i8o 



BIOLOGY AND HUMAN LIFE 



Fig. 94. An X-ray view of arteries of the arm 



The arteries of a new-born baby (dead) were injected 



with a mixture that is opaque to the X-rays, originated 



by Dr. Eben C. Hill. The blood vessels reach all parts 



of the organ. (Courtesy of Johns Hopkins Bulletin) 



solidifying is in the 

 form of fine fibers. 

 The ferment that 

 causes the coagula- 

 tion becomes active 

 when the lining of 

 a blood vessel is 

 injured ; possibly it 

 is formed only at 

 such times. At the 

 mouth of a small 

 cut this clot soon 

 stops the bleeding, 

 and it furnishes a 

 protective covering 

 until the wound is 

 healed. 



149. Serum. If 

 blood is allowed to 

 stand for a time in 

 a glass vessel, we 

 can see the mass of 

 fibers detach them- 

 selves from the 

 walls of the vessel 

 until the clot floats 

 at last in a clear 

 liquid which is al- 

 most colorless or 

 slightly yellowish. 

 This clear liquid is 

 called serum and is 

 practically the same 

 as blood plasma but 

 lacking the protein 

 fibrinogen. What- 



