BLOOD 



315 



being somewhat amoeboid in character (Fig. 79). They can 

 pass through the walls of the capillaries and wander through 

 the tissue fluids. These leucocytes are composed of proteins, 

 glycogen, lecithin, fat, and phosphates. They are said to 

 serve as blood scavengers, carry- 

 ing away and absorbing undis- 

 solved substances in the blood 

 such as bacteria. Apparently 

 whenever a wound gives entrance 

 to bacteria the white corpuscles 

 swarm to that place and help to 

 destroy them. Although this fact 

 has not been definitely proved, it 

 is still a reasonable belief. 



(d) Coagulation. — On expos- 

 ure to the air blood clots or coagu- 

 lates, and there is formed a mass 

 of corpuscles matted together 

 with a fibrous substance. The 

 cause of this clotting is not defi- 

 nitelv known, but the theory has 

 been advanced that an enzvme 

 with the help of a calcium salt 

 acts on fibrinogen, a soluble pro- 

 tein, changing it to fibrin, an 

 insoluble protein. Fibrin is a 

 white, fibrous material which en- 

 tangles the corpuscles into a clot. 

 The liquid remaining after the 

 clot forms is called serum, and is 

 merely plasma without the fibrin- 

 ogen. By beating fresh drawn 

 blood the fibrin can be obtained 

 free from corpuscles. The speed 



of coagulation is hindered by cold, by a 10 per cent, solution 

 of ammonium oxalate or of sodium chloride. It is hastened 

 by heat, ferric chloride, and alum. Speed of coagulation 

 also varies with animals. The blood of horses clots verv 

 slowly. 



Fig. 79. — Small blood-vessel, 

 showing how leucocytes pene- 

 trate the wall. (G. Bachman.) 



