BLOOD. 39 



and along the bottom, until it arrives at the entrance 

 of the heart, and rushes in to fill the vacuum produced 

 by the expansion of its walls after the periodic con- 

 traction. This is the perfect circle ; but the nainor 

 streams, that had forked off sideways in the course, as 

 those within the sac for example, find their way to the 

 entrance of the heart by shorter and more irregular 

 courses. 



One or two things connected with this circulatory 

 system are worthy of special notice. The first is, that 

 its direction is not constant but reversible. After we 

 have watched this course followed with regularity for 

 perhaps a hundred pulsations or so, all of a sudden the 

 heart ceases to beat, and all the globules rest in their 

 circling course, that we had supposed incessant. Strange 

 to behold, after a pause of two or three seconds, the 

 pulsation begins again, but at the opposite end of the 

 heart, and proceeds with perfect regularity, just as be- 

 fore, but in the opposite direction. The globules, of 

 course, obey the new impulse, enter at their former 

 exit, and pass out at their former entrance, and per- 

 form their circulation in every respect the same as be- 

 fore, but in the reverse direction. 



Those globules that pass through the vessel into the 

 foot-stalk appear to accumulate there as in a reservoir, 

 until the course is changed, when they crowd into the 

 heart again and perform their grand tour. Tet there 

 is a measure of circulation here; for even in the con- 

 necting vessel one stream ascends from the reservoir 

 into the body as the other (and principal one) descends 

 into it from the heart ; and so, vice versd. 



I have spoken of these motions as being performed 

 with regularity ; but, if you look closely, you will see 



