INFLUENCE OF VARIOUS CONDITIONS ON RESPIRATION. 633 



as modified respiratory movements, since the same group of muscles 

 come into play. These are all movements, with the exception of 

 yawning, which may be regarded as reflexes that have nothing to 

 do directly with the processes of respiration. A most interesting 

 variation of the normal type of respiration is known as the Cheyne- 

 Stokes respiration. It occurs in certain pathological conditions, 

 such as arteriosclerosis, uremic states, fatty degeneration of the 

 heart, etc. It is characterized by the fact that the respiratory 

 movements occur in groups (10 to 30) separated by apneic pauses, 

 which may last for a number (30 to 40) of seconds. After each pause 

 the respirations begin with a small movement, gradually increase 

 to a maximum, and then fall off gradually to the point of complete 

 cessation (see Fig. 252). The cause of this rhythm has not been 

 discovered. It is not certain whether the apnea between the 

 groups of respiration is a true apnea or an inhibitory apnea or an 

 apnea of some other kind resulting from some different kind of 

 action upon the respiratory center. A similar rhythm is often 

 observed in the beats of an isolated heart of the cold-blooded 

 animals under conditions which imply an insufficient supply of 

 oxygen. More or less rhythmical variations in the strength of the 

 breathing movements have been described also in normal sleep, 

 hibernation, chloral narcosis, etc., but nothing so definite and 

 characteristic as in these very interesting Cheyne-Stokes cases. 



