JVj THE BLOOD AND THE ORGANS OF CIRCULATION 59 



rate in any great quantity) in normal healthy tissues. To re- 

 capitixlate, when a vessel is ru])tured so that blood escapes from 

 it, the injured vessels will liberate thrombokinase. This thrombo- 

 kinase enters into the blood, and there it finds all the necessary 

 factors for the production of thrombin : it finds ]irothrombin and 

 calcium salts. Thrombin is therefore formed, and this causes the 

 fibrinogen to coagulate very quickly. This clotting helps to stop 

 bleeding, and if the injury is slight allows the \\ound to heal 

 without further loss of blood. 



The Heart. The blood is kept in constant circulation through 

 the vessels by the heart which acts as a double pump, the right 

 and left portions of which have no direct commiuiication with 

 each other. The heart is situated between the lungs in a chamber 

 called the pericardium. This is of the nature of a double bag, 

 one covering forming a thin layer closeh' adherent to the heart 

 itself, while the other or outer one envelops the heart more 

 loosely. Between these tMO coverings is the pericardial fluid 

 which is a form of lymph. The right and left portions of the 

 heart referred to above each consist of an auricle and a ventricle. 

 These are divided from one another by a movable transverse 

 partition and communicate with one another b}^ valves which admit 

 of the blood flowing in one direction only, namely from the aiiricle 

 to the ventricle. The apex of the heart is at the end of the ven- 

 tricular portion which has a much thicker wall than either of the 

 auricles. The valves just mentioned are of the same type on 

 each side of the heart, but whereas the one on the right side has 

 three flaps (the tricuspid valve) that on the left side has only two 

 flaps (the mitral valve). Each flap is fastened at the base to the 

 auriculo-ventricular junction, while its free end points towards the 

 ventricle. The free edges of the flaps are connected by tendons, 

 the chordae tendineae, to muscular processes (the musculi papil- 

 lares) on the inner walls of the ventricle, and these prevent the 

 flaps from being carried backwards into the auricular cavity 

 when the ventricle becomes full of blood. Consequently, whereas 

 increased pressure in the auricle forces the blood to flow into 

 the ventricle by the flaps opening, the reverse process cannot 

 take place excepting in certain kinds of heart disease when the 

 valves are damaged or do not properly close. Coming away from 

 each ventricle there is an artery (pulmonary artery on right side 

 and aorta on left) which is provided with \ alves of another type. 



