182 THE HUMAN BODY. 



fibrin, but this forms in it, sooner or later, when the blood 

 gets in any way out of the vessels or if the lining of these is 

 injured. By the clotting the mouths of the small vessels 

 opened in a wound are clogged up, and the bleeding, which 

 would otherwise go on indefinitely, is stopped. So too, 

 when a surgeon ties an artery, the tight ligature crushes or 

 tears its delicate inner surface, and the blood clots where 

 this is injured. The clot becomes more and more solid, 

 and by the time the ligature is removed has formed a firm 

 plug in the cut end of the artery, which prevents bleeding. 



Blood Compared with Water. " Leaving aside its color, 

 we all know that blood is thicker than water; this is true 

 not only in a metaphorical but in a literal sense. In the 

 first place, bulk for bulk, blood is heavier than water; ten 

 teaspoonfuls of blood weigh as much as ten and a half tea- 

 spoonfuls of water. Secondly, blood contains in it solid 

 corpuscles and when drawn from the body forms spon- 

 taneously a solid clot, while pure water has no solid bodies 

 floating in it, and can only be made solid by freezing. 

 Thirdly, the blood liquid itself, quite apart from the cor- 

 puscles, is thicker than pure water, because it contains a 

 great many things dissolved in it; things which are of great 

 importance, because they are the foods which the blood is 

 carrying to, and the wastes which it is carrying from, the 

 various organs of the body." 



The Composition of Blood-Serum. About one half of 

 the bulk of fresh blood is corpuscles and the other half 



When does blood clot? Illustrate the uses of the coagulating 

 property of blood. 



Compare blood with water, (1) as to the weight of equal bulks of the 

 two (specific gravity); (2) as to its microscopic structure; (3) as to its 

 tendency to solidify ; (4) as to the composition of its plasma. Why 

 arc the things dissolved in the plasma of great importance? 



What is the relative proportion of corpuscles and plasma in blood? 



