22O 



ESSENTIALS OF PHYSIOLOGY. 



The Vagus. When a weak stimulus is applied to the peripheral 

 portion (i.e. the end towards the heart) of the divided vago- sympathetic 

 nerve in the frog, the heart immediately beats more slowly : a stronger 

 stimulus brings the heart to a standstill. When the stimulus ceases, the 

 heart begins to beat again, at first feebly, but soon more strongly than 

 before the stimulus was applied. This effect of the vagus is known as 

 " inhibition," since the nerve, when stimulated, checks or inhibits the 

 normal rhythm of the heart. 



In the mammal, stimulation of the peripheral end of the vagus 

 produces the same effect, and, if the blood pressure is being recorded, 



Vagus. 



scapulae. 



Sympathetic. - 



FIG. 82. Origin of the nerves to the frog's heart. 

 1, 2, 3 and 4 are spinal nerves. 



the tracing shows a marked fall of pressure (fig. 83). When the 

 stimulus is removed, the heart begins to beat again, and the blood 

 pressure returns to, or even rises above, its original level. Sometimes, 

 especially if the stimulus is prolonged, the ventricle may again begin 

 to beat slowly even during the stimulation. This phenomenon is known 

 as " vagus escape," and is due to the fact that the ventricle is beginning 

 to beat independently with its own normal slow rhythm. 



In the frog the vagus fibres supply not only the sinus and auricles, 

 but also the ventricle. In many, and probably in all mammals, the 

 vagus fibres are distributed to the auricles, but not to the ventricles ; 

 stimulation of the vagus in this case affects only the auricles directly, 

 and the ventricles stop beating because they no longer receive the 

 normal stimulus from the auricles. As a rule the vagus affects chiefly 

 the sino-auricular node, inhibiting the impulses normally originating at 

 this point, and the whole heart is brought to a standstill. Occasionally 



