RESPIRATION. 369 



sent through them. There will follow disturbances of the vascular 

 system, particularly the heart; so that death follows in a short time. 

 If the central end of a divided vagus be irritated by a strong induc- 

 tion current, there is produced a strong degree of excitation in the 

 medulla oblongata. It sends out impulses along motor nerves which 

 arrest respiration in a state of inspiration, due to tetanus of the dia- 

 phragm. Stimulation of the central end of the superior laryngeal 

 calls out an expiratory arrest. Each half of the respiratory district, 

 termed a center, consists of two minor centers, which are in an 

 alternate state of activity. The one center is inspiratory; the 

 other, expiratory. Each one forms the motor central point for the 

 acts of inspiration and expiration. The co-ordinated impulses pro- 

 ceed from these centers in the medulla along the nerves which supply 



Fig. 133. Arrest of Respiration in State of Expiration. (HEDON.) 

 By irritation of the central end of the vagus in a chloralized dog. 



the muscles of respiration and the associated muscles of the face, 

 nose, and larynx. 



The activity of the respiratory center is excited by irritation of 

 the sensory nerves, either cutaneous or pulmonary. It may also be 

 stimulated by a strong excitation of the nerves which convey pain 

 ducing dyspnoea; diminution of oxygen and the presence of heat are 

 also noticeable factors. According to some observers, the acid sub- 

 stance formed in the blood when the muscles are greatly exercised 

 also stimulates the inspiratory center. 



The functions of the expiratory center, on the contrary, are 

 stimulated by a strong excitation of the nerves which convey pain 

 sensations. 



The consensus of opinion among physiologists now seems to be 

 in favor of considering the activities of the respiratory center as 

 partly automatic, partly reflex, and that the vagus is the principal 

 nerve concerned in the reflex activities. 



The true causes of the rhythmic action of the centers of respiration 



