SEC. 8. THE EFFECTS OF CHANGES IN THE COMPO- 

 SITION AND PRESSURE OF THE AIR BREATHED. 



376. The preceding sections have shewn us that the respira- 

 tory mechanism is arranged to work satisfactorily when the lungs 

 are adequately supplied with air of the ordinary composition 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 condition 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 ^ 334 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 m 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 con- 

 vulsions, 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 



