THE NERVOUS REGULATION OF THE HEART 1023 



is due to universal dilatation of the blood vessels of the body, the greater 

 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 there- 

 fore 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. 462. 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. 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. 



There are two very important reflex mechanisms associated with the 

 heart itself. If the arterial pressure be raised, either by stimulation of the 

 splanchnic nerves or by obstruction of the aorta, the heart is slowed. In 

 this slowing, which is effected through the vagus, two factors are concerned. 

 In the first place any rise of arterial pressure within the skull raises the intra- 

 cranial pressure and excites the vagus centre directly. In the second place 

 impulses starting in the root of the aorta and in the left ventricle travel to 

 the central nervous system chiefly by way of the depressor fibres and cause 

 a reflex slowing of the heart. According to ' Marey's law ' the pulse rate 



