290 



HUMAN PHYSIOLOGY 



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 ner- 



R.C. 



Diaph 



FIG. 130. Nervous Mechanism of Respiration. R.C., Respiratory Centre; Cut., 

 Cutaneous Nerves; Ph., Phrenics; In. C., Intercostal Nerves; P., Pul- 

 monary Branches of Vagus; S.L., Superior Laryngeal Branch of Vagus; 

 I/a., the Larynx; O. Ph., Glossopharyngeal Nerve; Diaph., Diaphragm. 



vous 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 



