50 EVENINGS AT THE MICROSCOPE 



proceeds with perfect regularity, just as before, 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 perform their circulation in every 

 respect the same as before, but in the reverse direction. 



Those globules that pass through the vessel into the 

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

 til the course is changed, when they crowd into the heart 

 again and perform their grand tour. Yet there is a meas- 

 ure of circulation here; for even in the connecting 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 

 that this must be understood with some qualification. The 

 pulsations are not quite uniform, being sometimes more 

 languid, sometimes more vigorous; perhaps forty beats 

 in a minute may be the average; but I have counted 

 sixty, and presently after thirty; I have counted twenty 

 beats in one-half minute, and only fifteen in the next. 

 The period during which one course continues is equally 

 uncertain; but about two minutes may be the usual time. 

 Sometimes the pulsation intermits for a second or so, and 

 then goes on in the same direction; and sometimes there 

 is a curious variation in the heart's action a faint and 

 then a strong beat, a faint and a strong one, and so alter- 

 nately for some time. 



The phenomena of respiration are so closely connected 

 with those of circulation that it is not at all malapropos to 

 turn from the latter to the former; not to say that it would 

 be high treason against scientific curiosity if I were to re- 



