162 ELEMENTARY PHYSIOLOGY 



access of air to the lungs is interfered with ; as, for 

 instance, when a man is strangled, drowned, or choked by 

 food or other obstacle in the trachea. But the blood 

 may become unusually venous by means less violent than 

 the above. Since the rapidity of difl'usion between two 

 gaseous mixtures depends on the difference of the propor- 

 tions in which their constituents are mixed, it follows that 

 the more nearly the composition of the tidal air aj)proaches 

 that of the stationary air, the slower will be the diffusion 

 of oxygen inwards, and of carbonic acid outwards, and 

 the more deficient in oxygen and overcharged with carbonic 

 acid will the air in the alveoli become. Thus by breathing 

 in a confined space, the oxygen in the tidal aii- is gradually 

 diminished and the carbonic acid grathialbj increased until 

 at length a point is reached when the change effected in 

 the stationary air is too slight to enable it to supply 

 the pulmonary blood with oxygen, and to relieve it of 

 carbonic acid to the extent required for its proper 

 arterialisation. 



When from any of the above causes the blood sent to 

 the respiratory centre is more venous than usual, the 

 centre is stimulated and the respiratory movements be- 

 come quicker and more forcible. This condition is usually 

 spoken of as dyspnoea, or laboured breathing. It is 

 characterised by the increased force and frequency with 

 which both the inspiratory and expiratory muscles con- 

 tract. If the offending cause of dyspnuia be not removed, 

 the blood becomes more and more venous. By this means 

 the respiratory centre is spurred on to still greater activity. 

 Not only do the ordinary muscles of respiration contract 

 more vigorously, but the accessory muscles (p. 145) come 

 into more prominent play, and chiefly those ivhich assist 

 expiration. StiU later, nearly all the muscles of the body 

 are thrown into a state of violent contracting activity, and 

 with the onset of these convulsions dyspnossi passes 

 over into asphyxia. The violence of the convulsive 

 movements speedily leads to exhaustion, and the con- 



