CIRCULATION OF BLOOD 1 05 5 



circulation of the blood in the capillary blood-vessels which dis- 

 tribute the fluid through the tissues it nourishes. This, of course, 

 can only be observed in such parts of animal bodies as are sufficiently 

 thin and transparent to allow of the transmission of light through 

 them, without any disturbance of their ordinary structure ; and the 

 number of these is very limited. The web of the frog's foot is per- 

 haps the most suitable for ordinary purposes, more especially since 

 this animal is to be easily obtained in almost every locality ; and the 

 following is the simple arrangement preferred by the Author : A 

 piece of thin cork is to be obtained, about nine inches long and three 

 inches wide (such pieces are prepared by cork-cutters, as soles), and a 

 hole about |ths 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, but 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 ex- 

 tended, so that the foot can be laid over the central aperture. 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 be- 

 neath, or by tying the ends of the toes with 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 suffering to the animal than 

 the latter, the confinement being obviously that which is most felt. 

 A few turns of tape, carried loosely around the calico bag, the pro- 

 jecting 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 appearance 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 produced by 

 undue pressure upon the body or leg of the animal. It will not un- 

 frequently 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 

 also must 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 ; and even this may be avoided by 

 previously subjecting it to the influence of ether or chloroform, which 

 may be renewed from time to time whilst it is under observation. 



