522 EMBRYOLOGY OF THE CHICK 



tube, and that the blood is formed in the blood-islands by the cavities 

 flowing together. As these cavities fuse they became tubular, forming 

 the vitelline veins which carry the blood to the heart. 



It is of the utmost importance to remember that this early heart 

 tube, even before the blood passes through it, has a slow, irregular 

 "beat." This, however, is not a true heart-beat, but merely the func- 

 tional movement of living muscle. 



The true heart-beat is established at that particular moment when 

 the thin membrane which separates the anterior from the posterior por- 

 tion in the tubular heart breaks through by the greater pressure of the 

 blood from the posterior region pressing forward. The tiny membrane 

 can be seen to bulge out toward the head region until it finally breaks. 



From that moment on, the blood forces its way through the heart 

 and begins a rhythmic muscular reaction on the part of the heart. 



The architecture of all muscles is such that various muscle cells 

 are antagonistic to other muscle cells in the same group, so that each 

 muscle can, if it elongates, also contract and shorten, the two sets of 

 fibers being mutually antagonistic, so as to retain a normal balance. 

 The heart muscle shows this principle admirably in that it is composed 

 of two groups of spirally wound muscle fibers, the one unwinding as 

 the other winds up, thus causing a mutual interaction which keeps up 

 by the rhythm of the heart-beat. 



From the study of physics we know that when two streams which 

 run in different directions meet, a vortetf is formed. If we now turn to 

 our earlier description of the development of the circulatory system 

 during the first two days, we shall find that there are two openings into 

 the heart from which streams of blood are brought into that organ. As 

 these two blood vessels send their streams together, a vortex is formed. 

 We thus find a physical explanation as to why the heart muscles follow 

 in their growth the optimum stretching caused by the spirally running 

 stream of blood. 



From all that has been said above, it follows that when a heart is 

 removed from an animal body and kept "alive" for days or weeks, it 

 is but the physical continuation of the normal muscular antagonistic re- 

 action of the two spiral shaped groups which have been wound up quite 

 as a clock is wound. 



As months and years have elapsed in the winding of these spiral 

 muscles, it is quite natural to understand that they are still sufficiently 

 wound when removed from the body so that they will continue in action 

 for some days if no external conditions exist to cause a stoppage sooner. 

 Such external conditions may be pressure, friction of various kinds, or a 

 drying up of the tissues when not retained in proper media. 



If the immediately preceding paragraph be remembered, one can 

 always explain such objections as this: "If potassium is removed from 

 the medium in which a heart is placed, it ceases to function, thereby 



