660 NERVOUS SYSTEM. 



when the pleura' is exposed in living animals. As a result of this distention of the air- 

 cells, the pulmonary capillaries are ruptured in different parts, the blood becomes coagu- 

 lated, and the lungs are finally carnified. This cannot occur in birds, because the lungs 

 are fixed, and their relations are such that they are not exposed to excessive distention in 

 inspiration. 



There is no satisfactory explanation of the remarkable changes in the respiratory 

 movements that follow section of the pneumogastrics. 



In this connection we may note a curious fact, observed by Prof. Balton and others, 

 that the pneumogastrics sometimes reunite after division. In January, 1874, we divided 

 both pneumogastrics in a medium-sized dog. The pulse was immediately increased from 

 one hundred and twenty to two hundred and forty in the minute, and the number of 

 respirations fell from twenty-four to four or six. In ten days, the pulse and respirations 

 had become normal. The dog was then killed by section of the medulla oblongata, and 

 the reunion of the divided ends of the nerves was found to be nearly complete. 



Sense of Want of Air. The pneumogastrics may regulate the respiratory acts, but 

 they are not the medium through which the sense of want of air (besoin de respirer), 

 which gives rise to the movements of respiration, is conveyed to the nerve-centres. If 

 it be true, as it undoubtedly is, that section of both pneumogastrics in the neck modifies 

 the number and the character of the respirations, and that, after division of the nerves, 

 galvanization of their central ends arrests respiration, it is probable that this function is 

 normally influenced through these nerves, by impressions conveyed to the centres ; but 

 what this influence is or what is the mechanism of its action, we do not know. 



The positive statement that the sense of want of air is not conveyed to the nerve- 

 centres through the pneumogastrics is based, to a great extent, upon our own experi- 

 ments, which have been fully detailed under the head of respiration ; and we shall here 

 give simply their results and the conclusions to which they lead. 



The acts of respiration are involuntary, although they may be modified, within cer- 

 tain limits, through the will ; and they are due to an impression made upon the respira- 

 tory nervous centre, the medulla oblongata, which gives rise to the stimulus that ex- 

 cites the action of the inspiratory muscles. It has been conclusively shown by experi- 

 ments that, if artificial respiration be efficiently carried on in a living animal, so as to 

 supply air fully to the system, the sense of want of air is not appreciated, and the animal 

 makes no effort to breathe ; but, if respiration be imperfectly performed, the animal 

 almost immediately feels the want of air, and, in our experiments, the exposed respira- 

 tory muscles were thrown into violent but ineffectual contraction. 



The principal points with reference to the location of the sense of want of air and its 

 action upon the nerve-centres, developed by our own experiments, are the following : 



A dog was etherized, the chest was opened, exposing the heart and lungs, and arti- 

 ficial respiration was carried on by means of a bellows secured in the trachea. So long 

 as the supply of air was sufficient, the animal made no respiratory effort. 



An artery was then exposed and the color of the blood noted. When the artificial 

 respiration was arrested, the animal made efforts to breathe as soon as the blood became 

 dark in the arterial system. We concluded from this, that the impression made upon 

 the respiratory nervous centre, giving rise to the movements of respiration, was due to 

 the action of non- oxygenated blood. 



We assume, as a conclusion drawn from experiments upon the different nerve-centres, 

 that the medulla oblongata is the sole centre presiding over the respiratory acts. In dogs 

 prepared as indicated above, when the vessels given off from the arch of the aorta were 

 constricted so as to cut off the supply of oxygen-carrying fluid to the medulla oblongata, 

 the trunk and lower extremities being still supplied with arterial blood and the lungs 

 being efficiently supplied with fresh air by the bellows, the animals began to make 

 respiratory efforts in a little more than two minutes after constriction of the vessels. 



These experiments demonstrate that the sense of want of air is felt when the supply 



