CIRCULATION OF THE BLOOD. 129 



of fibrinogen previously present. The origin of the ferment is 

 obscure, but there is reason to believe that it comes from the injured 

 vascular coats or from the breaking of the white corpuscles. 



Conditions Influencing Coagulation. The process is retarded by 

 cold, retention within living vessels, neutral salts in excess, inflam- 

 matory conditions of the system, imperfect aeration, exclusion from 

 air, etc. 



It is accelerated by a temperature of 100 F., contact with air, 

 rough surfaces, and rest. 



Blood coagulates in the body after the arrest of the circulation 

 in the course of twelve to twenty-four hours ; local arrest of the cir- 

 culation, from compression or a ligature, will cause coagulation, thus 

 preventing hemorrhages from wounded vessels. 



The composition of the blood varies in different portions of the 

 body. The arterial differs from the venous, in being more coagulable ; 

 in containing more oxygen and less carbonic acid ; in having a 

 bright scarlet color, from the union of oxygen with hemoglobin, 

 the purple hue of venous blood results from the deoxidation of the 

 coloring-matter. 



The blood of the portal vein differs in constitution, according to 

 different stages of the digestive process ; during digestion it is 

 richer in water, albuminous matter, and sugar ; occasionally it con- 

 tains fat ; corpuscles are diminished, and there is an absence of 

 biliary substances. 



The blood of the hepatic vein contains a larger proportion of red 

 and white corpuscles ; the sugar is augmented, while albumin, fat 

 and fibrin are diminished. 



CIRCULATION OF THE BLOOD. 



The circulatory apparatus by which the blood is distributed to 

 all portions of the body consists of a central organ, the heart, 

 with which is connected a system of closed vessels known as 

 arteries, capillaries, and veins. Within this system the blood is 

 kept, by the action of the heart, in continual movement, distributing 

 nutritive matter to all portions of the body and carrying waste 

 matters from the tissues to the various eliminating organs. 



The heart is a hollow, muscular organ, pyramidal in shape, meas- 

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