Il6 HUMAN PHYSIOLOGY. 



are always present in the blood. The formation of thrombin which would 

 cause coagulation is prevented by the presence of an anti-thrombin. As 

 soon as blood is shed or tissues are injured a new substance thrombinoplastin 

 is developed which neutralizes the anti-thrombin. This having been accom- 

 plished the calcium is enabled to activate the pro thrombin with the produc- 

 tion of thrombin and hence fibrin (Howell.) 



Conditions Influencing Coagulation. The process is retarded by cold, 

 retention within living normal vessels, neutral salts in excess, the injection 

 of commercial peptone, etc. 



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

 the presence of foreign bodies, whipping, etc. 



Blood coagulates in the body after the arrest of the circulation in the 

 course of twelve to twenty-four hours; local arrest of the circulation, 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 bluish red color 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, protein 

 matter, and sugar; occasionally it contains 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 protein and fat 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, measuring 

 about 5$ inches in length and about 3$ in breadth, weighing from 10 

 to 12 ounces in the male and from 8 to 10 in the female. Situated in the 

 thoracic cavity, between the lungs, its base is directed upward, backward, 

 and to the right; its apex is directed downward and to the left. 



