102 CIRCULATION 



10 mm. Hg. They form therefore, but a feeble force pump 

 which completes the filling of the ventricles. Their value in 

 this respect becomes important when the heart rate increases, 

 for then the ventricular pause is shortened and the auricles 

 form an efficient mechanism for quickly charging the ventricles. 

 The negative pressure of the auricles has been found to be as 

 low as minus 10 mm. Hg. This is caused by the elasticity of 

 the lung, which just at this moment is heightened because 

 the contracting ventricle increases the negative pressure in 

 the chest. The mouths of the great veins emptying into the 

 auricles are unprovided with valves, but are rich in circular 

 muscular fibers. 



The systole of the heart begins in a small mass of embryonic 

 tissue, the sino-auricular node, situated near the beginnings 

 of the great veins. Owing to its more rapid rate of rhythmic 

 discharge this part of the heart sets the pace for the remainder. 

 Under special conditions other parts of the heart may initiate 

 the rhythm. The diversity of opinion with reference to auto- 

 maticity and rhythmical power of the heart residing in nervous 

 or muscular structures has given rise to rival sets of theories 

 the neurogenic and myogenic respectively. In Limulus, at 

 least, Carlson has definitely shown these powers to reside 

 in a nervous apparatus but whether the same holds true for 

 mammalia is still an open question. 



The cause of the rhythmic movements of the heart lies within 

 itself, since it can be severed from the central nervous system 

 without necessarily destroying its activity. 



A strip of muscle cut from the apex of a tortoise's heart 

 in a zigzag manner, and suspended in a moist chamber, may 

 beat as long as thirty hours with a slow rhythm. Very small 

 microscopic pieces from the bulbus aortse of the frog, which 

 are probably devoid of nerve cells, contract rhythmically. Cu- 

 rarized striated muscle placed in certain saline solutions will 

 show a regular rhythm for hours. Many invertebrates have 

 hearts that are not provided with nerve cells. The heart of 

 the embryo beats before the nerves have grown into it. 



The cardiac contraction is preceded by a change of electrical 

 potential, which sweeps over it in the form of a wave. Both 

 normally take the same course, beginning at the great veins 

 and spreading rapidly over the auricles, then a short pause, 



