RESPIRATION 527 



of the vagus arrests respiration, and it may therefore be con- 

 cluded that the nervous mechanism presiding over this act is 

 situated there. 



This centre sends fibres down the lateral column of the cord 

 to act upon the outgoing neurons to the muscles of respiration, 

 and it is by influencing the activity of these that the respiratory 

 centre controls the act of respiration. 



Outgoing Nerves. — The diaphragm is supplied by the 

 phrenic nerves arising 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 or the neck broken below the fifth 

 cervical nerves, the intercostal muscles cease to act. If the 

 section be made above the third cervical nerves, the diaphragm, 

 too, is paralysed, and the animal dies of suffocation. 



B. Mode of Action. — The respiratory centre is under the 

 control of higher nerve centres, and, through these, it may be 

 thrown into action at any time, or prevented from acting for 

 the space of over a minute and, under certain conditions, for two or 

 three minutes. But, sooner or later, the respiratory mechanism 

 acts in spite of the most powerful attempts to prevent it. 



Pearl divers rarely are able to stay under water for more 

 than 85 seconds ; the record period of submergence is 4 minutes 

 45 seconds. 



1. Chemical Regulation. 

 A. Carbon dioxide. 



The activity of the centre is chiefly regulated by the tension of 

 CO2 in the blood going to it (p. 495), and everything which leads 

 to an increase in the CO2 increases the activity of respiration, 

 while everything which decreases it decreases the activity of 

 breathing. 



The tension of C0„ in the blood is directly proportional to 

 the partial pressure of COj in the medium to which the blood is 

 exposed (p. 496). The amount and tension of C0„ in the blood 

 and therefore its action upon the respiratory centre, depend 

 upon its production in the muscles on the one hand, and on its 

 tension in the alveolar air on the other. The latter may be 

 raised by breathing air with an increased amount of COg, e.g. 

 5 per cent. 



