RESPIRATION. 2<J5 



consequence the filling of the systemic veins, (b) The increased arterial 

 tension is followed by inhibition of the action of the heart, and the 

 heart, contracting less frequently, and also gradually enfeebled by defi- 

 cient supply of oxygen, becomes over-distended with blood which it 

 cannot expel. At this stage the left as well as the right cavities are 

 over-distended. 



The ill effects of these conditions are to be looked for partly in the 

 heart, the muscular fibres of which, like those of the urinary bladder or 

 any other hollow muscular organ, may be paralyzed by over-stretching; 

 and partly in the venous congestion, and consequent interference with 

 the function of the higher nerve-centres, especially the medulla ob- 

 longata. 



(3) The passage of non-aerated blood through the lungs and its dis- 

 tribution over the body are events incompatible with life in one of the 

 higher animals for more than a few minutes; the rapidity with which 

 death ensues in asphyxia being due, more particularly, to the effect of 

 non-oxygenized blood on the medulla oblongata, and, through the 

 coronary arteries, on the muscular substance of the heart. The excita- 

 bility of both nervous and muscular tissue is dependent on a constant 

 and large supply of oxygen, and, when this is interfered with, excita- 

 bility is rapidly lost. 



Effects of breathing gases other than the atmosphere. The diminu- 

 tion of oxygen has a more direct influence in the production of the usual 

 symptoms of asphyxia than the increased amount of carbon dioxide. 

 Indeed, the fatal effect of a gradual accumulation of carbon dioxide 

 in the blood, when a due supply of oxygen is maintained, resembles 

 rather the action of a narcotic poison than it does asphyxia. 



Then again we must carefully distinguish the asphyxiating effect of 

 an insufficient supply of oxygen from the directly poisonous action of 

 sucn gases as carbonic oxide, which is contained to a considerable 

 amount in common coal-gas. The fatal effects often produced by this 

 gas (as in accidents from burning charcoal stoves in small, close rooms) 

 are due to its entering into combination with the haemoglobin of the 

 blood-corpuscles and thus expelling the oxygen. The partial pressure 

 of oxygen in the atmosphere may be considerably increased without 

 much effect. Hydrogen may take the place of nitrogen if the oxygen 

 is in the usual proportion with no marked ill effect. Sulphuretted 

 hydrogen destroys the haemoglobin of blood. Nitrous oxide acts 

 directly on the nervous system as a narcotic. Certain gases, such as 

 carbon dioxide in more than a certain proportion ; sulphurous and 

 other acid gases, ammonia, and chlorine produce spasmodic closure 

 of the glottis, and are irrespirable. 



As conditions causing asphyxia in addition to the obstruction to the 

 trachea or elsewhere, and the prevention of the meeting of the blood 



