PNEUMOGASTRIC, OR PAR VAGUM NERVE. 653 



As we should naturally expect from what has already been said, section of the infe- 

 rior laryngeal nerves paralyzes both the vocal and the respiratory movements of the 

 larynx. It is not necessary to refer in detail to the ancient and modern experiments 

 illustrating this point, the former dating from the time of Galen. In adult animals, the 

 cartilages of the larynx are sufficiently rigid to allow of inspiration after the organ has 

 been completely paralyzed ; but, in young animals, the glottis is closed, and suffocation 

 ensues. We have generally observed in cats, that suffocation follows immediately upon 

 section of the recurrents or of the pneumogastrics in the neck. 



The impediment to the entrance of air into the lungs is a sufficient explanation of the 

 increase in the number of the respiratory acts after division of both recurrents. The 

 acceleration of respiration is much greater in young than in adult animals. This does 

 not apply to very young animals, in which section of the recurrents produces almost in- 

 stant death. 



Feeble galvanization of the central ends of the inferior laryngeals, after their division, 

 produces rhythmical movements of deglutition, generally coincident with arrest of the 

 action of the diaphragm. These phenomena are generally observed in rabbits, but they 

 are not constant. The reflex action of these nerves in deglutition is probably dependent 

 upon the communicating filaments which they send to the superior laryngeal nerves. 



Properties and Functions of the Cardiac Nerves, and Influence of the Pneumogastrics 

 upon the Circulation. One of the most interesting questions connected with the physi- 

 ology of the pneumogastric nerves is their action upon the heart ; and the results of 

 experiments, which will be fully detailed hereafter, are precisely the opposite of what 

 would be expected in the case of a nerve containing motor filaments and distributed to 

 a muscular organ. Section of the pneumogastrics in the neck, far from arresting the 

 action of the heart, increases the rapidity of its pulsations ; and galvanization of the 

 nerves arrests the heart's action in diastole. 



"Within the past few years, some very remarkable experiments have been made upon 

 the influence of certain nerves given off near the superior laryngeals, which have been 

 called the depressors of the circulation ; but most observations have been made upon 

 the trunks of the pneumogastrics in the cervical region, as it is exceedingly difficult to 

 isolate the thoracic cardiac branches and to operate upon them without involving other 

 nervous filaments. In galvanizing the nerves in the neck, we have to consider both the 

 direct influence of the current and the phenomena due to reflex action. 



Effects of Section of the Pneumogastrics upon the Circulation. It is not necessary 

 to cite in detail the various experiments upon the effects of section of the pneumogastrics 

 in the neck upon the action of the heart. The division of these nerves in living animals 

 is sufficiently easy, and all who have performed this operation have noted the same re- 

 sults. By section of these nerves, the heart is at once separated from one of the most 

 important of its nervous connections ; and the effects show that, as far as this organ is. 

 concerned, the motor filaments present great differences from the ordinary motor nerves 

 of the cerebro-spinal system. Most of the observations made by dividing the nerves 

 have been upon dogs, and the differences in the effects upon other animals are slight and 

 unimportant. The following are the important phenomena presented in typical experi- 

 ments : 



Section of one of the pneumogastrics in the neck does not produce any very marked 

 effect upon the action of the heart, after the slight disturbance which usually follows the 

 operation has passed away. The number of pulsations is slightly increased, and the car- 

 diac pressure, as shown by a cardiometer fixed in the carotid artery, is slightly dimin- 

 ished ; but this is insignificant as compared with the effects of dividing both nerves. 



Section of both pneumogastrics usually produces immediate and serious disturbance 

 in the respirations, which are momentarily accelerated. The animal usually becomes 

 agitated and suffers from want of air; and, when it is desired especially to note the car- 



