36 EVENINGS AT THE MICE0SC0PE. 



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 pulsa- 

 tions 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, pre- 

 sently 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- 



