1098 PHYSIOLOGY 



a load too great for its powers, and therefore dangerous to its future 

 welfare. In many animals the depressor fibres are bound up with 

 the trunk of the vagus and cannot be excited separately. 



Stimulation of the central end of the vagus generally causes reflex 

 slowing of the heart through the cardiac centre and the other vagus. 

 In this reflex inhibition the chief fibres stimulated are those coming 



O 



from the lungs (Brodie). Inflation of the lungs causes acceleration of 

 the heart whether due to diminution of the tonic action of the vagi, 

 or to reflex excitation of the accelerator nerves, is not known. 

 Most sensory nerves of the body when stimulated give either a slowing 

 or a quickening of the heart. Stimulation of the fifth nerve, as in the 

 nasal mucous membrane, always causes reflex inhibition. 



The rate of the heart-beat in the normal animal is closely connected 

 with the blood pressure. Increase in blood pressure due to a large 

 vaso-constriction is associated as a rule (but not invariably) with a 

 slowing of the heart-beat. In fact, ' Marey's law ' states that the pulse- 

 rate varies inversely as the blood pressure. In this slowing of the heart 

 the vagus nerves are of course active. Whether the blood pressure acts 

 directly on the cardiac centre in the medulla, or reflexly through afferent 

 nerves distributed to the aorta and heart cavities, is not yet fully made 

 out. The reflex slowing of the heart often fails to accompany rise of 

 blood pressure. Thus the rise in blood pressure and the increased 

 filling of the heart associated with muscular exercise are attended by an 

 increased pulse-rate. 



THE PULSE-RATE IN MAN 



The normal pulse-rate in man is about 72 per minute. It is largely 

 influenced by bodily movements. The pulse-rate varies considerably 

 with age. The following Table represents the average pulse-rate in 

 man at different ages : 



Pulse-rate 

 Age in years per minute 



. . 136 



5 .. 88 



10-15 . . 78 



15-60 . . 68-72 



It must be remembered that marked differences in pulse-rate 

 may be found in different individuals without having any patho- 

 logical significance. Thus pulse-rates of 30 per minute and 120 per 

 minute have been observed in men who were otherwise perfectly 

 healthy. The pulse-rate is raised by warmth and diminished by 

 cold applied to the surface of the skin. It is also increased some- 

 what by the taking of food. The act of swallowing causes a reflex 

 quickening of the rate through the vagi. 



