The Heart. 59 



SectioD of the sympathetic reduces the number of beats 

 of the heart, and, as we might expect, stimulation of 

 the cut end causes the heart to beat with increased rapidity. 

 Both the pneumogastric and sympathetic have a central 

 origin in the medulla, but in addition the sympathetic has 

 extensive connection with the spinal cord. We have, there- 

 fore, in the medulla an inhibitory and augmenting centre ; 

 these are both differently affected by the gases brought to 

 them by the blood. CO, in large quantities excites the in- 

 hibitory centre, whilst oxygen stimulates the augmenting 

 centre. The inhibitory or controlling centre is in the con- 

 stant exercise of its function throughout the whole life of 

 the animal ; this is not the case with the augmenting or 

 accelerating centre. 



The inhibitory action of the vagus may be excited by 

 reflex impressions conveyed to the medulla — for instance, 

 fainting through shock or through blows on the abdomen, 

 are instances of impressions conveyed to the medulla 

 exciting the cardio-inhibitory centre. 



The fibres conveying inhibition to the heart from the 

 centre in the medulla are in company with the spinal 

 accessory, for if these be divided, stimulation of the vagi 

 has no action on the heart. 



If the vagus of an animal be stimulated during the ex- 

 periment of graphically recording the blood pressure, it is 

 found that almost immediately after the stimulation the 

 blood pressure falls, the curve being a sudden one ; when 

 the current is shut off, the beats of the heart return, and 

 the pressure rises rapidly until the normal condition is 

 obtained. 



Contrary to what might be imagined, repeated excitation 

 of the vagus, when reaction sets in after withdrawing the 

 current, rather increases than decreases the force of the 

 contraction ; and, on the other hand, repeated stimulation 

 of the sympathetic is followed in the reactionary period by 

 a weakening of the heart's beats. 



Gaskell does not regard either the cardiac ganglia or the 

 nerves supplying the heart as the cause of the beat of the 



