INNERVATION OF THE RESPIRATORY MOVEMENTS. G91 



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 active expirations. Such movements 

 may occur independently of the respirations proper, as in cough- 

 ing 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, there- 

 fore, as to the existence of what might be called an expiratory 

 center, a group of nerve cells controlling the co-ordinated activ- 

 ity of the expiratory muscles. The mere fact that in dyspnea we 

 have a rhythmical and co-ordinated 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 concUtions 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 characteristics may be assigned 

 to it: In the first place, it is not automatic; at least not under 

 normal conditions. In the second place, its activity must be 

 dependent in some way upon that of the inspiratory 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 



