1 89 1 .] A nccxtlictic . 1 ction of Pure Nitrogen. 145 



consequence of obstruction in front. That the arrest of the circula- 

 tion is not due to paratysis of the heart's walls, by the circulation 

 of venous blood through its tissues, seems to be proved by the follow- 

 ing 1 experiment. 



Into the trachea of a small dog, with the chest and pericardium 

 opened and kept alive by artificial respiration, a glass ~f"-tube was 

 introduced, through which pure nitrous oxide was passed into the 

 lungs, whilst the expired gases escaped into the air. As usual, first 

 the left then the right cavities became distended, and in one minute the 

 heart's action had nearly ceased, with over-distension of the right side. 

 Then inhalation of nitrous oxide, impregnated with the vapour of nitrite 

 of amyl, was substituted for pure N 2 O, by means of a two-way stopcock, 

 and the result was that almost immediately the distension of the right 

 cavities began to subside, and in two minutes they had nearly re- 

 gained their normal size. 



The explanation is, that the circulation, having been arrested by 

 the contraction of the arterioles, was, for a time, restored by the 

 paralysing influence of nitrite of amyl upon those vessels, while 

 atmospheric air was strictly excluded. 



Additional evidence of the influence of the arterioles in arresting 

 the circulation during the progress of asphyxia is derived from the 

 fact that a sufficient dose of such agents as are known to paralyse the 

 arterioles, e.g., curara and atropine, prevents over-distension of the 

 heart's cavities, and considerably prolongs the life of the animal. 



This is conclusively shown by experiments performed by Mr. 

 Martin, the details of which are given in the paper of which this is 

 an abstract. 



It has been suggested that the distension first of the left then of 

 the right side of the heart in asphyxia is the result solely of systemic 

 arterial contraction, the impediment acting backwards from the left 

 side of the heart, through the lungs, to the right cavities and the 

 systemic veins. The main objection to this theory is the fact that, 

 when the chest is opened immediately after death from asphyxia, the 

 lungs are found extremely pale, from anaemia of their minute vessels, 

 and in a corresponding degree collapsed. Backward pressure from 

 the left side of the heart, sufficient to greatly distend the right 

 cavities, must of necessity involve engorgement of the pulmonary 

 capillaries. 



That there is a certain amount of backward pressure from the 

 primary distension of the left heart, extending as far as the pulmonary 

 veins, would seem to be proved by observations made by Mr. Martin 

 to the effect that a manometer in a branch of a pulmonary vein 

 indicates an early and continuous increase of pressure during the 

 progress of asphyxia ; but that this backward pressure does not 

 extend to the right side of the heart is shown by the fact that in the 



