The kidneys may be called complex tubular glands, because the tubes of which they are composed are made up of a number of parts essentially differing from one another both in their structure and in their relation to the blood vessels.
The tubes begin by a small rounded dilatation (Malpighian capsule), which is lined by thin flattened epithelium. Opening from this capsule, Fig. 171 (g), is found a tortuous tubule (/), lined by peculiar large, rod-beset, epithelial cells, which occupy the greater portion of its diameter. This convoluted tubule (f) leads into a tube (e) of much less external diameter, but about equal lumen, owing to the thinness of its lining epithelium, the cells of which are more flattened and much thinner than those in the tortuous tubes. This thin tube forms a loop extending down to the medullary pyramid and returning to the cortex, where it can be seen to become again convoluted (d) and then to open into a straight collecting tube. The collecting tubes (c, b) receive many similar tributary tubes on their way toward the apex of the medullary pyramid, where they pour their contents into the pelvis of the kidney. The epithelial lining of these collecting tubes is of the ordinary cylindrical type.)
Fig. 170. Section of Kidney of Man.
a. Cortical substance composed chiefly of convoluted tubules; the portions between the medullary pyramids form the columns of Benin b. Pyramids of medullary substance, composed of straight tubes, etc., radiating toward cortex. d. Commencement of ureter leading from central sac or pelvis.
c. Papillae, where the tubes open into pelvis.
Fig. 171. Diagram of the Tubules of the Kidney. (Cadiat).
a. Large duct opening at papillae. b and c. Straight collecting tubes. d and e. Looped tubule of Henle.
f. Convoluted tubules of cortex.
g. Capsule from which the latter spring.
Fig. 172. Portions of various Tubules highly magnified, showing the relation of the lining epithelium to the wall of the tube. (Cadiat).
a. Large duct near the papilla.
b. Commencement of Henle's loop.
c. Thin part of Henle's loop.
We thus find four kinds of epithelial cells in the various parts of the urinary tubules, viz., scaly cells in the capsule; peculiar rod-beset glandular cells in the convoluted tubes; flattened cells in a great part of the loop; and ordinary cylindrical cells in the large straight tubes. (Figs. 172 and 173).