PNEUMOGASTRIC, OR PAR VAGUM NERVE. 65? 



clear and satisfactory, and they afford, perhaps, the only positive explanation we have of 

 reflex action upon the heart. The substance of these observations is briefly as follows : 



In the rabbit, is a nerve, arising by two roots, one coming from the trunk of the 

 pneumogastric and the other from its superior laryngeal branch, passing then toward the 

 carotid artery and taking its course down the neck by the side of the sympathetic as far 

 as the thorax. In the chest, it joins with sympathetic filaments to pass with them to the 

 heart, by little branches between the origin of the aorta and the pulmonary artery. 

 This nerve can be completely isolated in the neck from the sympathetic and the trunk 

 of the pneumogastric. If it be divided in this situation, after the irritation produced by 

 the operation has subsided, very distinct and important modifications in the circulation 

 may be produced by its galvanization. 



In the first place, it was noted in all the experiments, that galvanization of the periph- 

 eral extremities produced no change, either in the number of the pulsations of the 

 heart or in the pressure of blood in the vascular system ; which points to the fact that 

 its action is not direct, but reflex, and that it is due to an impression conveyed to the 

 nerve-centres. 



If the central ends of the nerves be galvanized, the pressure in the arteries dimin- 

 ishes little by little, until it may be reduced to one-half or two-thirds of the pressure 

 before the irritation was applied. This low pressure continues so long as the interrupted 

 current is applied ; but, when the galvanization is arrested, it gradually returns to the 

 normal standard. These phenomena are observed in all the large arterial trunks. The 

 length of time required to produce the greatest diminution in the pressure is somewhat 

 variable, but the experimenters have never seen it reach its minimum before fifteen pul- 

 sations of the heart. 



" The diminution in the pressure is attended with a reduction of the pulse in the 

 instances in which the depressor-nerve only has been divided. The irritated nerve is 

 isolated in a manner so complete that we cannot fear the passage of the exciting current 

 in the trunk of the pneumogastric. The changes in the number of pulsations persist 

 even when the pneumogastric has been excited by the side where the irritation has been 

 applied, from the point where the superior laryngeal is given off to the point where the 

 pneumogastric enters the thoracic cavity. 



" From the foregoing it is evident that the changes taking place in the number of 

 pulsations are due to excitation of the depressor-nerve. If we study attentively the 

 progress of the cardiac pulsations during the excitation, we observe always that the most 

 considerable reduction takes place at the beginning of the experiment ; that is to say, at 

 the moment when the blood-pressure descends from its normal standard to the lowest 

 point. When the pressure is completely depressed, the pulse is accelerated again and 

 even reaches almost completely the numbers presented before the oscillations. When 

 the irritation ceases, after a shorter or longer period, the heart generally beats more 

 rapidly than before the irritation, and this during all the time that is occupied in the 

 return of the pressure to the normal standard. This observation in itself refutes the 

 idea that the diminution in the pressure may depend upon the diminished number of pul- 

 sations. If the reduction in the rate of the pulse produced a diminished pressure, it 

 should be increased when the pulsations of the heart become accelerated. 



" The manner in which the pulse is reduced leads to the supposition that it is due to 

 a reflex action of the pneumogastric. 



" It was easy to verify this last opinion, and we have been able to confirm it by first 

 cutting the pneumogastrics on both sides, and afterward irritating the central end of the 

 depressor-nerve. In this case, the pressure fell to 0*62, 0*55, etc., while the number of 

 pulsations remained the same, or at least oscillated very slightly above and below the 

 number observed before the irritation." 



The above extract from the observations of Cyon shows two important points : 



First, galvanic stimulation of the central extremities of the divided depressor-nerves 

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