618 



VERTEBRATES ANIMALS. 



across between these, it may not unfrequently be seen that the 

 direction of the movement changes from time to time. The 

 larger vessels (Fig. 327), with which the capillaries are seen to 



FIG. 327. 



Capillary circulation in a portion of the web of a Frog's foot: 1, trunk of vein; 2, 2, its branches; 



3, 3, pigment-cells. 



be connected, are almost always veins, as may be known from 

 the direction of the flow of blood in them from the branches 

 (2, 2) towards their trunks (1) ; 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 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 colorless corpuscles which are 

 occasionally discernible, move slowly in the clear stream near 

 its margin. 



432. The circulation may also be displayed in the tongue of the 

 Frog, by laying the animal down 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. This method, however, 

 is so much more distressing to the animal, that its employment 

 seems scarcely justifiable for the mere purpose of display; and 

 nothing but some anticipated benefit to science, can justify the 

 laying open of the body of the living animal, for the purpose of 



