PNEUMOGASTRIC, OR PAR VAGUM NERVE. 659 



the lungs themselves being supplied by the pulmonary branches alone. The sensibility 

 of the mucous membrane of the trachea and bronchi is due to the pneumogastrics, for 

 these parts are insensible to irritation when the nerves have been divided in the neck. 

 Longet has shown that, while an animal coughed and showed signs of pain when the 

 mucous membrane of the respiratory passages was irritated, after division of the pneumo- 

 gastrics there was no evidence of sensibility, even when the tracheal mucous membrane 

 was treated with strong acid, or even cauterized. He also saw the muscular fibres of the 

 small bronchial tubes contract when a galvanic stimulus was applied to the branches of 

 the pneumogastrics. 



The main interest, in this connection, is attached to the pulmonary branches and their 

 relations to the respiratory acts. These are undoubtedly connected with important reflex 

 phenomena, acting as centripetal nerves ; and their direct action in respiration is probably 

 much less important. They are exposed and operated upon in living animals with so 

 much difficulty, that we know little of the direct effects of their irritation and must judge 

 of their general properties chiefly by experiments showing their action upon respiration. 

 Wa shall have to study, in connection with the functions of these nerves, the effects of 

 their division, upon the lungs and the respiratory acts, and the phenomena, referable to 

 the respiratory organs, which follow their galvanization. We shall also consider certain 

 theoretical views with regard to their action in the automatic processes of respiration, and 

 with the sense of want of air (besoin de respirer), which gives rise to the reflex respira- 

 tory acts. 



Effects of Division oftlie Pneumogastrics upon Respiration. Section of both pneumo- 

 gastrics in the neck, in mammals and birds, is usually followed by daath, in from two to 

 five days. In young animals, death may occur almost instantly, from paralysis of the 

 respiratory movements of the glottis, a fact which we have already noted in connection 

 with the recurrent laryngeal nerves. 



Very little of importance, with regard to the functions of the pneumogastrics in con- 

 nection with respiration, has been ascertained by the numerous experiments on record of 

 section of one or both of these nerves in the cervical region. It has been found by all 

 experimenters, that animals survived and presented no very distinct abnormal phenomena 

 after section of one nerve. Longet states that animals operated upon in this way present 

 hoarseness of the voice and a slight increase in the number of respiratory acts. Some 

 observers have found the corresponding lung partly emphysematous and partly engorged 

 with blood, and others have not noted any change in the pulmonary structure. 



When both nerves are divided in full-grown dogs, an experiment which we have often 

 repeated, the effect upon the respiratory movements is very marked. For a few seconds, 

 the number of respiratory acts may be increased ; but, as soon as the animal becomes 

 tranquil, the number is very much diminished, and the movements change their character. 

 The inspiratory acts become unusually profound and are attended with excessive dilata- 

 tion of the thorax. The animal is generally quiet and indisposed to move. We have 

 seen, under these conditions, the number of respirations fall from sixteen or eighteen to 

 four per minute. 



In most animals that die from section of both pneumogastrics, the lungs are found 

 engorged with blood, and, as it were, carnified, so that they sink in water. This curious 

 foot, although its physiological significance is not apparent, has been the subject of much 

 speculation and experimental research. Bernard found that the pulmonary lesion did not 

 exist in birds, although section of both nerves was fatal. It had previously been ascer- 

 tained that, in some animals, death takes place with no alteration of the lungs. When 

 the entrance of the secretions into the air-passages was prevented by the introduction of 

 n canula into the trachea, the carnification of the lungs was nevertheless observed. 

 Without detailing all o the experiments upon which the explanation offered by Bernard 

 is based, it is sufficient to state that he observed a traumatic emphysema as a consequence 

 of the excessively labored and profound inspirations. Indeed, this can be actually seen 



