SEC. 7. THE EFFECTS OF CHANGES IN THE COMPO- 

 SITION AND PRESSURE OF THE AIR, BREATHED. 



306. The preceding sections have shewn us that the res- 

 piratory mechanism is arranged to work satisfactorily when 

 the lungs are adequately supplied with air of the ordinary com- 

 position of, and at the ordinary pressure of the atmosphere. 

 We have further seen that the mechanism can adapt itself 

 within certain limits to changes in the composition and pressure 

 of the air supplied. We may now consider briefly what takes 

 place when those limits are overstepped. The most striking 

 effects are seen, when, on account of occlusion of the trachea, 

 or by breathing in a confined space, or for other reasons, a due 

 supply of air not being obtained, normal respiration gives 

 place, through an intermediate phase of dyspnoea, to the condi- 

 tion known as asphyxia; this, unless remedial measures be 

 taken, rapidly proves fatal. 



Asphyxia. As soon as the blood becomes less arterial, more 

 venous than normal, the respiratory movements become deeper 

 and at the same time more frequent ; both the inspiratory and 

 expiratory phases are exaggerated, the supplementary muscles 

 spoken of 265 are brought into play, and the rate of the 

 rhythm is hurried. These effects, as we have seen, are chiefly 

 to be ascribed to the deficiency of oxygen in the blood. 



As the blood continues to become more and more venous the 

 respiratory movements continue to increase both in force and 

 frequency, a larger number of muscles being called into action 

 and that to an increasing extent. Very soon, however, it may 

 be observed that the expiratory movements are becoming more 

 marked than the inspiratory. Every muscle which can in any 

 way assist in expiration is in turn brought into play ; and at 

 last almost all the muscles of the body are involved in the 

 struggle. The orderly expiratory movements culminate in 

 expiratory convulsions, the order and sequence of which are 

 obscured by their violence and extent. That these convulsions, 

 through which dyspnoea merges into asphyxia, are due to a 

 stimulation (by the venous blood) of the spinal bulb, is proved 



491 



