980 PHYSIOLOGY 



part, however, being played by the splanchnic area. This nerve is probably 

 brought into action whenever the pressure in the aorta is so high as to con- 

 stitute a serious check to the expulsive action of the heart. It is stated that 

 under these conditions a current of action may be detected in the trunk of the 

 depressor nerve, and that if both depressor nerves be cut when the aortic 

 pressure is high the blood- pressure rises still higher. It presents a means by 

 which the heart can be relieved of 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. 



FIG. 453. Blood-pressure curve from rabbit showing effect of excitation of central 

 end of depressor nerve (mercurial manometer). (BAYLISS.) 



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 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, ' Mafey'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 



