SENSE OF SUFFOCATION. 167 



and the animal made respiratory efforts in two minutes and seven seconds. Both vertebral 

 arteries and both carotids were constricted, and the animal made no respiratory efforts 

 for five minutes ; but respiratory efforts were made in one minute and thirty-five seconds 

 after both subclaviaus had been constricted in addition to the vertebrals and carotids. 



It seems, from these experiments, that, in order to induce respiratory efforts in an 

 animal under the influence of ether and with the lungs supplied with air by artificial res- 

 piration, either the innominate artery and the left subclavian artery, or both subclavians, 

 both carotids, and both vertebral arteries, must be tied. In other words, according to 

 our view of the cause of these respiratory efforts, the supply of blood to the medulla ob- 

 longata cannot be cut off completely except by tying all the vessels given off from the 

 arch of the aorta. 



As the result of these experiments, we must now modify the view advanced in 1861 

 as a conclusion from experiments then published, which we have maintained up to the 

 present time; viz., that the sense of want of air, which is the starting-point of the move- 

 ments of respiration, is due to want of oxygen in the general system. The experiments 

 made in 1861 were accurate, and the conclusions from them seemed to be legitimate ; but 

 these experiments were incomplete. Our more recent experiments, taken in connection 

 with the experiments of 1861, lead to the conclusion that the sense of want of air is due 

 to a want of circulation of oxygenated blood in the medulla oblongata. 



If we regard the sense of want of air as due primarily to a deficiency of oxygen in 

 the medulla oblongata, which can hardly be doubted, it becomes an important and inter- 

 esting question to determine, whether the normal respiratory movements be actually re- 

 flex in their character, as has been generally supposed, or whether they be due to direct 

 excitation of the nerve-cells in the respiratory centre. The latter seems, at present, to 

 be the more reasonable supposition. 



Sense of Suffocation. "We must separate, to a certain extent, the respiratory sense 

 from the sense of distress from want of air, and its extreme degree, the sense of suffoca- 

 tion. The first is not a sensation, but an impression made upon the medulla oblongata, 

 giving rise to involuntary respiratory movements. The necessities for oxygen on the part 

 of the system regulate the supply of air to the lungs. We have already seen that, once in 

 every five to eight respirations, or when the respiratory movements are a little restricted 

 under the influence of depressing emotions, an involuntary deep or sighing inspiration is 

 made, for the purpose of changing the air in the lungs more completely. The increased 

 consumption of oxygen, and a certain amount of interference with the mechanical process 

 of respiration during violent muscular exercise put us " out of breath," and for a time 

 the respiratory movements are exaggerated. This is perhaps the first physiological way 

 in which the want of air is appreciated by the senses. A deficiency in hasmatosis, either 

 from a vitiated atmosphere, mechanical obstruction in the air-passages, or grave trouble 

 in the general circulation, produces all grades of sensations, from the slight oppression 

 which is felt in a crowded room, to the intense distress of suffocation. When ha3matosis 

 is but slightly interfered with, only an indefinite sense of oppression is experienced, and 

 the respiratory movements are a little increased, the most marked effect being an increase 

 in the number and extent of sighing inspirations. 



Experiments have failed to show that either the respiratory sense or the sense of suf- 

 focation is due to irritation produced by carbonic acid in the non-oxygenated blood. 



Respiratory Efforts before Birth. 



It is generally admitted that one of the most important functions of the placenta, and 

 the one which is most immediately connected with the life of the fetus, is a respiratory 

 interchange of gases, analogous to that which takes place in the gills of aquatic animals. 

 The vascular prolongations from the fetus are continually bathed in the blood of the 

 mother, and this is the only way in which it can receive oxygen. Notwithstanding the 



