6 Whitney^ 'on the Circulation in the Tadpole. 
observe that the blood-current in the tail^ even in the main 
artery or vein_, is sluggish or even still. This occurs inde- 
pendently of the hearty which may continue to beat as 
usual ; and it happens_, because the circulation in the tail 
depends very much on the motion of the organ. When this 
is suspended (as in confining the tadpole under the micro- 
scope), the blood moves sluggishly,, or stops, till the tail 
regains its freedom and motion, when the activity of the 
current is restored. This principle is thus alluded to by Dr. 
Grant : — ^' It is the restless activity of the worm and of the 
insect that makes every fibre of their body, as it were, a heart 
to propel their blood and circulate their fluids, while the slow- 
creeping snail that feeds upon the turf has a heart as com- 
plicated as that of the red-blooded, vertebrated fish, that 
bounds with such velocity through the deep. It is because 
the fish is muscular and active in every point that it requires 
no more heart than a snail to keep up the necessary move- 
ments of its blood.'^ 
Having arrived at the end of the arterial system, which 
conveys the blood from the heart to the extremities, we will 
now trace its return by the veins back again to the heart. 
The caudal vein runs near to the artery during the greater 
part of its course, with its stream, of course, towards the 
heart. This stream is swollen by perpetual tributaries flowing 
into it at all points of its course, from vessels so numerous 
that their loops form a network which covers the entire surface 
of the tail. As the vein approaches the root of the tail it 
lies superficial to the artery, and diverges from it at the point 
of entering the abdomen. Here it approaches the kidney, 
sends off" a branch which encloses that organ on the one side, 
while the main trunk continues its course on the other, re- 
ceiving tributaries from the kidney as it passes. By this 
time this vein has become the chief river of venous blood, the 
vena cava h^ferior. Passing upwards behind a coil of intes- 
tine, it approaches the liver, and runs in a curved course 
along the margin of that organ, where it receives the large 
residue of the blood from which the bile has been secreted. 
This blood is seen to enter the vena cava by numerous fine 
channels, which converge towards the great vein as it passes 
in close proximity to the organ (figs. 2, 3). Beyond the 
liver the vena cava continues its course upwards and inwards 
to its terminus in the sinus venosus or rudimentary auricle 
of the heart. This terminus is the junction of not less than 
six distinct venous trunks, incessantly pouring their blood 
into the heart. Figure 2 illustrates that the junction of 
these venous trunks, and, in part, the vessels themselves 
were revealed under the influence of the low-diet system 
