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AN AMERICAN TEXT-BOOK OF PHYSIO/A OG V. 



in the width of the stream-bed, and a high blood-pressure as compared with 

 ordinary capillaries. Surrounding; this glomerulus is the double- walled capsule. 

 < me wall of the capsule is closely adherent to the capillaries of the glomerulus; 

 it not only covers the structure closely, but dips into the interior between the 

 small lobules into which the glomerulus is divided. This layer of the capsule 

 is composed of flattened endothelial-like cells, the glomerular epithelium, to 

 which great importance is new attached in the formation of the secretion. It 

 will benoticed that between the interior of the blood-vessels of the glomerulus and 



Fig. 65.— Portions of the various divisions of the uriniferous tubules drawn from sections of human 

 kidney: .1, Malpighian body ; x, squamous epithelium lining the capsule and reflected over the glomer- 

 ulus ; //, e, a Hi rent and efferent vessels of the tuft; e, nuclei of capillaries; n, constricted neck marking 

 passage of capsule into convoluted tubule ; B, proximal convoluted tubule : C, irregular tubule; D and 

 F, spiral tubules ; E, ascending limb of Henle's loop; G, straight collecting tubule (Piersol). 



the cavity of the capsule, which is the beginning; of the uriniferous tubule, there 

 are interposed only two very thin layers, namely, the epithelium of the capil- 

 lary wall and the glomerular epithelium. The apparatus would seem to afford 

 most favorable conditions for filtration of the liquid parts of the blood. The 

 epithelium clothing the convoluted portion- of the tubule, including under this 

 designation the so-called irregular and spiral portions and the loop of Henle, is 

 of a character quite different from that of the glomerular epithelium (Fig. 65, B, 

 C, D, E, F, G). The cells, speaking generally, are cuboidal or cylindrical, proto- 

 plasmic, and granular in appearance; on the side toward the basement mem- 

 brane they often show a peculiar >t nation, while on the lumen side the extreme 

 periphery presents a compact border which in some cases shows a cilia-like 

 itriation. These cells have the general appearance of active secretory struc- 

 tures, and recent theories of urinary secretion attribute this importance to them. 

 Composition of Urine. — The chemical composition of the urine is very 

 complex, as we should expect it to be when we remember that it contains most 

 of the end-products of the varied metabolism of the body, its importance in 

 this respect being greater than the other excretory organs such as the lungs, skin, 

 and intestine. The secretion is a yellowish liquid which in carnivorous ani- 

 mal- and in man has normally an acid reaction, owing to the presence of acid 



