ACTION OF THE HEART. 313 



oblongata contains a cardio-motor centre, the centripetal fibres of which run 

 upwards in the vagus, whilst the efferent or motor fibres descend through the 

 spinal cord and are distributed to the heart. Irritation of the upper cut 

 surface of the vagus he thought excites the centre, which acting in a reflex 

 manner stimulates the heart to beat more energetically, and the blood-pres- 

 sure rises, whilst section of the spinal cord divides the motor filaments pro- 

 ceeding from the ganglion, and the heart beats with less force, and the 

 blood-pressure consequently diminishes. Ludwig and Thiry on repeating v. 

 Bezold's experiments substantiated their accuracy, but demonstrated the in- 

 correctness of his conclusions. They noticed that even when all the fibres 

 pa>sing from the spinal cord to the heart were destroyed by means of a gal- 

 vano-caustic wire, the effects observed by v. Bezold to result from irritation 

 of the cord could still be manifested, and they concluded that the rise in the 

 blood-pressure and augmentation in the frequency of the beats of the heart, 

 were due to constriction of the small vessels in the body generally, and in 

 this conclusion they were supported by the results of ligatures applied to the 

 aorta or larger arteries. Further investigations, the details of which were 

 published in Ludwig's Arbeiten in 1866, enabled them to announce their 

 discovery of the functions of a small nerve given off from the vagus, and 

 to which they applied the name of Nervus Depressor. This nerve arises in 

 the rabbit from the vagus, immediately below the superior laryugeal nerve, 

 from which part it often receives a reinforcing filament ; and descending 

 through the neck it is distributed to the heart. If this nerve be divided 

 and the lower cut surface be irritated, no effects are observed, but if the 

 upper cut surface be irritated its inhibitory action immediately becomes 

 manifest, and the animal gives indications of pain. The direction in which 

 it conducts impulses is therefore exactly the opposite to that of the cardio- 

 iuhibitory nerve of the vagus: it is a sensory or afferent nerve. If the irri- 

 tation be continued, the slowing action on the heart passes off, and the heart 

 beats as fast or even faster than before, and coiucidently the blood-pressure 

 in the vessels falls. The mechanism of the phenomena observed as deduced 

 from their experiments seems to be that when a stimulus is first applied to the 

 depressor nerve, an impulse is conducted straight to the vagal centres, from 

 whence an inhibitory influence is directed through the cardio-inhibitory nerves 

 to the intracardiac inhibitory centre, and the heart is slowed ; but the depres- 

 sor nerve also contains fibres terminating in a vaso-inhibitory centre in the 

 medulla oblongata, which exerts a restraining or inhibitory power over a vaso- 

 motor centre in close relation with it. Under ordinary conditions a nervous 

 influence is discharged from the vaso-motor centre by fibres.which, descending 

 through the spinal cord, emerge with the several roots of the spinal nerves, 

 and are distributed to the smaller vessels throughout the body (one only 

 being shown in the figure). This nervous influence maintains the vessels in 

 a state of permanent tonic contraction, which keeps up the blood-pressure to 

 its normal height. But when the vaso-inhibitory of the vagus (depressor 

 nerve) is stimulated, an impulse is conveyed to the vaso-iuhibitory centre of 

 the medulla oblougata, which reacting on the vaso-motor centre inhibits its 

 action, and the downward current from this through the spinal cord being 

 consequently arrested, the vessels dilate, the blood-pressure sinks to one-half 

 or two-thirds of its normal amount, and the medulla receiving less blood, 

 the vagal inhibitory centres become less active and the heart resumes its ordi- 

 nary activity, or if the dilatation of the vessels and the consequent relief 

 from the retracing power be very considerable, may even beat faster than 

 before. It has been pointed out by Dr. Rutherford, that the inhibitory fibres 

 of the vagus may be excited to action by stimulation applied to: 1. The 

 central end of the vaso-iuhibitory or superior cardiac branch of the vagus 



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