52 



ELEMENTARY PHYSIOLOGY 



LESS. 



the width of the heart fx'oin side to side becomes less ; pro- 

 bably also the depth from back to front is at the same 

 time slit^htly increased. The result of tliis is that whereas 

 during diastole tlie shape of a section of the ))ase <}f the 

 ventricles is elliptical, during systole it becomes much 

 more nearly circular. 



Fio. 20.— Tkansvlusk Skction ■iiiu<>i(iii the .middli-: of the Ventricles 

 OF A Doo's Heart in Diastole and in SvsTt)LE. (After Uekse.) 



R. V. right ventricle ; Z. V. left ventricle. 



Tlie length of the heart is very slightly lessened, if at 

 all, during systole, but the heart as a whole is twisted to 

 a certain extent on its long axis, from the left and beliind 

 towards the front and riglit. The apex is at the same, 

 time tilted slightly forward and is hence pressed rather 

 more firmly against the wall of the thorax, a fact of some 

 impoi'tance in connection vvitli what we shall describe 

 presently as the "cardiac impulse" (see p. 57). 



7. The Action of the Valves. — Having now acejuired 

 a notion of tlie arrangement of the difierent pipes and 

 reservoirs of the circulatory system, of the position of the 

 valves, and of the rhythmical contractions of the heart, it 

 will be easy to comprehend what must happen if, when 

 the whole apparatus is full of l)lood, the first step in the 

 pulsation of the heart occurs and the auricles contract. 



By this action each auricle tends to scjueeze the fluid 

 which it contains out of itself in two directions — the one 

 towards the great veins, the other towards the ventricles ; 

 and the direction which the blood, cis a whole, will take, 



