DR. STEVENS ON THE THEORY OF RESPIRATION. 353 



two gases act upon each other, the one penetrates the membrane at the same mo- 

 ment that the other is removed. But if this were the case, why did the membrane 

 become convex in experiment 10, and concave in experiment 11 ? If the action were 

 equal, the membrane would remain unchanged ; but this is so far from being the 

 case, that in some experiments the membranes became distended to such an extent 

 that they actually burst. 



It was supposed by Spallanzani, and afterwards by Dr. Edwards, that in the pro- 

 cess of respiration the carbonic acid was merely exhaled from the lungs. We have 

 seen, however, that venous blood so retains that acid that it cannot be removed, 

 even with the aid of an air-pump ; consequently, were there not an active agent for 

 the purpose of removing the carbonic acid from the venous blood as it circulates 

 through the lungs, it would remain unchanged, and almost instantly cause death. 

 It is the power which oxygen possesses of attracting carbonic acid, which renders 

 oxygen essential to life. As hydrogen also possesses this power, it supports life for a 

 longer period than most of the other gases ; but hydrogen has a deleterious effect 

 on the blood ; and when animals are forced to breathe it, though the carbonic acid 

 is removed, a part of the hydrogen is at the same time absorbed, which blackens 

 the blood, and the animals soon die. 



The property which oxygen possesses of attracting carbonic acid, furnishes the 

 following explanation of the process of respiration. When the venous blood arrives 

 in the lungs, the oxygen of the atmosphere is, in the first instance, the active or at- 

 tracting agent. It removes the carbonic acid, which had been the cause of the dark 

 colour of the blood. When this is removed, or perhaps in proportion as it is re- 

 moved, the blood becomes the attracting agent, and a portion of oxygen is attracted 

 into the blood, and takes the place of the carbonic acid. From the peculiar struc- 

 ture of the lungs, these changes are rapidly effected, particularly at the high tempe- 

 rature of 98° ; and when the process is fully completed, we know from the great 

 discovery of Harvey, that the blood which has received the pure air passes rapidly 

 on to the arterial, and from this again to the capillary system. 



If the above theory be correct, it follows that the blood is converted from arterial 

 to venous in the extreme circulation, by the loss of oxygen and the addition of car- 

 bonic acid ; whilst the venous blood is converted into arterial, by the loss of carbonic 

 acid, and the addition of oxygen ; consequently, the essential difference betwixt venous 

 and arterial blood is, that the former contains carbonic acid, and the latter oxygen. 



I have said that black is the natural colour of the colouring matter ; but when this 

 agent is diffused in a saline fluid, such as the serum, it is of a bright scarlet tint, 

 which is, in fact, the natural colour of arterial blood. When carbonic acid is added 

 to this blood in the extreme circulation, it becomes dark red ; but when this acid is 

 removed in the pulmonary organs, the blood then resumes its natural scarlet or 

 arterial colour ; and this, as I have said, is produced not directly by oxygen, but 

 chiefly, if not entirely, by the action of the salts of the blood on the colouring matter. 



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