390 OF RESPIRATION. 



or complete inactivity of the ganglionic centre, which is concerned in 

 the respiratory actions ; or from interruption to the transmission of its 

 influence along the nervous trunks. Further, when there is no obstacle 

 to the free ingress or egress of air, Asphyxia may be produced by the want 

 of oxygen in the atmosphere that is respired, or by the presence of car- 

 bonic acid in too large an amount. And the presence of other gases, 

 which exert a directly poisonous influence on the blood, such as sulphu- 

 retted hydrogen, produces a state, which may be included under the 

 same general description. 



705. Now when, from any of these causes, the free exchange of car- 

 bonic acid for oxygen in the pulmonary capillaries is checked, the first 

 effect of the interruption appears to be, the stagnation of the blood in 

 the pulmonary capillaries. This stagnation is evidently due, not to any 



' deficiency of power in the heart ; for that organ is not yet affected ; but 

 to the insufficiency of the heart's power, acting alone, to drive the blood 

 through the pulmonary capillaries : the force which should be generated 

 by chemical changes in them ( 598), being deficient. The stagnation 

 is not, however, complete at first ; since the quantity of oxygen contained 

 in the lungs is sufficient to produce an imperfect arterialization of the 

 blood ; and the blood thus partially changed is transmitted to the left 

 side of the heart, and is thence propelled to the system. Owing to its 

 half-venous condition, it cannot exert its usual stimulating influence on 

 the tissues, especially the muscular and nervous ; and their powers are 

 consequently weakened. For the same reason, it does not receive its 

 usual auxiliary force in the systemic capillaries ( 599) ; since the 

 changes, which it ought to undergo in them, can only be partially per- 

 formed. 



706. As the air included in the lungs loses more and more of its oxy- 

 gen, and becomes more and more charged with carbonic acid, the aeration 

 of the blood in the pulmonary capillaries becomes more and more im- 

 perfect ; the quantity of blood which is allowed to return to the heart is 

 gradually diminished, and its condition becomes more and more venous ; 

 and at last, the pulmonary circulation is altogether suspended. From 

 the relation which the respiratory circulation bears to the systemic, in 

 all the higher classes of animals, save Reptiles, it follows that the sys- 

 temic circulation must in like manner be brought to a stand. The venous 

 blood accumulates in the pulmonary artery, in consequence of the ob- 

 struction of its capillaries ; it distends the right cavities of the heart ; 

 and the accumulation extends to the venous system of the body in 

 general, especially affecting those organs which naturally receive a large 

 quantity of venous blood, such as the liver and spleen. The arterial 

 system, on the other hand, is emptied in a corresponding degree; nearly 

 all its blood having passed through the systemic capillaries ; and no 

 fresh supplies being received from the heart. From this deficiency, 

 and from the venous character of the blood which the systemic arteries 

 do contain, it results that the nervous and muscular systems lose their 

 power ; insensibility comes on, at first accompanied with irregular con- 

 vulsive movements ; but in a short time there is a total cessation of all 

 movement except that of the heart ; and the pulsations of that organ be- 

 come feebler and feebler, until they cease altogether. The immediate 

 cause of the cessation of the heart's action appears to be different on the 



