VERTEBRATED ANIMALS. 287 



arrangement preferred by the Author: A piece of thin Cork is ta 

 be obtained, about 9 inches long and 3 inches wide (such pieces are 

 prepared by Cork-cutters, as soles), and a hole about 3-8th 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 corre- 

 spond 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 man- 

 ner as to confine its movements, but not to press to 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 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 beneath, 

 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 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 ex- 

 hibits 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 unfrequently 

 be found, however, that the current of blood is nearly or altogether stag- 

 nant 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 chloroform, which may be 

 renewed from time to time whilst it is under observation. The move- 

 ment of the Blood will be distinctly seen by that of its corpuscles (Fig. 

 478), which course after one another through the network of Capillaries 

 that intervenes between the smallest arteries and the smallest veins; in 

 those tubes Vhich pass most directly from the veins to the arteries, the 

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

 between these, it may not unfrequently be seen that the direction of the 

 movement changes from time to time. The larger vessels with which 

 the capillaries are seen to be connected, are almost always veins, as may 

 be known from the direction of the flow of blood in them from the 

 branches (#, b) towards their trunk (); the arteries, whose ultimate 

 subdivisions discharge themselves into the capillary network, are for the 

 most part restricted to the immediate borders of the toes. When a 

 power of 200 or 250 diameters is employed, the visible area is of course 

 greatly reduced; but the individual vessels and their contents are much 



