Chap. XIII] RESPIRATORY SYSTEM 243 



Mechanism of inspiration and expiration. — During inspiration 

 the cavity of the chest is enlarged in all three diameters : 

 (1) antero-postepior, (2) lateral, and (3) vertical. This is brought 

 about by the action of the intercostal and other muscles, which 

 elevate the ribs and thereby increase the antero-posterior and 

 lateral diameters. The descent of the diaphragm increases the 

 vertical diameter. The lungs are correspondingly distended to 

 fill the enlarged cavity. To prevent a vacuum in the lungs, air 

 rushes in by way of the trachea to the bronchi. Upon the relaxa- 

 tion of the inspiratory muscles, the elasticity of the lungs and the 

 weight and elasticity of the chest walls cause the chest to return 

 to its original size, in consequence of which the air is expelled from 

 the lungs. As in the heart, the auricular systole, the ventricular 

 systole, and then a pause follow in regular order, so in the lungs the 

 inspiration, the expiration, and then a pause succeed one another. 



Control of respiration. — Respiration is both a voluntary and 

 an involuntary act. It is possible for a short time to increase or 

 retard the rate of respiration within certain limits by voluntary 

 effort, but this cannot be done continuously. If we intentionally 

 arrest the breathing or diminish its frequency, after a short time 

 the nervous impulse becomes too strong to be controlled, and the 

 movements will recommence, as usual. If, on the other hand, we 

 purposely accelerate respiration to any great degree, the exertion 

 soon becomes too fatiguing for continuance, and the movements 

 return to their normal standard. 



Cause of respiration. — The nervous impulses which cause the 

 contractions of the respiratory muscles are entirely dependent on 

 the nervous system, especially that part known as the respiratory 

 centre, which is located in the medulla oblongata. Efferent nerves 

 from the respiratory centre travel down the spinal cord and end 

 at different levels, where they connect with the fibres of the pneu- 

 mogastric and sympathetic nerves that are distributed in the lung 

 tissue. Afferent nerves lead from these different levels to the res- 

 piratory centre. 



The consensus of opinion at the present time seems to be that 

 the action of the respiratory centre is automatic, but that the rate 

 and rhythm of the respiratory movements is controlled (1) by 

 the pneumogastric nerve, and (2) by the chemical condition of the 

 blood. 



