibilities have to be kept in mind when we attempt to 

 determine the exciting cause for an observed change in the heart rate in 

 man. Thus, a slowing of the rate may be due to mechanical stimulation 

 of the vagus trunk, as in pressure on the nerves by a tumor or aneurism 

 in the neck. That such mechanical irritation may stimulate the vagus 

 is easily demonstrated in many individuals by applying pressure to the 

 vagus where it lies in the neck in front of the sixth cervical vertebra. 

 Such pressure sometimes produces so profound an inhibition of the heart 

 that temporary loss of consciousness occurs. It is often an unsafe ex- 

 periment to perform. 



Change in the heart rate in man may be caused by direct stimulation 

 of the vagus center, as by the presence of a tumor or a blood clot in the 

 medulla, or by the action on the center of some unusual hormone in the 

 blood. A general increase in intracranial pressure also stimulates the 

 vagus center. The slowing of the heart which occurs in asphyxia might 

 be due either to the action of hormones (hydrogen-ion concentration) 

 in the blood as the result of the asphyxia, or to the increased intra- 

 cranial pressure. That the latter is the more important cause is shown 

 by the fact that, if the rise in blood pressure is prevented by connecting 

 an artery with a mercury valve, that is, with a tube dipping into a 

 cylinder of mercury to a depth corresponding to the normal blood 

 pressure, so that when the pressure tends to rise the blood escapes, 

 the slowing of the heart is not observed. The excitability of the afferent 

 vagus fibers in the lungs is greatly increased during the earlier stage 

 of chloroform administration. 



Finally it should be pointed out that, although we have no voluntary 

 control of the activity of the vagus center, its activities are subject 

 to great variation as a result of impulses transmitted from centers higher 

 up in the cerebrospinal axis. It is by the operation on the vagus center of 

 such impulses that changes in heart rate occur during emotional ex- 

 citement, fright, etc. The increased heart rate in muscular exercise is 

 probably dependent upon a number of causes, such as the irradiation of 

 the motor impulses on to the cardiac centers (see page 412), the rise in 

 temperature and changes in the hydrogen-ion concentration of the blood, 

 etc. 



Mechanism of Action of Vagus on the Heart. Physiologists have nat- 

 urally been curious as to the exact manner in which the vagus nerve 

 brings about inhibition of heart action. Similar inhibition as a result 

 of stimulation of efferent nerves exists in the case of the dilator fibers 

 to the blood vessels (page 234) and the sympathetic nerve to the intes- 

 tine (page 467). Inhibition of voluntary muecles can be produced only 

 through the central nervous system by stimulation of afferent nerves 



