36 EVENINGS AT THE MICROSCOPE. 



the opposite end of the heart, and 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, 

 until the course is changed, when they crowd into the 

 heart again, and perform their grand tour. Yet there is a 

 measure 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 versa. 



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 

 alternately for some time. 



The phenomena of respiration are so closely connected 

 with those of circulation that it is not at all out of the way 

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

 would be high treason against scientific curiosity if I were 

 to remove this object without explaining to you that mar- 

 vellous play of wheels that occupies the largest part of the 

 area that you behold. As you look on the globe, you ob- 



