THE BLOOD. 81 



and coagulates more firmly than that which is venous. The 

 difference in colour is due to the presence in the former of 

 oxygen, and in the latter of carbon in a state of combination 

 not yet well determined. Venous blood, when exposed to 

 the action of oxygen, soon acquires the vivid red colour of 

 arterial blood, and this, when submitted to the influence of 

 carbonic acid, as speedily assumes the dark hue of venous 

 blood. 



The greater or less firmness in the clot formed is owing to 

 the different amount of fibrin contained in the two fluids, and 

 which is greatest in that which is arterial, the coagulum 

 of which, therefore, possesses the greatest density. The dif- 

 ferences detected by the microscope in the blood corpuscles 

 of arterial and venous blood are scarcely appreciable. Gerber 

 states that the " tint of colour exhibited is various ; bright in 

 the globule of arterial blood, dark red and somewhat streaky 

 in that of venous blood :" this difference of colour, which doubt- 

 less exists, it is easier to infer than positively to demonstrate 

 by means of the microscope. While arterial blood is richer 

 in salts, venous blood contains a greater proportion of fatty 

 matter. 



There are several substances which effect a change in the 

 colour of the blood : thus oxygen, the concentrated solutions 

 of salts with an alkaline base, and sugar turn dark venous 

 of a bright florid or arterial red, this reddening being ac- 

 complished by the salts and sugar, even when the blood is 

 placed in a vacuum, or an atmosphere of hydrogen, nitrogen, 

 or carbonic acid gases. 



Newbigging* hath also remarked that venous blood takes 

 the tint of vermilion in a cup, at those situations at which it 

 is painted with the green oxide of chrome, and Taylor f has 

 confirmed the observation that the colours which contain the 

 oxide of chrome brighten the tint of blood. 



On 'the other hand, bright or arterial blood is darkened, 

 or even blackened, by contact with carbonic and oxalic acids, 



* Edinburgh New Philosophical Journal, October, 1839. 

 f Lancet, February, 1840. 



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