VI 



FORM AND FUNCTION 



73 



classes of animals, mammals and birds, have each 

 separately developed a perfect type of heart from 

 some lower form which allowed the pure and impure 

 blood to mix. On the left side of the heart also the 

 passage between the upper and lower cavities is 

 guarded by a valve. Both in birds and mammals it 

 is formed of two membranous flaps fastened to the 

 walls below by strong cords of the nature of tendons. 



■ M^ 



TV - 



Fig. 21. — (it) Bird's heart showing valve between right auricle and right ventricle. 

 (b) — modified from Quain — Man's heart showing the same, lv, wall of left ventricle ; 

 RV, right ventricle ; TV, tricuspid valve ; v, valve. 



In the human heart it is called the mitral valve from 

 its fancied resemblance to a bishop's mitre. There 

 are other valves as well without which the heart would 

 be very imperfect. There must be some means of pre- 

 venting the blood when it is driven into the two great 

 arteries, the aorta and the pulmonary, from returning 

 to the heart. The entrance to each, therefore, is 

 guarded by three " semilunar valves," little pockets 

 which look outwards, away from the heart, and, con- 



