540 NERVOUS SYSTEM 



nary branches alone. The sensibility of the mucous membrane of the 

 trachea and bronchia is due to the pneumogastrics ; and these parts are 

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



Effects of Division of the Pneumogastrics on Respiration. Section of 

 both pneumogastrics in the neck usually is followed by death in two to 

 five days. In very young animals, death may occur almost instantly 

 from paralysis of the respiratory movements of the glottis. It has been 

 found by all experimenters that animals survived and presented no very 

 distinct abnormal phenomena after section of one nerve. Animals 

 operated on 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, the effect on 

 the respiratory movements is very marked. For a few seconds the 

 number of respiratory acts may be increased ; but so soon as the ani- 

 mal becomes tranquil, the number is very much diminished and the 

 movements change their character. The inspiratory acts become unusu- 

 ally profound and are attended with excessive dilatation of the thorax. 

 The animal usually is quiet and indisposed to move. Under these con- 

 ditions the number of respirations may 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 condition probably is not the result of inflam- 

 mation of the pulmonary parenchyma, although this was the view for- 

 merly entertained, and it is now held by some physiologists. Bernard 

 found that the pulmonary lesion did not exist in birds, although section 

 of both nerves was fatal. It had previously been ascertained that in 

 some animals death takes place with no alteration of the lungs. When 

 the entrance of the secretions into the air-passages is prevented by the 

 introduction of a canula into the trachea, solidification of the lungs is 

 nevertheless observed. Those who regard the condition as inflamma- 

 tory call it a "vagus pneumonia," due to division of "trophic" fibres 

 of the pneumogastrics ; but the mechanism and immediate causes of 

 the pulmonary changes are obscure and have not been satisfactorily 

 explained. 



The pneumogastrics sometimes reunite after division. The following 

 observation (Flint, 1874) illustrates this fact, which has frequently been 

 noted : Both pneumogastrics were divided in the neck in a medium- 

 sized dog. The pulse was immediately increased from one hundred and 



