3 o6 VETERINARY PHYSIOLOGY 



VI. Controlling Mechanism of Respiration. The rhythmic 

 movements of respiration require the harmonious action of 

 a number of muscles, and this is directed by the nervous 

 system. 



The diaphragm is supplied by the phrenic nerves rising 

 from the third and fourth, and partly from the fifth cervical 

 nerves. The intercostals are supplied by branches from their 

 corresponding dorsal nerves. 



If the spinal cord be cut below the fifth cervical nerve the 

 intercostal muscles cease to act. If the section is made above 

 the third cervical nerve, the diaphragm, too, is paralysed, and 

 the animal dies of suffocation. 



Respiratory Centre. Obviously, then, there is some nervous 

 mechanism above the spinal cord presiding over these 

 muscles. 



Removal of the brain above the medulla oblongata does not 

 stop the respiratory rhythm. 



The mechanism must, therefore, be situated in the medulla 

 oblongata. 



If the medulla is 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, but normally the two parts are connected, and thus 

 act together. 



Destruction of the part of the medulla lying near the root 

 of the vagus arrests respiration, and it may therefore be 

 concluded that the nervous mechanism presiding over this 

 act is situated there. 



It must not be imagined that this centre sends fibres directly 

 to the muscles concerned in respiration. The nerves passing 

 to these come from the cells in the grey matter of the spinal 

 cord, and it is by influencing the activity of these cells that 

 the respiratory centre controls the act of respiration. 



Since expiration, when forced, is a complex muscular act, 

 it is reasonable to suppose that the respiratory centre contains 

 two parts one presiding over inspiration, one presiding over 

 expiration. While the inspiratory centre is constantly in 

 rhythmic action, the expiratory centre is only occasionally at 

 work. 



Mode of Action of the Respiratory Centre. Both parts of 



