74 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



blood leaves the branchiae and passes into the sinuses which 

 connect them with the pericardial sinus (fig. 15, lev), and 

 thence into that cavity. At the end of the contraction, 

 or systole, of the heart, its volume is of course diminished 

 by the volume of the blood forced out, and the space 

 between the walls of the heart and those of the pericardial 

 sinus is increased to the same extent. This space, how- 

 ever, is at once occupied by the blood from the branchiae, 

 and perhaps by some blood which has not passed through 

 the branchiae, though this is doubtful. When the systole 

 is over, the diastole follows ; that it to say, the elasticity 

 of the walls of the heart and that of the various parts 

 which connect it with the walls of the pericardium, bring 

 it back to its former size, and the blood in the pericardial 

 sinus flows into its cavity by the six apertures. With a 

 new systole the same process is repeated, and thus the 

 blood is driven in a circular course through all parts of 

 the body. 



It will be observed that the branchiae are placed in the 

 course of the current of blood which is returning to the 

 heart; which is the exact contrary of what happens in 

 fishes, in which the blood is sent from the heart to the 

 branchiae, on its way to the body. It follows, from this 

 arrangement, that the blood which goes to the branchiae 

 is blood in which the quantity of oxygen has undergone 

 a diminution, and that of carbonic acid an increase, as 

 compared with the blood in the heart itself. For the 



