526 VETERINAKY PHYSIOLOGY 



ten to twelve, or about one to every four or five beats of the 

 heart. Deep breathers are slow breathers, and shallow breathers 

 are quick breathers, and in the latter a smaller part of the lung 

 is ventilated. 



The most important factor modifying the rate of respiration 

 is muscular exercise. After galloping the respirations may be 

 over 60 per minute. 



The other modifications in the rate of breathing will be 

 better understood after studying the nervous mechanism of 

 respiration. 



Inspiration is more rapid than expiration (see fig. 216). 

 As soon as it is completed, a reverse movement occurs, which 

 is at first rapid, but gradually becomes slower, and may be 

 followed by a pause, during which the chest remaios in the 

 collapsed condition. The existence and duration of this pause 

 varies much, and it may really be regarded as the terminal 

 period of expiration. Considering it in this light, we may say 

 that inspiration is to expiration as 6 is to 7. 



VI. The Nervous Control of Respiration. 



The rhythmic movements of respiration require the har- 

 monious action of a number of muscles, and this is directed by 

 the nervous system. 



The Respiratory Centre. 



If the spinal cord be cut above the third cervical nerves the 

 movements of respiration at once stop. Obviously there is 

 some nervous mechanism above this level presiding over these 

 muscles. 



A. Position. — Removal of the brain above the medulla 

 oblongata does not stop the respiratory rhythm. The mechan- 

 ism must therefore be situated in the medulla oblongata. 



If the medulla be split into two by an incision down the 

 middle line, respiration continues, but the two sides do not 

 always act at the same rate. The mechanism, then, is bi- 

 lateral. Normally the two parts are connected, and thus act 

 together. 



Destruction of the part of the medulla lying near the root 



