206 HANDBOOK OF PHYSIOLOGY. 



depends upon the manner of the deprivation of oxygen, whether sudden 

 or gradual. The convulsive stage is short, lasting, it may be, only one 

 minute. The third stage may last three minutes and upwards. 



The circulatory conditions which accompany these symptoms are 



(1) More or less interference with the passage of the blood through 

 the pulmonary blood-vessels. 



(2) Accumulation of blood in the right side of the heart and in the 

 systemic veins. 



(3) Circulation of impure (non-aerated) blood in all parts of the 

 body. 



Cause of death. The causes of these conditions and the manner in 

 which they act, so as to be incompatible with life, may be here briefly 

 considered. 



(1) The obstruction to the passage of blood through the lungs is not 

 very great ; and such as there is occurs chiefly in the later stages of 

 asphyxia, when, by the violent and convulsive action of the expiratory 

 muscles, pressure is indirectly made upon the lungs, and the circulation 

 through them is proportionately interfered with. 



('2) Accumulation of blood, with consequent distention of the right 

 side of the heart and of the systemic veins, is the direct result, at least 

 in part, of the obstruction to the pulmonary circulation just referred to. 

 Other causes, however, are in operation, (a) The vaso-motor centres 

 stimulated by blood deficient in oxygen, causes contraction of all the 

 small arteries with increase of arterial tension, and as an immediate 

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

 rial 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 

 oblongata. 



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

 bution 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 coro- 

 nary arteries, on the muscular substance of the heart. The excitability 

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

 large supply of oxygen, and, when this is interfered with, excitability is 



