an
capsules_ (_A_, Fig. 88). Each capsule incloses a cluster of looped
capillaries and connects with a single tubule (Fig. 89). From the capsule
the tubule extends toward the concave side of the kidney and, after
uniting with similar tubules from other parts, finally terminates at the
pyramid. Between its origin and termination, however, are several
convolutions and one or more loops or turns. After passing a distance many
times greater than from the surface to the center of the kidney, the
tubule empties its contents into the expanded portion of the ureter.

[Fig. 88]

Fig. 88--*Sectional view of kidney.* 1. Outer portion or cortex. 2.
Medullary portion. 3. Pyramids. 4. Pelvis. 5. Ureter. _A._ Small section
enlarged to show the tubules and their connection with the capsules.

[Fig. 89]

Fig. 89--*Malpighian capsule* highly magnified (Landois). _a._ Small artery
entering capsule and forming cluster of capillaries within. _e._ Small
vein leaving capsule and branching into _c_, a second set of capillaries,
_h._ Beginning of uriniferous tubule.

The uriniferous tubules are lined with secreting cells. These differ
greatly at different places, but they all rest upon a basement membrane
and are well supplied with capillaries. These cells provide one means of
separating wastes from the blood (Fig. 90).

[Fig. 90]

Fig. 90--*Diagram illustrating renal circulation.* 1. Branch from renal
artery. 2. Branch from renal vein. 3. Small artery branches, one of which
enters a Malpighian capsule (5). 6. Small vein leaving the capsule and
branching into the capillaries (7) which surround the uriniferous tubules.
4. Small veins which receive blood from the second set of capillaries. 8.
Tubule showing lining of secreting cells.

*Blood Supply to the Kidneys.*--The method by which the kidneys do their
work is suggested by the way in which the blood circulates through them.
The renal artery entering each kidney divides into four branches and these
send smaller divisions to all parts of the kidney. At the outer margin of
the kidney, called the _cortex_, the blood is passed through _two sets of
capillaries_. The first forms the clusters in the Malpighian capsules and
receives the blood directly from the smallest arteries. The second forms a
network around the uriniferous tubules and r