258 TEXT-BOOK OF PHYSIOLOGY. 



General Composition. The results of the analyses of the blood will 

 vary with the animal and the methods employed. The following table, 

 taken from Gad, shows the average composition, expressed in whole numbers, 

 of horse's blood. In essential respects the ratio of the constituents in human 

 blood would not be materially different. 



One thousand parts of blood contain: 



Cells.. 



Plasma. 



CHEMISTRY OF COAGULATION. 



The changes which eventuate in the formation of fibrin, and hence all 

 the subsequent phenomena of coagulation, are chemic in character; but as 

 these changes take place in organic compounds the composition of which is 

 but imperfectly known, the intimate nature of the process is quite obscure. 

 All the theories which have been advanced in explanation, though approxi- 

 mating the truth, are more or less incomplete and in some respects contra- 

 dictory. Since the coagulation is coincident with the appearance of the 

 fibrin, the antecedents of this substance, the physical and chemic conditions 

 which condition its development, and the succession of chemic changes 

 involved must be determined, before any consistent theory can be established. 



Extra-vascular Coagulation. At present it is generally believed 

 that the immediate factors concerned in extra-vascular coagulation are 

 fibrinogen, a calcium salt, and an agent thrombin. As to the manner in 

 which these three bodies react one with another there is a diversity of opinion. 



As an outcome of a long series of experiments that have been performed 

 to determine the nature and the succession of the chemic phenomena 

 underlying the coagulation of the blood, the following facts seem to be well 

 established, viz: the immediate cause of the coagulation is the appearance 

 of fibrin, a derivative of an antecedent substance always present in the blood 

 termed fibrinogen; the cause of the conversion of the soluble fibrinogen into 

 the insoluole fibrin is the presence and activity, under the circumstances, of 

 an agen^ termed thrombin, the chemic nature of which is a subject of discus- 

 sion. /By some chemists it is regarded as a ferment which causes a mo- 

 lecular rearrangement of the fibrinogen; by others it is regarded as a definite 

 organic colloidal body which unites in some physico-chemic manner with 

 tiie fibrinogen to form fibrin. 



The crux of the problem is the source and the conditions necessary for 

 the production of the thrombin. It is generally conceded that thrombin is a 

 derivative of an antecedent substance prothrombin or thrombogen, a substance 

 always present in the blood plasma, a product of the decomposition of blood- 

 platelets and leukocytes. With prothrombin there is physiologically associ- 

 ated a calcium salt, the presence of which is absolutely essential for coagula- 



