VIII 



THE EXCRETION OF URINK. 



457 



the formation of urine. Of course this would not suffice to 

 decide between the theory of Bowman and Heidenhain and 

 that maintained by Sobieranski. Against this last theory, how- 

 ever, we may adduce almost if not quite all the objections made 

 by Heidenhain to Ludwig's contention. It further seems to us 

 improbable on the argument by analogy, in view of the fact that 

 the epithelial cells in all other tubular glands function as organs 

 of external secretion, although this does not preclude the possi- 

 bility of their simultaneously affecting the constitution of the 

 blood, by an outpour into the lymph of other special products of 

 an internal secretion. But this view, which was held by Brown- 

 Sequard, Fra'nkel, Meyer and others is far from proved, since both 

 the experimental facts and the clinical data on which it is 

 founded lend themselves to quite another interpretation. 



Among the later histological researches into the activity of 



i: 



Fir;. 125. Epithelium from pelvis of human kidney. (Kulliker.) 350 diameters. A, different 

 kinds of epithelial cells ; B, the same, in situ. 



the kidney, we must cite the work of Gurwitsch (1902), who 

 investigated the elimination of indifferent aniline pigments on 

 frogs. He saw that they were excreted by means of vacuoles, 

 similar to those observed in Protozoa. The vacuoles, loaded with 

 pigment, gradually advance to the surface of the epithelial cells, 

 where they discharge their content into the lumen of the duct. 

 The latest experiments of Hober and Konigsberg (1905) on 

 pigment excretion by the kidneys has completely confirmed and 

 extended the results of Gurwitsch, so that histology also pro- 

 nounces decidedly in favour of the Bowman-Heidenhain theory. 



VIII. The urine flowing from the collecting tubules of Bellini 

 is conducted from the renal pelvis by the Ureters, which are ducts 

 about the width of a goose-quill, 30-40 cm. long, with strong walls 

 consisting of an external fibrous coat, a middle coat of plain 

 muscular tissue (with two layers of longitudinal fibres, and one 

 thicker intermediate circular layer) ; and an internal mucous 

 coat with an epithelium composed of four layers in which the 

 cells differ in size and shape (Fig. 125). 



