THE CIRCULATION. Ill 



21. Action of the Heart. The substance of the 

 heart is of a deep red color, and its fibres resemble those 

 of the voluntary muscles by which we move our bodies. 

 But the heart's movements are entirely involuntary. The 

 advantage of this is evident; for if it depended upon 

 us to will each movement, our entire attention would be 

 thus engaged, and we would find no time for study, pleas- 

 ure, or even sleep. The action of the heart consists in 

 alternate contractions- and dilatations. During contrac- 

 tion the walls come forcibly together, and thus drive out 

 the blood. In dilatation, they expand and receive a renewed 

 supply. These movements are called systole and diastole. 

 The latter may be called the heart's period of repose ; and 

 although it lasts only during two-fifths of a heart-beat, or 

 about a third of a second, yet during the day it amounts 

 to more than nine hours of total rest. 



22. A remarkable property of the tissue of the heart is 

 its intense vitality. For while it is more constantly active 

 than any other organ of the body, it is the last to part 

 with its vital energy. This is especially interesting in 

 view of the fact that after life is apparently extinguished, 

 as from drowning, or poisoning by chloroform, there yet 

 lingers a spark of vitality in the heart, which, by contin- 

 ued effort, may be fanned into a flame so as to revivify the 

 whole body. In cold-blooded animals this irritability of 

 the heart is especially remarkable. The heart of a turtle 

 will pulsate, and the blood circulate for .a week after its 

 head has been cut off; and the heart will throb regularly 

 many hours after being cut out from the creature's chest. 

 The heart of a frog or serpent, separated entirely from the 

 body, will contract at the end of ten or twelve hours: that 

 of an alligator has been known to beat twenty-eight hours 

 after the death of the animal. 



21. Substance of the heart? Its fibres? Its movements? The advantage 

 of such movements ? Action of the heart ? Its period of repose ? 



22. Remarkable property of the tissue of the heart ? How shown ? How in- 

 teresting? In cold-blooded animals ? Heart of a turtle ? Of a frog? Alligator? 



