THE VASCULAR MECHANISM. 22$ 



vagus trunk, to end in the medulla oblongata. If the nerve is 

 divided, stimulation of its central portion (i.e. the end towards the 

 brain) causes a fall of blood pressure and slowing of the heart. The 

 slowing of the heart is due to a reflexly produced increase of vagus 

 tone, and does not occur when the depressor nerve is stimulated after 

 section of the vagi. 



(2) Stimulation of the central end of one vagus usually causes slowing 

 of the heart, provided the other vagus is intact (fig. 85). 



(3) The stimulation of the central end of almost any sensory nerve, 

 which in a conscious animal would give rise to pain, causes reflex 



FIG. 86. Reflex acceleration of the heart and rise of blood pressure 

 caused by stimulation of central end of the (divided) sciatic nerve. 



quickening of the heart (fig. 86), owing mainly to diminution of the tone 

 of the vagus centre and partly to stimulation of the accelerator nerves. 

 Stimulation of the central end of the splanchnic nerves or of the fifth 

 nerve, however, causes slowing of the heart, an -.effect which is some- 

 times seen to follow a severe blow on the abdomen, and which is also 

 readily produced by irritation of the nasal mucous membrane. 



The tonic action of the vagus centre is also increased when the 

 general blood pressure is raised, and the heart is slowed, possibly as the 

 direct effect of the increased blood pressure on the vagus centre. 

 This relationship between the blood pressure and the pulse rate is 

 known as Marey's law, which states that "the pulse rate varies 

 inversely with the blood pressure." Exceptions to this law are observed 

 (1) during muscular exercise and (2) as a result of painful stimuli. 



