CIRCULATION OF THE BLOOD. 127 



inhibitory ganglia in the heart-walls, and that the inhibition of the heart's 

 lii'iits bv means of the vagus, is not a simple action, but that it is pro- 

 duced by stimulating centres in the heart itself. These inhibitory centres 

 are paralyzed* by atropin, and then no amount of stimulation of the vagus, 

 or of the heart itself, will produce any effect upon the cardiac beats. 

 "Urari in large doses paralyzes the vagus fibres, but in this case, as the 

 inhibitory action can be produced by direct stimulation of the heart, it is 

 inferred that this drug does not paralyze the ganglia themselves. Mus- 

 carin and pilocarpin appear to produce effects similar to those obtained 

 by stimulating the vagus fibres. 



If a ligature be tightly tied round the heart over the situation of the 

 ganglia between the sinus and the auricles, the heart stops beating. 

 This experiment (Stannius') would seem to stimulate the inhibitory gan- 

 glia, but for the remarkable fact that atropin does not interfere with its 

 success. If the part (the ventricle) below the ligature be cut off, it will 

 begin and continue to beat rhythmically, this may be explained by sup- 

 posing that the stimulus of section induces pulsation in the part which 

 is removed from the influence of the inhibitory ganglia. ' 



So far, the effect of the terminal apparatus of the vagi has been con- 

 sidered; there is, however, reason for believing that the vagi nerves are 

 simply the media of an inhibitory or restraining influence over the action 

 of the heart, which is conveyed through them from a centre in the me- 

 dulla oblongata which is always in operation, and, because of its restrain- 

 ing the heart's action, is called the car dio -inhibitory centre. For, on 

 dividing these nerves, the pulsations of the heart are increased in fre- 

 quency, an effect opposite to that produced by stimulation of their 

 divided (peripheral) ends. The restraining influence of the centre in the 

 medulla may be increased reflexly, producing slowing or stoppage of the 

 heart, through influence passing from it down the vagi. As an example 

 of the latter, the well-known effect on the heart of a violent blow on the 

 epigastrium may be referred to. The stoppage of the heart's action is due 

 to the conveyance of the stimulus by fibres of the sympathetic to the 

 medulla oblongata, and its subsequent reflection through the vagi to the 

 inhibitory ganglia of the heart. It is also believed that the power of the 

 medullary inhibitory centre may be reflexly lessened, producing acceler- 

 ated action of the heart. 



Acceleration of Heart's Action. Through certain fibres of the 

 sympathetic, the heart receives an accelerating influence from the medulla 

 oblongata. These accelerating nerve-fibres, issuing from the spinal cord 

 in the neck, reach the inferior cervical ganglion, and pass thence to the 

 cardiac plexus, and so to the heart. Their function is shown in the 

 quickened pulsation which follows stimulation of the spinal cord, when 

 the latter has been cut off from all connection with the heart, excepting 

 that which is formed by the accelerating filaments from the inferior cer- 



