CIRCULATORY SYSTEIVI. 109 



air. If a clot of blood in a basin be examined, its upper sur- 

 face, which has been exposed to the air, will be found to possess 

 a bright scarlet colour, while the lower or unexposed part will 

 appear of a dark Modena purplish hue : only invert the clot, 

 however, and in a short time that part which is dark will turn to 

 a bright red, while the other (now excluded from the air) will 

 change to a dark purple aspect. This change of colour is wholly 

 ascribable to the action of the oxygenous part of the air. Expe- 

 riment has fully proved this fact; and also, that the other ingre- 

 dient of the atmosphere, viz. azote (as well as the carbonic acid 

 gas), has quite the contrary effect upon the blood, converting its 

 scarlet hue into purple. 



Notwithstanding the fluctuating and contradictory accounts of 

 chemical inquirers into the composition of the blood, there appears 

 little reason to doubt the existence of iron in it, and in the red 

 globules in particular the presence of the metal has been proved : 

 through whose metallic agency, we may add, it is, that oxygen 

 produces the remarkable change above alluded to. For the pre- 

 sent, let this much suffice. I shall have occasion to amplify much 

 on this part of my subject when I come to speak of respiration. 



SERUM 



Is the yellow or straw-coloured fluid which gradually exudes after 

 coagulation from the crassamentum. It has a saline taste, and is 

 adhesive in its nature, on which account it is found somewhat 

 specifically heavier than water. 



Superficial observation and analogical inference have given rise 

 to serious error in regard to the proportionate quantities of serum 

 and crassamentum in the blood of the horse. For some hours 

 after a horse's blood is coagulated, it' exhibits one uniform gela- 

 tinous mass whose surface is scarcely moistened by serous exuda- 

 tion ; whereas that of a man in the same interval of time would 

 discover the clot actually swimming in serum. The truth of the 

 matter however, is, that so far from there being a comparatively 

 less quantity of serum in horses' blood, there is actually a taroer 

 proportion ; the difference being that it requires a much lonoer 

 time for its evolution in the graminivorous than in the carnivorous 

 animal. 



Take a pint of blood from a man, and place it in a temperature 

 of 50*, and in the course of three days it will not only have per- 

 fectly resolved itself into its component parts, but will be o-rowino- 

 putrid. But draw a pint from a horse, and place it in a medium 

 of the same temperature, and serum will continue to ooze from it 

 even for a week afterwards. 



In fact, the two most essential differences between human and 



