THE CIRCULATION 155 



function of nutrition, being carriers of peptone and fat. 

 Others claim that they act as destroyers of bacteria, acting 

 in this way as general scavengers of the body. But neither 

 these nor any other of the many theories advanced as to the 

 functions of the white corpuscles is capable of positive demon- 

 stration. (Read Note 1.) 



11. Coagulation. The blood, in its natural condition in the 

 body, remains perfectly fluid ; but within a few minutes after 

 its removal from its proper vessels, a change takes place. It 

 begins to coagulate, or assume a semi-solid consistence. If 

 allowed to stand, after several hours it separates into two 

 distinct parts, one of them being a dark red jelly, called 

 the coagulum, or clot, which is heavy and sinks ; and the 

 other, a clear, straw-colored liquid, called serum, which covers 

 the clot. This change is dependent upon the presence in the 

 blood of fibrin, which possesses the property of solidifying 

 under certain circumstances, one of them being the separation 

 of the blood from living tissues. The color of the clot is due to 

 the entanglement of the corpuscles with the fibrin. 



12. In this law of the coagulation of the blood is our safe- 

 guard against death by hemorrhage, or undue loss of blood. If 



I. The Blood. " You feel quite sure that blood is red, do you not ? 

 Well, it is no more red than the water of a stream would be if you were 

 to fill it with little red fishes. Suppose the fishes to be very, very small 

 as small as a grain of sand and closely crowded together through the 

 whole depth, of the stream, the water would look red, would it not? 

 And this is the way in which the blood looks red : only observe one 

 thing a grain of sand is a mountain in comparison with the little red 

 bodies that float in the blood, which we have likened to little fishes. If 

 I were to tell you they measured about the 3200th part of an inch in 

 diameter, you would not be much the wiser ; but if I tell you that in a 

 single drop of blood, such as might hang on the point of a needle, there 

 are a million of these bodies, you will perceive that they are both very 

 minute and very numerous. Not that any one has ever counted them, 

 as you may suppose, but this is as close an estimate as can be made in 

 view of what is known of their minute size." Mace's History of a 

 Mouthful of Bread. 



II. The blood in its natural condition in the body ? Describe the process by which the 

 coagulation of blood takes place. 



12. If coagulation were impossible ? How is it in fact ? 



