150 GENERAL ZOOLOGY 



ments made in 1875. Using Rana esculenta, Nussbaum cut off 

 the circulation to the glomeruli by completely ligating the renal 

 arteries. If a solution of a dye, indigo-carmine, is then injected 

 into the venous system, the color is later found only in the 

 loop of the tubule (proximal convoluted part) near the dorsal 

 surface of the kidney, the other segments of the tubule remaining 

 free of color. Practically no water is excreted. This shows that 

 dyes are absorbed from the blood by the portion of the tubule 

 supplied by the renal portal circulation. If a 10 per cent solution 

 of urea is injected into the venous system instead of dye, the 

 bladder becomes filled with urine in the course of two or three 

 hours. This experiment shows that the uriniferous tubule, 

 deprived of its glomerulus, is able to excrete urine under the 

 stimulus of urea in the blood. Further, more recent work indi- 

 cates that the urine is concentrated by the resorption of water 

 in the distal convoluted tubule. Thus it would seem that the 

 fluid absorbed from the glomeruli, largely water, as it flows down 

 the tubule receives the specific excretion of the proximal con- 

 voluted region, the mixture being further modified by the resorp- 

 tion of water as it passes through the distal convoluted region. 



Most of the nitrogen leaves the body in the form of urea, which 

 is largely formed in the liver. Urine also contains inorganic salts 

 such as sulphates, chlorides and phosphates of sodium, potassium, 

 calcium and magnesium, as well as other substances in small 

 quantities. There is evidence to show that in addition to most 

 of the water, the glomeruli also excrete albumin, sugar, and 

 inorganic salts. 



The Bladder of the Frog. — The ureters open close together in 

 the dorsal wall of the cloaca. The bladder arises as an evagina- 

 tion of the ventral wall of the cloaca, opposite the mouths of the 

 ureters. In its fully developed state the bladder is attached to 

 the cloaca at its point of origin by a narrow pedicle, beyond 

 which it expands into a thin-walled, bilobed distensible vesicle 

 (Fig. 17). It is lined with a stratified epithelium and contains 

 plain muscle in its wall. The external opening of the cloaca is 

 closed by a sphincter muscle. Urine passes from the ureters into 

 the cloaca and collects in the bladder, from which it is expelled 

 rather abruptly by the contraction of the muscles of the body wall. 



The Human Kidney.— The human kidneys (Fig. 97) are 

 bean-shaped retroperitoneal organs, attached to the dorsal body 



