THE BLOOD 159 



While the blood is in motion within a healthy blood 

 tube, no clotting occurs, but as soon as blood is drawn, 

 it clots, or if a blood vessel is wounded, a clot forms at 

 the wounded spot. The use of clotting is to stop bleeding. 

 Sometimes no clot will form, but a wound will keep on 

 bleeding until it is healed. This is a disease called hemo- 

 philia, and may cause death. 



278. Anemia. Sometimes there are too few red cor- 

 puscles in the blood. Then the skin appears pale and 

 there is shortness of breath, because too little oxygen is 

 carried by the diminished number of red blood cells. The 

 disease is called anemia, meaning lack of blood. It is 

 mainly a lack of red corpuscles. 



279. Good and bad blood. The terms good and bad blood 

 are remnants of the old idea that disease was caused by watery sub- 

 stances, called humors, in the blood. From their supposed influence 

 on the mind the terms good and bad humored are derived. 



For many years attempts have been made to inject healthy blood 

 into the veins of sick persons. Injecting a liquid into the veins of a 

 living person is transfusion. In bleeding, the loss of water is one of 

 the greatest dangers, and to replace it water is sometimes injected into 

 the veins. It answers better than blood itself. 



260. The blood in lower animals. All living beings 

 possess some form of fluid circulating in their interior. 

 In higher animals, birds, reptiles, and fishes, the fluid is 

 red, and contains both red and white corpuscles. In 

 insects the blood is usually white or colorless. In worms 

 the blood is sometimes colorless and sometimes red or 

 green. In shellfish the blood is colorless, and contains no 

 corpuscles. In animals which are made up of a single 

 microscopic speck of matter, there seems to be a continual 

 motion of fluid within their bodies, although they are so 

 extremely small that nothing definite can be seen. 



