94 ELEMENTARY PHYSIOLOGY 



To the naked eye the layer of blood upon the first slide 

 will appear of a pale reddish colour, and quite clear and 

 homogeneous. But on viewing it with even a pocket lens 

 its apparent homogeneity will disapj)ear, and it will look 

 like a mixture of excessively fine yellowish-red particles, 

 like sand, or dust, with a watery, almost colourless, fluid. 

 Immediately after the blood is drawn, the particles will 

 appear to be scattered very evenly through the fluid, but 

 by degrees the}^ aggregate into minute patches, and the 

 layer of blood becomes more or less spotty. 



The " particles " are what are termed the corpuscles 

 of the blood ; the nearly colourless fluid in which they 

 are suspended is the plasma. 



The second slide may now be examined. The drop of 

 blood will be unaltered in form, and may perhaps seem to 

 have undergone no change. But if the slide be inclined, 

 it will be found that the drop no longer flows ; and, 

 indeed, the slide may be inverted without the disturbance 

 of the drop, wliich has become solidified, and may be 

 removed, with the point of a penknife, as a gelatinous 

 mass. The niass is quite soft and moist, so that this 

 setting, the clotting or coagulation, of a drop of 

 blood is sometliing very different from its drying. 



On the third slide, this process of clotting will be found 

 not to have taken place, the blood remaining as fluid as it 

 was when it left the body. The s<ilt therefore has 

 prevented the coagulation of the blood. Thus this very 

 simple investigation teaches that blood is comp(jsed of a 

 nearly colourless plasma, in which many coloured corpuscles 

 are susjjended ; that it has a remarkal)le power of 

 clotting ; and that tliis clotting may be prevented by 

 artificial means, such as the addition of salt. 



If, instead of using tlie hand lens, the drop of blood on 

 the first slide be placed under the microscope, the 

 particles, or corpuscles, of the blood will be found to be 

 bodies with very definite characters, and of two kinds, 

 called respectively the red corpuscles and the white 



