CIRCULATION AND RESPIRATION 



135 



which precipitates the calcium and prevents coagulation. Blood 

 drawn into an oil-coated vessel may be kept from clotting partic- 

 ularly if kept at a temperature near 0°C. Blood heated to near 

 100°C. loses its power to clot, owing apparently to the destruc- 

 tion of an enzyme, thrombin, which is necessary in addition to 



Fig. 84. — Diagram of a lymph gland, a, afferent lymph vessels; c, capsule, 

 from which traveculae extend inward; e, efferent lymph vessels; n, nodule, com- 

 posed mostly of lymphocytes. (Modified after Stohr.) 



calcium to produce clotting. However, if a calcium-free solution 

 of fibrinogen is brought into reaction with a calcium-free solution 

 of thrombin, a clot is formed, the fibrin of which is practically 

 free of calcium. From this it would seem that calcium is not 

 essential to the process of clotting after the thrombin is once 



Fig. 85. — Silver nitrate preparation of a lymphatic vessel from a rabbit's 

 mesentery, showing the boundaries of the endothelial cells and a bulging just 

 beyond a valve. (From Stohr' s Textbook of Histology, by Lewis. P. Blakiston's 

 Son and Company. By permission.) 



formed and that, therefore, the role of the calcium is in the pro- 

 duction of thrombin. Thrombogen, the inactive form of thrombin, 

 is thought to occur in blood plasma and tissues. 



Corpuscles of the Frog's Blood. — Three kinds of corpuscles 

 can be distinguished in the frog's blood, viz., red corpuscles or 



