154 EXCRETION. 



been called the neck of the capsule. The tube soon dilates 

 to the diameter of about -^J-g- of an inch, when its course be- 

 comes exceedingly intricate and convoluted. These are 

 what have been known as the convoluted tubes of the 

 kidney. The membrane of these tubes is transparent and 

 homogeneous, but quite firm and resisting. It measures 

 about 40000 of an inch in thickness. It is lined throughout 

 with a single layer of rounded or irregularly polygonal epi- 

 thelial cells, from -j-fVtr to 10 1 00 of an inch in diameter, some- 

 what larger, consequently, than the cells lining the straight 

 tubes. These cells are nucleated and usually quite granular. 

 It has been found that in many of the lower orders of ani- 

 mals, the cells lining the neck of the capsule are provided 

 with vibratile cilia. Bowman has described ciliated epi- 

 thelium in the kidneys of reptiles, 1 and Johnson speaks of 

 the cilia as found in other classes. 2 Isaacs has observed 

 feeble movements in cells from the kidneys of some of the 

 mammalia, 3 and it is possible that they may exist in man, 

 though their presence has never been actually demonstrated. 



The course of the tubes, after they have lost the charac- 

 ters which were formerly supposed to be peculiar to the tubes 

 of the cortical substance, and their anastomoses, have attracted 

 much attention within the last few years. It has been shown 

 by Henle, and the most important points in his observations 

 have been confirmed by numerous anatomists, that the con- 

 voluted tubes, instead of connecting directly with the tubes 

 of the pyramidal substance, are continuous with a system of 

 smaller tubes, which pass into the pyramids in the form of 

 loops. 4 



Narrow Tubes of Henle. According to the most re- 

 cent observations, the convoluted tubes above described, 



1 Op. cit., p. 73. 



3 JOHNSON, Cydopcedia of Anatomy and Physiology, London, 1847-1849, 

 vol. iv., part i., p. 246, Article, Ren. 



3 Op. cit., p. 383. 



4 Henle first described looped tubes of very small diameter projecting into 

 the pyramidal substance, but did not fully recognize the connections of these 



