THE KIDNEYS. 



1881 



scribes a wavy or spiral course in its descent to the pyramid. This, the spiral tubule of Schachowa, 

 differs from the preceding in the gradual reduction of its diameter (.35-040 mm.) and in the 

 thickness of the epithelial lining, the cells of which, although retaining the general character of 

 those of the convoluted tubule, exhibit a distinct demarcation from one another and a narrow 

 homogeneous inner zone. The spiral tubules are distinguishable from the surrounding collecting 

 tubules by the lighter sharply defined cuboidal lining cells of the latter. Just before passing into 

 the medulla to become the descending limb of Henle's loop, the spiral tubule diminishes in width 

 and in consequence ends as a canal of conical form. 



4. The Loop of Henle. — The descending limb of this U-like segment is distinguished not 

 only by the conspicuous reduction in its diameter (.012-015 mm.), being the narrowest part of 

 the entire uriniferous tubule, but also by the altered character of its epithelium. The latter 

 consists of low elements, so thin that the oval nuclei cause distinct elevations in the cells which 

 project beyond the general level of the epithelium. Since' the nuclei usually do not lie exactly 



Fig. 1605. 



Fig 1606. 



Ascending 

 limb 



Blood- 

 vessel 



Descending 

 limb 



Ascending/' 

 limb 



Blood- 

 vessel 



Henle's loop 



Collecting tubule 



Longitudinal section of medulla passing through 

 Henle's loop. ■; 400. 



Ascending limb 



Longitudinal section of medulla, showing parts 

 of limbs of Henle's loop. X 400. 



opposite each other, the projections on 

 one wall alternate with those of the 

 other, in consequence of which dispo- 

 sition the lumen appears wavy and 

 irregular, although not much reduced 

 below the diameter of that of the pre- 

 ceding spiral segment and generous in 

 proportion to the entire width of the 



tubule. The flattened cells consist of clear, slightly granular cytoplasm, in which is embedded 

 a distinct elliptical nucleus of relatively large size. 



The ascending limb differs from the descending in its increased diameter (.024-.028 mm.), 

 which depends upon sudden augmented thickness of the walls and not upon the width of the 

 lumen, the darker and striated appearance of its epithelium, and its extension from the medulla 

 into the cortex. The outlines of the individual lining cells are not sharply defined in well-pre- 

 served organs, although the readiness with which these elements undergo post-mortem change 

 often results in their artificial separation. The cells are often irregular in height, the lumen, in 

 consequence, varying and in places, especially within the cortex, being almost obliterated. The 

 nuclei often occupy a clear area, and are separated by striations of unusual length. Although 

 the cells exhibit a differentiation into an outer rodded zone, a finely striated inner border, as seen 

 in the epithelium of the convoluted tubules, is wanting ; where an inner zone is represented, it 

 assumes a variable vesicular rather than a striated character. The length of the loop of Henle 

 is influenced by the level of the corresponding Malpighian body within the cortex — the nearer 

 the latter lies to the medulla the greater the descent of the loop towards the papilla, and vice 

 versa, this relation probably depending upon the intimate association between the termination 

 of the ascending limb and the Malpighian body. According to the reconstructions of Huber,' 



^ Amer. Journ. of Anatomy, vol. iv.. Supplement, 1905. 



