74 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
blood leaves the branchie and passes into the sinuses which 
connect them with the pericardial sinus (fig. 15, bev), 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, 18 at once occupied by the blood from the branchie, 
and perhaps by some blood which has not passed through 
the branchiz, 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 branchix 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, i in which the blood is sent from the heart to the 
| branchie, on its way to the body. It follows, from this 
arrangement, that the blood which goes to the branchie 
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 
