352 DR. STEVENS ON THE THEORY OF RESPIRATION. 



parently there that animal heat is evolved. When carbonic acid, which is the result 

 of the above process, is added to the venous blood, it not only blackens the colour, 

 but renders it incapable of supporting life. For this reason warm-blooded animals 

 have a double circulation, one for circulating the arterial, and another for purifying 

 the venous blood*. 



When we obtain hcematosine, or the colouring matter of the blood, in a pure state, 

 it is black ; but a solution of any neutral salt possesses the peculiar property of 

 striking a beautiful scarlet or arterial colour with it. When we make an incision 

 into a clot of blood which has just coagulated, we find that the clot is then all equally 

 red : when we cut out a thin slice of this red clot, and immerse it in distilled water, 

 the salt serum oozes into the water. In proportion as this takes place the clot be- 

 comes darker ; and when the whole of the serum is removed, perfectly black. When 

 in this state neither atmospheric air, nor even pure oxygen, is capable of changing 

 its colour ; but when we immerse this black clot in a clear saline solution, it in- 

 stantly changes from jet black to a scarlet, or arterial colour. From these facts we 

 may conclude, that oxygen is a secondary agent in the change of colour from venous 

 to arterial ; and that if the scarlet colour of the blood be essential to life, it is pro- 

 duced, not by oxygen, but by another cause. The mere removal of the carbonic acid 

 from venous blood would not produce any change of colour, were it not that there 

 is in the blood itself another agent which produces the arterial tint, the moment 

 that the blackening effect of the carbonic acid is removed : this is effected by the 

 action of the natural saline ingredients of the blood on the colouring matter. When 

 oxygen is added to blood it may have a slight share in brightening the colour, 

 but it can only perfectly effect this when the colouring matter is in contact with a 

 saline fluid. Oxygen is so far from being the sole cause of the arterial colour, that 

 even pure oxygen is of itself inert as a colouring agent ; whilst a saline fluid changes 

 the colour of the blood from venous to arterial even in an atmosphere of carbonic 

 acid. 



Many authors describe the changes which occur in respiration, by asserting " that 

 oxygen disappears, and carbonic acid is emitted." But from some of the experiments 

 which I have detailed, it is evident that the removal of the acid is the first part of 

 the process, and the addition of oxygen the last. Others have maintained that when 



* It is well known that cold-blooded animals use very little food. If a rattlesnake gets one good meal 

 in three months, it is all that he requires : but even this is not actually necessary ; for I have seen one of these 

 animals that had not tasted food or water for twelve months, as plump, active, and venomous as those in the 

 wild state. On the other hand, all those animals that have warm blood require an immense quantity of food, 

 and if they do not receive this they soon perish ; but nineteen twentieths of this appears to be taken into the 

 system for the evolution of animal heat. The carbon is ultimately derived from the nourishment that we use, 

 and the oxygen is directly derived from the arterial blood : a constant supply of nourishment is therefore 

 necessary in warm-blooded animals, but a very small part of the blood which is formed from this is required 

 for nuttition, and if the whole of it were expended in this way, it is very clear that there would be none left to 

 return by the veins. 



