THE BLOOD. 13 



ART. II. THE BLOOD. 



OF all the fluids in the animal economy, the most interest- 

 ing and the most important is the Blood : and it is an appre- 

 ciation of this fact which has led to the concentration upon 

 its study, in times past as well as present, of the powers of a 

 host of able and gifted observers, whose labours have not 

 been without their reward. 



The knowledge of this fluid acquired by the early physician 

 was of a very limited character, it being confined to the 

 observance of a certain number of external and obvious 

 appearances, such as the colour, consistence, and form of the 

 effused blood. Limited as this knowledge was, however, 

 compared with that which, in our favoured day, we enjoy, it 

 was yet not without its practical utility. 



More recently the chemist, who is in these times extend- 

 ing in all directions so rapidly the boundaries of his domain, 

 has cast upon this particular portion of it a flood of light. 

 Who, to look upon a dark and discoloured mass of blood, 

 could imagine that the magic power of chemistry could 

 reveal in it the existence of not less than forty distinct and 

 essential substances? 



Lastly, the micrographer, with zeal unweariable, has even 

 outstripped the progress of his rival the chemist, and brought 

 to light results of the highest importance. It is these results 

 that in this work we have more especially to consider. 



In the following pages we shall have to treat of the blood 

 under various aspects and conditions ; we shall have to regard 

 it alive and dead, circulating within its vessels, and motion- 

 less without them ; as a fluid and as a solid ; healthy and 

 diseased ; or, in other words, we shall have to consider the 

 blood physiologically, pathologically, and anatomically. 



DEFINITION. 



The blood may be defined as an elaborated fluid, having 

 usually a specific gravity of about 1*055, that is, heavier than 



