474 A TEXTBOOK OF PHYSIOLOGY 



In the metazoa, the taking up of waste products is done by special- 

 ized organs the nephridia which are bathed by the body fluid. 



In the vertebrates there are three stages of renal development. 

 First, the pronephros which represents a collection of primitive 

 nephridia and excretes the waste products into the coelom. The 

 pronephros is represented by the kidney of fishes. Secondly, the 

 mesonephros appears ; thirdly, the metanephros. The mesonephros is 

 represented by the kidney of the amphibia, the metanephros by the 

 kidney of birds and mammals. In the development of the human 

 embryo, all three stages are represented, a transitory pronephros at 

 the third week, then the mesonephros, from which the genital organs 

 are developed, and lastly, the metanephros or permanent kidney. 

 The last is formed by the combination of two elements : a nephric or 

 secretory part, a duct or excretory part (Fig. 230). The kidney is a 

 collection of these elements, the whole being held together by con- 

 nective tissue, and compactly bound up in a covering capsule. 



The Minute Structure of the Kidney Tubule. Each tubule starts 

 in the cortex of the kidney as an expansion the capsule of Bowman 

 into which dips a tuft of capillaries the glomerulus. The wall of 

 the capsule is formed of flattened endothelium, and is involuted by 

 the tuft of the capillaries. The cells covering the glomerulus form a 

 syncytium. The glomerulus itself is a tabulated structure. The 

 endothelium of the capsule and other parts of the renal tubule rests 

 upon a homogeneous basement membrane. The capsule narrows to 

 a neck, also lined by flattened epithelium, and this passes into the 

 first convoluted tubule. Here the epithelium consists of cubical granu- 

 lar cells, called, on account of the rod-like disposition of the granules, 

 *' rodded epithelium." Leaving this convoluted portion, the tubule 

 narrows and passes down into the medulla as a long, straight limb, 

 kiied with flattened epithelium the descending loop of Henle. Turn- 

 ing suddenly upwards, it again passes into the cortex as the ascending 

 loop of Henle. Here the epithelium is cubical and granular. In the 

 cortex the tubule again expands as the distal convoluted tubule, where 

 the epithelium once more becomes wi rodded.'' Finally, the tubule 

 opens into a collecting tubule lined with a more flattened epithelium. 

 This conveys the urine to the pelvis of the kidney (Fig. 231 A). 



The Arrangement of the Blood-Supply. In the mammalian kidney 

 the renal artery, after entering the substance of the kidney at the 

 hilum, breaks up in the boundary zone between cortex and medulla 

 into a number of small vessels, which anastomose with each other, 

 and give off branches both to the cortex and to the medulla. Of 

 these, the arteriee rectae pass downwards into the medulla, and form 

 capillaries around the descending and ascending loops of Henle. The 

 main blood-supply, however, passes by straight (interlobular) branches 

 into the cortical zone. These give off on all sides small branches, 

 which pass to the glomeruli. From the glomeruli pass efferent veins, 

 which are of smaller calibre than the afferent arteries. These veins 

 partake of the nature of a " portal circulation," since from the glomeruli 



