REGULATION OF RESPIRATION 115 



imposed upon a normal inspiration {Complemental air) = 

 1500-2000 c.c. 



The vokinie which can be exhaled by an effort after a 

 normal expiration {Sitpple^nenfal air) = 1500-2000 c.c. 



The total change of capacity, full inspiration and full 

 expiration {Vital capacity) = 3300-4500 c.c. 



Even after the greatest expiratory effort, the residual 

 air remains, measuring 1500-2000 c.c. 



THE REGULATION OF RESPIRATION 



For the proper ventilation of the lungs two things are 

 necessary, an orderly alternation of inspiration and expira- 

 tion and an adaptation either of the extent or the rapidity 

 of the movement to the needs of the body. We therefore 

 have to consider how the rhythm is maintained and how 

 it undergoes variation. 



Respiration is dependent ultimately upon the integrity 

 of a centre situated in the floor of the fourth ventricle 

 near the nuclei of the vagus. When this centre is destroyed 

 respiration immediately ceases. But respiration is a co- 

 ordinated muscular act, and must therefore be due to the 

 stimulation of motor centres in the cord — ^the centre for 

 the phrenic nerve in the third, fourth and fifth cervical 

 segments, those for the intercostal nerves in the thoracic 

 region. The co-ordinated action of these centres is due 

 to stimuli which they receive from the medulla, since the 

 intercostal movements are abolished' after section of the 

 cord in the lower part of the cervical region, and both 

 intercostal and diaphragmatic movements are paralysed 

 after section through the upper part of the cervical region. 

 Section of the brain-stem above the medulla is without 

 effect upon the respiratory movements. 



Respiration is under the control of the will only to a 

 Umited extent. We may cease breathing for a time or 

 we may breathe excessively, but in either case the effort is 

 short-hved and is followed by a compensatory effect, 



