734 CIRCULATION OF BLOOD IN FROG'S FOOT. 



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 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 produced by undue 

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

 quently 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 moviug, or 

 will only give an occasional twitch. Even this may be avoided by 

 previously subjecting the animal to the influence of Chloroform ; 

 and this may be renewed from time to time whilst it is 

 under observation. The movement of the Blood will be distinctly 

 seen by that of its Corpuscles (§ 555), which course after one 

 another through the network of Capillaries that intervenes be- 

 tween the smallest Arteries and the smallest Veins : in those tubes 

 which 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 

 (Fig. 393), 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 (6, b) towards their 

 trunks (a) ; the Arteries, whose tdtimate subdivisions discharge 

 themselves into the capillary network, are for the most part re- 

 stricted 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 more plainly seen ; and it may then be observed that whilst 

 the Red corpuscles (§ 555) flow at a very rapid rate along the 

 centre of each tube, the Colourless corpuscles (§ 556) which are 

 occasionally discernible, move slowly in the clear stream near its 

 margin. 



