1056 VERTEBRATED ANIMALS 



The movement of the blood will be distinctly seen by that of its 

 corpuscles (fig. 791), which course after one another through the 

 network of capillaries that intervenes between 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 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 b) towards their trunks (a) ; the arteries, whose ultimate sub- 

 divisions 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 

 6 b 



FIG. 791. Capillary circulation in a portion of the web of a frog's foot : 

 , trunk of vein ; b, b, its branches ; c, c, pigment-cells. 



are much more plainly seen : and it may then be observed that whilst 

 the ' red ' corpuscles flow at a very rapid rate along the centre of each 

 tube, the * white ' corpuscles, which are occasionally discernible, move 

 slowly in the clear stream near its margin. 



The circulation may also be displayed in the tongue of the frog 

 by laying the animal (previously chloroformed) on its back, with its 

 head close to the hole in the cork-plate, and, after securing the body 

 in this position, drawing out the tongue with the forceps and fixing 

 it on the other side of the hole with pins. So, again, the circula- 

 tion may be examined in the lungs where it affords a spectacle of 

 singular beauty or in the mesentery of the living frog by laying 

 open its body and drawing forth either organ, the animal having 

 previously been made insensible by chloroform. The tadpole of the 

 frog, when sufficiently young, furnishes a good display of the capillary 

 circulation in its tail ; and the difficulty of keeping it quiet during 



