124 PHYSIOLOGY FOR BEGINNERS 



Quantity of Air respired. At each inspiration an adult 

 man will take into the lungs 20 to 30 cubic inches of air, and 

 pass it out again at the succeeding expiration. This is called 

 the tidal air. By the most powerful expiration from 75 to 100 

 cubic inches of air may be driven out in addition to the tidal 

 air. This is called the supplemental air. What is then left 

 in the lungs, which is about loo cubic inches, is called the 

 residual air. By the deepest inspiration 100 cubic inches of 

 air may be taken in in addition to the tidal air. This is called 

 the complemental air. Thus at the end of an ordinary 

 expiration there are about 200 cubic inches of air in the 

 lungs, and this air, which is called the stationary air, is 

 only renewed by being mixed with the tidal air in the 

 smaller bronchial tubes. We have seen that the air in the 

 alveoli of the lungs is separated from the blood capillaries 

 by nothing more than a thin layer of flattened cells, and 

 that oxygen diffuses from the air into the blood, and carbonic 

 acid diffuses from the blood into the air ; so that it is with the 

 air in the alveoli that the interchange takes place, and the 

 alveolar air in turn interchanges gases with the tidal air by 

 rapid diffusion. 



Dependence of Respiration on the Central Nervous 

 System. The movements leading to inspiration are brought 

 about by the contraction of certain skeletal muscles, and we 

 have seen that the skeletal muscles of the body contract only 

 when they receive nervous impulses along their motor nerves. 

 The motor nerves for the intercostal muscles come from the 

 thoracic region of the spinal cord, and the motor nerves for the 

 diaphragm, of which there are two, one on each side, called 

 the phrenic nerves, come from the cervical region of the spinal 

 cord. If the phrenic nerves are cut, the diaphragm does not 

 contract, because no motor impulses reach it ; similarly, the 

 intercostal muscles can be put at rest by dividing their motor 

 nerves. Thus respiration is caused by impulses passing at 

 regular intervals from the central nervous system to the 

 appropriate muscles. The impulses start from the spinal bulb, 

 pass down the spinal cord, and then out along the motor nerves 

 in question. The part of the spinal bulb from which these 

 impulses start is called the respiratory centre. If this part 

 is injured respiration stops. 



