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. 
Fic. 21.—(a) Bird's heart showing valve between right auricle and right ventricle. 
(4)—modified from Quain—Man’s heart showing the same. Lv, wall of left ventricle ; 
kv, 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- 
