262 THE BLOOD AND THE LYMPH 



One of these is the insoluble fibrin, while the other is a globulin called 

 fibrin-globulin. This latter is a mere by-product, so far as known, 

 of this reaction, whose sole purpose is to construct the fibrin-mesh. 

 This is substantially the doctrine developed especially by Schmidt, Pekal- 

 haring, and Hammarsten. More lately evidences of a precursor of the 

 thrombin have been discovered which is perhaps also an enzyme. Thus 

 Morawitz supposes that a substance called prothrombin or thrombogen 

 (occurring only in blood and lymph) under the influence of thrombo- 

 kinase (an enzyme occurring in all tissues), unites with the calcium salts 

 to form the ferment thrombin (this "thrombogen" being the nucleo- 

 proteid spoken of above). Besides the thrombocytes, the leukocytes 

 may take part in the reaction by furnishing part of the nucleo-proteid. 

 It is very doubtful, however, if this substance exists in solution in the 

 plasma, because some of the body-fluids, such as hydrocele fluid, con- 

 taining no platelets or leukocytes, fail to coagulate, although rich in 

 fibrinogen. The precise reaction of the calcium in coagulation has not 

 yet been demonstrated; it is not strictly indispensable to coagulation, for 

 barium or strontium may be made artificially to replace it. These 

 act much less well than calcium, however, and do not occur in appreciable 

 amount in the blood or the tissues. It is possible that it may be the 

 ionic values of these three melats which are the determining factors in 

 their still mysterious function. Whatever be the mode of action, in 

 normal coagulation calcium salts of some kind are necessary. They 

 seem to be antagonized in this function by sodium and potasium, a fact 

 of possible importance in explaining the inhibitory influence of the 

 endothelial protoplasm. The relations of the gross elements entering 

 into coagulation are almost graphically represented thus : 



f Serum 



The plasma with the corpuscles constitutes blood, and in coagulation 

 divides into serum and fibrin, which with the corpuscles forms the clot. 



Besides blood, lymph, muscle-plasma, and milk normally coagulate. 

 The lymph clots in much the same way as does blood, but with a less 

 solid coagulum because of its lack of erythrocytes, and somewhat more 

 slowly. Muscle-plasma coagulates in a similar way, the clot being 

 called myosin. Milk coagulates in the stomach and duodenum by about 

 the same sort of reaction as obtains in the case of blood; rennin is the 

 active agent instead of thrombin, but calcium is similarly necessary. 

 The aqueous humor of the eye and the pericardial fluid contain no 

 thrombin, but having all the other essentials for coagulation, clot on the 

 addition of this ferment. (See Plate V.) 



The Physical Constitution of the Blood and Lymph. The morphology 

 of the blood and lymph embraces mainly the description and the functions 

 of the corpuscles contained in them and the mechanical relations of these 

 corpuscles to the circulating liquids. We have already seen how com- 



