10 RESPIRATION 



It was evident from these experiments, and from the marked 

 slowing and deepening of breathing after the vagi are cut, that 

 distention of the lungs stimulates the nerve endings of the vagi 

 in the lungs in such a way as to terminate inspiration and initiate 

 expiration, while deflation of the lungs produces a corresponding 

 stimulus acting so as to terminate expiration and initiate inspira- 

 tion. Thus inspiration seems to be the cause of expiration, and 

 expiration of inspiration. Hering described this as the "self-regu- 

 lation" of breathing. 



Another series of observations relates to chemical stimulation 

 of the respiratory center. It was found that if air containing very 

 little oxygen is breathed, or a small volume of ordinary air is 

 repeatedly rebreathed, great panting ensues, followed by general 

 convulsions and final cessation of breathing. The same result was 

 found by Kiissmaul and Tenner to follow if the blood supply to 

 the brain is completely cut off, so that the blood remaining in the 

 vessels becomes venous. The respiratory center is thus first stimu- 

 lated to excessive action by imperfectly oxygenated or venous 

 blood, and later becomes exhausted and finally ceases to act. But 

 another most significant fact was definitely discovered by Rosen - 

 thai in i862. 15 If in an animal artificial respiration is pushed so 

 that the ventilation of the lungs is abnormally great the activity 

 of the respiratory center ceases entirely for a time, and this 

 condition he designated as apnoea. In most persons apnoea can be 

 produced easily by voluntarily forcing the breathing for a short 

 time. After a few deep and rapid breaths it will be noticed that 

 all natural tendency to breathe ceases for a time. 



These observations suggested that ordinary breathing is de- 

 termined by the degree of arterialization of the blood supplying 

 the respiratory center. If the degree of arterialization is dimin- 

 ished the breathing is increased, and vice versa, so that the 

 respiratory center automatically maintains a normal degree of 

 arterialization. When the venous blood is arterialized in the lungs 

 two changes occur, as we have already seen. The blood takes up 

 oxygen, and also loses carbonic acid. It might be one or the other, 

 or else both, of these changes that determines the activity of the 

 respiratory center. The most immediately evident change in the 

 blood during its passage through the lungs is its change in color 

 from a bluish to a bright scarlet color, and this change, as already 

 seen, is due solely to its oxygenation and not to loss of carbonic 

 acid. We thus naturally tend to think of blue blood as venous and 



"Resentful, Z>* At**mb*totg**g**, 1862. 



