CIRCULATION OF BLOOD IN FROG'S FOOT. 617 



the arrangement which the author has found most convenient 

 for the purpose. A piece of thin cork is to be obtained, about 

 9 inches long and 3 inches wide (such pieces are prepared by the 

 cork-cutters, as soles), and a hole about 3-8ths of an inch in 

 diameter is to be cut at about the middle of its length, in such a 

 position that, when the cork is secured upon the stage, this 

 aperture may correspond with the axis of the microscope. The 

 body of the frog is then to be folded in a piece of wet calico, 

 one leg being left free, in such a manner as to confine its move- 

 ments, bat not to press too tightly upon its body; and being 

 then laid down near one end of the cork plate, the free leg is to 

 be extended, so that the foot can be laid over the central aper- 

 ture. The spreading out of the foot over the aperture is to be 

 accomplished, either by passing pins through the edge of the 

 web into the cork beneath, or by tying the ends of the toes by 

 threads to pins stuck into the cork at a small distance from the 

 aperture ; the former method is by far the least troublesome, and 

 it may be doubted whether it is really the source of more suffer- 

 ing to the animal than the latter is, the confinement being 

 obviously that which is most felt. A few turns of tape, carried 

 loosely around the calico bag, the projecting leg, and the cork, 

 serve to prevent any sudden start; and when all is secure, the 

 cork plate is to be laid down upon the stage of the microscope, 

 where a few more turns of the tape will serve to keep it in place. 

 The web being moistened with water (a precaution which should 

 be repeated as often as the membrane exhibits the least appear- 

 ance of dryness), and an adequate light being reflected through 

 the web from the mirror, this wonderful spectacle is brought 

 into view on the adjustment of the focus (a power of from 75 to 

 100 diameters being the most suitable for ordinary purposes), 

 provided that no obstacle to the movement of the blood be pro- 

 duced by undue pressure upon the body or leg of the animal. 

 It will not unfrequently be found, however, that the current of 

 blood is nearly or altogether stagnant for a time; this seems 

 occasionally due to the animal's alarm at its new position, which 

 weakens or suspends the action of its heart, the movement 

 recommencing again after the lapse of a few minutes, although 

 no change has been made in any of the external conditions. 

 But if the movement should not renew itself, the tape which 

 passes over the body should be slackened; and if this does not 

 produce the desired effect, the calico envelope must also be 

 loosened. When everything has once been properly adjusted, 

 the animal will often lie for hours without moving, or will only 

 give an occasional twitch. The movement of the blood will be 

 distinctly seen by that of the corpuscles, which course after one 

 another through the network of capillaries that intervenes be- 

 tween the smallest arteries and the smallest veins; in those 

 tubes that pass most directly from the veins to the arteries, the 

 current is always in the same direction ; but in those which pass 



