RESPIRATION. 373 



an excitation of one set of fibers and the diminution of a permanent 

 excitation in another set. 



Synchronous with the respiratory movements they noticed a 

 variation of the current of demarcation of the vagus. The different 

 facts lead to the original view of Hering-Breuer, that certain stimuli 

 of inspiration induce expiration with inhibition of inspiration., whilst 

 others in the collapse of the lungs inhibit expiration and induce 

 inspiration. 



Apnoea. When a dog has frequent insufflations of air through 

 a tracheal cannula by means of a bellows, there ensues an arrest of 

 respiratory movements for a short time. Rosenthal believed this to 

 be due to an excess of oxygen in the blood and that the respiration 

 centers were not excited because of this excess in the tissues. Fred- 

 ericque lately, by cross-circulation in the head of one dog with blood 

 from another dog, has been able to produce apnoea which remains a 

 long time if the other dog continues to receive exaggerated pulmonary 

 insufflations. This apnoea is not due to an augmentation of the 

 oxygen, but to a deficiency of tension of carbonic acid. The arrest that 

 ensues in a dog by a frequent insufflation of hydrogen instead of oxygen 

 is, according to Fredericque, due to irritation of the vagus fibers, which 

 calls out an expiration-arrest and which is a simulated apnoea. 



Thus, apnoea is produced by two factors, lessened tension of C0 2 

 and irritation of the terminals of the vagi. Mosso calls the diminished 

 tension of C0 2 in the blood by the name of acapnia. The frequent 

 ventilations of the lungs removes the C0 2 from the blood, and thus 

 lowers the stimulation of the center of respiration. These rapid 

 insufflations, also stimulating the ends of the vagi in the lungs, call 

 out an inhibition of the respiration center. After section of the vagi 

 it is difficult to produce apnoea. 



Asphyxia. In considering the phenomena of asphyxia, it is nec- 

 essary to distinguish between rapid asphyxia, produced by complete 

 obstruction to the entrance of air, and slow asphyxia, which is grad- 

 ually established. The phenomena of asphyxia are divisible into 

 three stages, which are easily observed in animals, especially in the 

 dog. 



In the first stage, called the stage of exaggerated breathing, hyper- 

 pnoea, the respiration is more rapid and deeper, due to the C0 2 ; then 

 the phenomena of dyspnoea appear, the extraordinary muscles of 

 inspiration and expiration are called into play, the abdominal muscles 

 contract forcibly, the pupils are small. This stage lasts about a 

 minute. 



