694 PHYSIOLOGY OF RESPIRATION. 



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. So also when we describe the respiratory 

 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. 

 The expiratory center may or may not be located in the same 

 region as the inspiratory center, but the following general char- 

 acteristics may be assigned to it : In the first place, it is not auto- 

 matic ; at least not under normal conditions. In the second place, 

 its activity must be dependent in some way upon that of the in- 

 spiratory center. Even our most violent respiratory movements 

 show an orderly sequence of inspiration and expiration, and we 

 may believe that the action of the expiratory center is conditioned 

 by the previous discharge of the inspiratory center, just as in the 

 heart the beat of the ventricle depends upon the previous systole 



