742 THE UEINAEY TRACT. 



The convoluted tubules of the first order, having an average 

 diameter of 0.0045 mm., are lined by polyhedral epithelia, the 

 cement-substance between them often being illy defined, or, espe- 

 cially in kidneys of children, absent. In these epithelia R. Heiden- 

 hain discovered a rod-like structure similar to that observed in the 

 epithelia of the ducts of salivary glands. The ascending and de- 

 scending portions of the narrow tubules have a diameter of 0.0020 

 to 0.0025 mm., and are lined by cuboidal epithelia, which also 

 exhibit the rod-like structure ; this peculiarity is particularly well 

 marked in the irregular portions of the tubules. The descending 

 portion gradually becomes narrow, and its epithelium passes by 

 degrees into the flat variety, while the ascending portion often ap- 

 pears abruptly widened close above the loop, or in the depth of the 

 loop itself. Along the course of the ascending tubule within the 

 cortical substance the epithelium again may become flat, corre- 

 sponding to a narrowing of the caliber. The narrow portions 

 have a diameter of 0.0014 ; their caliber is comparatively wide, 

 and the flat epithelia are finety granular and supplied with a dis- 

 tinct nucleus. In .edge-view these epithelia appear spindle- 

 shaped, closely resembling the endothelia of capillaries. The 

 convoluted tubules of the second order have only a few convolu- 

 tions ; their caliber is somewhat wider than that of the convo- 

 luted tubules of the first order ; their epithelia, however, being 

 identical with those of the latter. In the irregularly winding 

 portions the epithelia show slight differences in their depths. 

 The collecting tubules have the widest caliber, their diameter 

 being, at the apex of the medullary ray, the same as that of the 

 convoluted tubules, while in their course toward the papilla they 

 gradually assume a diameter of 0.020 to 0.030 mm. Their epi- 

 thelia are at first cuboidal, but with increasing caliber the epi- 

 thelia become distinctly columnar, being finely granular and 

 obliquely arranged in the lower portions, after the manner of 

 shingles on a roof. According to C. Ludwig, the membrana 

 propria near the papillae is fused with the surrounding connect- 

 ive tissue. 



All tubular formations of the kidney are ensheathed by deli- 

 cate connective tissue, which carries the blood-vessels and nerves. 

 The peculiarities above described may be shown by antero-poste- 

 rior (sagittal) sections of the kidney, which sever the tubules in a 

 transverse direction. (See Fig. 334.) 



Sagittal sections of the cortical substance will exhibit, within 

 the medullary ray, transverse sections of the narrow tubules, 



