STUDY OF BLOOD 161 



lost their nucleus. The presence of these cells allows us 

 to consider the blood as a true tissue. 



In general, then, blood is a tissue consisting of a yellowish 

 liquid (plasma) in which float a large number of non- 

 nucleated cells (red corpuscles) and a few nucleated cells 

 (white corpuscles). 



Properties of blood. — If we mount a drop of blood on 

 a glass shde and allow it to stand for a few moments, at 

 the end of that time we note two distinct changes in its 

 appearance. First, it is darker in color, and second, its 

 surface is covered with a film which, when pricked with a 

 needle, shows an elastic consistency. This formation is 

 called a clot. If we examine the needle puncture on the 

 finger we shall find that it has ceased to bleed, and is 

 covered with a similar clot. If we wash this clot off the 

 finger, bleeding begins anew, and this gives us a hint of 

 an important use for this clot formation. An examination 

 with the microscope of the clotted blood on the slide 

 shows that it is actually a complicated network of slender 

 threads in which are entangled the corpuscles. Evidently 

 these threads must have their origin in the plasma, and to 

 understand their nature it is necessary to make a careful 

 examination of the plasma. For this purpose a larger 

 quantity of blood is necessary. 



Study of blood. — Fortunately for our purpose, the blood 

 of all warm-blooded animals is practically identical in 

 structure with human blood. This fact permits us to use 

 fresh-drawn blood of a pig or cow. (See Ex. XLIII.) 

 First, place some blood in a bottle and let it stand for a 

 time exposed to the air. Put the rest in a basin and whip 

 it vigorously with twigs. Pour the whipped blood into 

 another bottle. While these two bottles are standing we 



EDDr. PHYS. — 11 



