CIRCULATION 353 



The Heart in Action. — In a quiet room, the pulsation of the heart 

 may be distinctly heard. A long sound, lub, is followed by a short 

 one, dub. The first sound is caused by the contraction of the 

 muscles of the heart; the latter sound by the closing of the valves 

 in the heart. The action of the heart is somewhat like that occur- 

 ring when we squeeze water through a rubber bulb. Blood enters 

 the auricles from the veins because the muscles of that part of the 

 heart relax; this allows the space within the auricles to fill. Al- 

 most immediately the muscles of the ventricles relax, thus allow- 

 ing blood to pass into the chambers within the ventricles. Then, 

 after a short pause, during which time the muscles of the heart are 

 resting, a w^ave of muscular contraction begins in the auricles and 

 ends in the ventricles, with a sudden forceful contraction which 

 forces the blood out into the arteries. 

 This contraction of the heart is known 

 as a systole. The extension of the muscles, 

 to allow the auricles and ventricles to fill, 

 is called a diastole. Blood is kept on its 

 course by the valves, which act in the same 

 manner as do the valves in a pump, thus 

 forcing the blood to pass into the arteries ^ .• r . 



^ , ^ Transverse section or an artery, 



upon the contraction of ventricle walls. showing muscular walls. 



The Work of the Heart. — The work performed by the heart is consid- 

 erable. The two ventricles, at each pulsation, expel about a cup and a half 

 of blood into the arteries of the body. The average rate of the heart beat is 

 about seventy to the minute, so that the work of the ventricles, in a single 

 day, is estimated to release enough energy to lift 193 tons one foot from the 

 ground. The heart is estimated to do as much work in a single day as a 

 moderately heavy man would perform in climbing a mountain 3600 feet 

 in height. 



Demonstration. The Circulation of Blood in a Frog's Foot. — Bore a 

 half inch hole in one end of a shingle. Wrap a live frog in wet flannel or 

 absorbent cotton, and bind it, by means of elastic bands, upon the board 

 so that the web of the foot is stretched in a horizontal position over the hole. 

 Keep the web of the foot wet. Cover it with a large coverslip. Place it 

 on the stage of a compound microscope, focus with low and then with the 

 high power. A network of blood vessels will be seen which maj^ be partially 

 obscured by numerous pigment cells (dark-colored cells of irregular shape). 

 The blood vessels may easily be recognized by the fluid contents, the ovoid 

 corpuscles floating in the transparent plasma. Note that in some of the 

 blood tubes the blood moves in regular spurts. These are the arteries. 



hunter's BIOL. — 23 



