HUMAN STRUCTURE AND LIFE- ACTIVITIES 485 



Blood from a frog, fish, or bird, or permanent preparations of 

 these, may also be examined. Nuclei will be seen in the red cells 

 which are much larger than those in human blood. 



Blood from an earthworm, crayfish, insect, or other invertebrate 

 animal will show only white cells. 



407. Coagulation of Blood. Every one knows how 

 blood will flow from a cut for a time and then stop because 

 some of it has thickened into a jelly-like mass, which closes 

 over the cut and forms a scab. This process of thickening is 

 known in physiology as coagulation, or clotting. Examina- 

 tion of blood allowed to clot in a drop of normal salt solution 

 will show that coagulation is due to the formation of delicate 

 fibers which bind the corpuscles into a semi-solid mass. 

 These fibers are composed of a substance called fibrin, which 

 is simply the solid form of a protein called fibrinogen. This 

 is dissolved in the plasma of the blood and becomes hardened 

 into threads when blood escapes from a blood-vessel. If 

 fresh blood of any animal be stirred with a feather or camel 's- 

 hair brush, the fibrin will form on the feather and the de- 

 fibrinated blood will not coagulate, thus proving that the 

 formation of fibrin causes coagulation. 



The use of coagulation is obviously to prevent excessive 

 bleeding. If blood did not coagulate quickly, a slight in- 

 jury to even a small artery might cause the death of animals 

 or of men before surgical aid could be secured. 



408. Lymph. This is a liquid very much like blood, but 

 without red corpuscles. In fact, it is chiefly blood-plasma 

 which has osmosed out through the capillaries into the very 

 small spaces which exist between the cells in all tissues. 

 Thus lymph comes into direct contact with the cells, giving to 

 them food and oxygen, which are dissolved in plasma, and 

 receiving from the cells some excretions. These small 

 lymph-spaces -between the cells are united together into 

 larger lymph-capillaries, which in turn connect with the 

 large lymph-vessels. These pour their lymph into the veins 



