The convoluted portion of the tube which proceeds from, and is continuous with, the Malpighian capsule is composed of a delicate basement membrane in immediate relation_ externally with an abundant capillary net-work, and lined in its interior by the spher oidal or glandular variety of epithelium. The diameter of its central canal is about of an inch. The straight portion of the tubes of which the pyramids are com posed is lined with epithelium which approaches more nearly td the scaly or tesselated variety, and which seems to serve as a protecting layer, rather than to take part in the function of secretion. The tubes unite with one another to a great degree as they pass through the structure of the pyramids, so that at the base of a pyramid there may be many thousand tubes, while the number of openings at the extremity of a papilla are comparatively few.
It now remains to consider the respective functions of these two essential elements of the kidney (as it exists in the vertebrate animals), viz., the Malpighian bodies and the tubes. From the admirable researches of Mr. Bowman (Philosophical Transactions, 1842), and from the labors of subsequent anatomists, it appears that in animals in which the urinary excretion is passed in an almost solid form (as in birds and reptiles), the tufts are small and simple as compared with those of the kidneys of animals which (like man and most mammals) pass the urinary Constituents dissolved in a large quantity of water. On
these grounds, as well as from the fact that the anatomical arrangement of the tufts is well calculated to favor the escape of water from the blood, Mr. Bowman arrives at the conclusion, that the function of the Malpighian bodies is to furnish the fluid portion (the water) of the urine. The arrangement of the convoluted portion of the tubes, with a capillary net-work on one side ,of their basement membrane, and secreting epithelial cells on the other, is the exact counterpart of the arrangement in other secreting glands, and there can be no doubt that the functions of the cells in the convoluted portion of the tubes is to separate from the blood the various organic constituents (urea, uric acid, creatinine, etc,) and inorgani,c salts (chloride of sodium and phosphate of soda, etc.), which collectively form the solid constituents of the urine. It does not necessarily follow that these secreting cells undergo rapid decay and renewal; it is more probable that they have the power of selecting certain materials from the blood, and of transmitting them, without the disintegration of their own structure, to the interior of the tube.
The physical and chemical characters of the secretion yielded by the kidneys will be considered in the article URINE.