1132 



THE URINARY ORGANS. 



ment membrane lined with epithelium. The epithelium varies considerably in 

 different sections of the uriniferous tubes. In the neck the epithelium is con- 

 tinuous with that lining the Malpighian capsule, and, like it, consists of flattened 

 cells with an oval nucleus (Fig. 720 A). The cells are, however, very indistinct and 

 difficult to trace, and the tube has here the appearance of a simple basement 

 membrane unlined by epithelium. In the proximal convoluted tubule and the 

 spiral tubule of Schachowa the epithelium is polyhedral in shape, the sides of the 

 cells not being straight, but fitting into each other, and in some animals so fused 



FIG. 721. 1 Longitudinal section of Henle's 

 descending limb. a. Membrana propria. 6. 

 Epithelium. 



FIG. 722. Longitudinal section of straight 

 tube. a. Cylindrical or cubical epithelium. 

 b. Membrana propria. 



together that it is impossible to make out the lines of junction. In the human 

 kidney the cells often present an angular projection of the surface next the base- 

 ment membrane. These cells are made up of more or less rod-like fibres, which 

 rest by one extremity on the basement membrane, whilst the other projects toward 

 the lumen of the tube. This gives to the cells the appearance of distinct striation 

 (Heidenhain) (Fig. 720 B). In the descending limb of Henle's loop the epithelium 

 resembles that found in the Malpighian capsule and the commencement of the tube, 

 consisting of flat transparent epithelial plates with an oval nucleus (Figs. 720 A, 

 721). In the ascending limb, on the other hand, the cells partake more of the 

 character of those described as existing in the proximal convoluted tubule, being 

 polyhedral in shape and presenting the same appearance of striation. The 

 nucleus, however, is not situated in the centre of the cell, but near the lumen 

 (Fig. 720 c). After the ascending limb of Henle's loop becomes narrower upon 

 entering the cortical structure, the striation appears to be confined to the outer 

 part of the cell : at all events, it is much more distinct in this situation, the 

 nucleus, which appears flattened and angular, being still situated near the lumen 

 (Fig. 720 D). In the irregular tubule the cells undergo a still farther change, 

 becoming very angular, and presenting thick bright rods or markings, which 

 render the striation much more distinct than in any other section of the urinary 

 tubules (Fig. 720 H). In the distal convoluted tubule the epithelial cells are long 

 in shape, are highly refractive, and their nuclei are comparatively large. In other 

 respects they resemble those in the proximal convoluted tubule (Fig. 720 B). In 

 the curved tubule, just before its entrance into the straight collecting tube, the 

 epithelium varies greatly as regards the shape of the cells, some being angular 

 with short processes, others spindle-shaped, others polyhedral (Fig. 720 E). 



In the straight tubes the epithelium is more or less columnar ; in its papillary 

 portion the cells are distinctly columnar and transparent (Figs. 722, 723), but as 

 the tube approaches the cortex the cells are less uniform in shape ; some are 

 polyhedral, and others angular with short processes (Fig. 720 F and G). 

 1 From the Handbook for the Physiological Laboratory. 



