618 PHYSIOLOGY OF RESPIRATION. 



there is not simply an inhibition of the inspiratory movements, 

 but a reflex excitation of a peculiar type of expiratory movements. 



The Voluntary Control of the Respiratory Movements. 

 We can control the respiratory movements within wide limits, make 

 forced or feeble inspirations or expirations, accelerate the rhythm, 

 or completely inhibit the respirations in any phase. If, however, 

 the "breath is held," that is, if the respiratory movements are 

 inhibited and the glottis is closed, the increasing irritability of 

 the respiratory center eventually breaks through the voluntary 

 inhibition. How far this voluntary control is based upon direct 

 connections between the cerebrum and the respiratory center and 

 how far it depends upon voluntary paths to the separate spinal 

 nuclei of the muscles involved cannot be discussed profitably. 



The Nature of the Respiratory Center. The respiratory 

 center located in the medulla oblongata might with more propriety 

 be designated as the inspiratory center. Our normal respirations 

 throughout life consist of an active inspiration and a passive 

 expiration. It is the co-ordinated activity of the inspiratory 

 muscles that is characteristic of the respiratory movements. The 

 expiratory muscles come into action only occasionally and under 

 special conditions. It is, in reality, incorrect to speak of the normal 

 respirations as consisting of alternate inspiratory and expiratory 

 movements; as a matter of fact, they consist of rhythmical in- 

 spiratory movements alone. So also when we describe the respira- 

 tory center as essentially automatic we refer only to the action on 

 the inspiratory muscles, since a series of active inspiratory move- 

 ments is the essential feature of respiration. Under certain con- 

 ditions, however, we do have rhythmical expiratory movements, 

 active expirations. Such movements may occur independently 

 of the respirations proper, as in coughing and laughing, or in the 

 straining movements of defecation, micturition, and parturition; 

 or they may occur as an integral part of the respirations, as in the 

 forced movements of dyspnea. Under the conditions of partial 

 suffocation, for instance, as the blood becomes more and more 

 venous the respirations increase in force and active expirations 

 appear. It becomes a question, therefore, as to the existence of 

 what might be called an expiratory center, a group of nerve cells 

 controlling the co-ordinated activity of the expiratory muscles. 

 The mere fact that in dyspnea we have a rhythmical and co-ordi- 

 nated activity of these muscles seems to imply the existence of such 

 a center, but there is no definite experimental knowledge as to its 

 location. Assuming that there is such a center, it may be believed 

 that it exists in the medulla, since after section below the medulla 

 there is no evidence of the occurrence of rhythmical expiratory 

 movements even in extreme conditions of venosity of the blood. 



