iO EVENmGS AT THE MICROSCOPE. 



that this must be understood witli some qualification. 

 The pulsations are not quite uniform, being sometimes 

 more languid, sometimes more vigorous ; j)erhaps 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 fif- 

 teen 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 in- 

 termits 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 con- 

 nected with those of circulation that it is not at all vial- 

 ai^rojpos to turn from the latter to the former ; not to 

 say that it would be high treason against scientific cu- 

 riosity if I were to remove this object without explain- 

 ing to you that marvellous play of wheels that occupies 

 the largest part of the area that yon behold. As you 

 look on the globe, you observe, hanging down from the 

 upj)er extremity, and reaching nearly to the bottom in 

 one direction and almost from side to side in another, 

 a transparent square veil, which is indeed a flat mem- 

 branous bag, having its sides pretty close together, with 

 email openings along its edges, and an orifice at the 

 bottom leading into the stomach. 



The mouth of this sac is in close connection with 

 the upper or principal orifice, and therefore receives the 

 water, which is constantly flowing in, while that aper- 

 ture is expanded. Tliis fluid then bathes the whole in- 

 terior of the sac, but a portion of it escapes by the 



