48 EVENINGS AT THE MICROSCOPE 



that of the Frog which you have just been gazing at, nor 

 does it carry disks of the same elegantly regular form. 

 But you have the advantage here of tracing, at one view, 

 the whole course of the circulation, from its first rush out 

 of the heart to its return into that organ again. 



At the bottom of the interior, below the breathing sac, 

 there is an oblong cavity, through whose centre there runs 

 a long transparent vessel, formed of a delicate membrane, 

 the appearance of which resembles that of a long bag, 

 pointed (but not closed) at either end, and then twisted 

 in some unintelligible manner so as to make three turns. 

 This is the heart; and within it are seen many minute 

 colorless globules, floating freely in a subtile fluid: this is 

 the nutrient juice of the body, which we may, without 

 much violence, designate the blood. Now see the circu- 

 lation of this fluid. The membranous bag gives a spas- 

 modic contraction at one end, and drives forward the 

 globules contained there; the contraction in an instant 

 passes onward along the three twists of the heart (the 

 part behind expanding immediately as the action passes 

 on), and the globules are forcibly expelled through the 

 narrow but open extremities. Meanwhile, globules from 

 around the other end have rushed in as soon as that part 

 resumed its usual width, which in turn are driven forward 

 by a periodic repetition of the systole and diastole. 



The globules, thus periodically driven forth from the 

 heart, now let us watch and see what becomes of them. 

 They do not appear to pass into any defined system of 

 vessels that we may call arteries, but to find their way 

 through the interstices of the various organs in the gen- 

 eral cavity of the body. 



The greater number of globules pass immediately from 



