1056 



VEETEBRATED 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) towards their trunks () ; 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 

 b b 



a 



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

 a, 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 ra.te 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 tout/lie of the froi>- 

 by laying the animal (previously chloroformed) on its bade, with it* 

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

 in tin's position, drawing out the tongue with the forceps and tixiiiu 

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

 tion may be examined in the /tun/*- where it alford-. ;i spectacle of 

 singular beauty or in the /m'w///,-/-// of the living frog l>v laving 

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

 pi-e\ iously been made insensible l>y chloroform. The t(((fj)olf of the 

 frog, when siitlicieni ly young, furnishes a good display ofthe capillary 

 circulation in its tail : and the dilliculty of keeping it ipiiet during 





