GENERAL CONSIDERATIONS. 307 



undergoes shrinkage, and presses out from its meshes a clear, straw- 

 colored fluid, which is termed the blood-serum. This gradually 

 increases in amount, while the size of the clot diminishes, and may 

 then be utilized for the purpose of chemical examination. The 

 fibrous material which is formed during the process of clotting is 

 termed fibrin. Its formation is intimately associated with the death 

 of the organism, either local or general, and is dependent in the first 

 instance upon the presence of a specific ferment the so-called fibrin 

 ferment, or thrombin of Alexander Schmidt. In the circulating 

 blood fibrin is not found, but we here meet with its mother-substance, 

 fibrinogen, which is not present in the blood-serum. The blood- 

 plasma, viz., the fluid, non-cellular portion of the circulating blood, 

 thus differs from the blood-serum in containing fibrinogenic material, 

 but not the fibrin ferment, which is found in the serum. The fer- 

 ment itself results from decomposition of the cellular elements of 

 the blood, notably of the blood-plates. This may be seen when 

 the process of coagulation is observed through a microscope. After 

 a variable length of time, more rapidly when the blood-drop is not 

 too small and when the surface of the glass is a little uneven, fine 

 filaments of fibrin thus begin to appear, which usually have for their 

 starting-point those bunch-like conglomerations of the plaques which 

 have already been described. In these bodies the pro-enzyme of the 

 ferment, the so-called prothrombin of Alexander Schmidt, is proba- 

 bly contained, and gives rise to the ferment itself when the death 

 of the cell occurs. It should be stated, however, that the fibrin fer- 

 ment is not only derived from the plaques, but may also be formed 

 during decomposition of the remaining cellular elements of the 

 blood, and, to judge from recent observations, from protoplasmic 

 material in general (see page 325). 



PHYSICAL CHARACTERISTICS OF THE BLOOD. 



Color. The color of normal blood is referable to the presence 

 of a peculiar albuminous substance in the red corpuscles belong- 

 ing to the class of proteids which is termed haemoglobin. In 

 arterial blood this is principally found in combination with oxy- 

 gen as oxyhaernoglobin, while in venous blood a mixture of both 

 occurs. With a preponderance of oxyhaemoglobin over haemoglobin 

 the color of the blood tends toward a bright scarlet-red. In its 

 absence it assumes a dark-bluish color, and we accordingly find all 

 gradations in shade between the two. When venous blood is 

 exposed to the air the haemoglobin immediately absorbs oxygen, 

 and is transformed into oxyhaernoglobin. This actually takes place 

 in the alveoli of the lungs, and explains the difference in color 

 between the blood of the right and the left heart. 



Under pathological conditions we may find still other colors than 

 those which have been described. In coal-gas poisoning the blood 

 is thus of a bright cherry-red; in poisoning with potassium chlorate, 



