930 THE KIDNEYS. 



foveola found near the summit of the papilla, but most commonly the 

 surface is pitted over with about a score of minute depressions of this sort. 

 On tracing these minute openings into the substance of the pyramids, they 

 are discovered to be the mouths of small tubes or ducts, called the urini- 

 ferous tubes (tubuli uriniferi), which thus open upon the surface of the 

 several papillae into the interior of the calyces. 



As these tubuli pass up into the pyramidal substance, they bifurcate again 

 and again at very acute angles, their successive branches running close to- 

 gether in straight and slightly diverging lines, and continuing thus to 

 divide and subdivide until they reach the sides and bases of the pyramids, 

 whence they pass, greatly augmented in number, into the cortical sub- 

 stance, where they enter the pyramids of Ferrein. These straight tubules 

 continued up from the orifices in the papillae are sometimes called ducts of 

 Bellini : they are largest near their orifices, at a short distance from which, 

 within the papillae, their diameter varies, according to Huschke, from ^th 

 to ^th of an inch. Further on in the pyramid they become smaller, 

 measuring about ^th of an inch in diameter, and then do not diminish as 

 they continue to bifurcate, but remain nearly of the same uniform average 

 diameter. 



The convoluted tubes, tubuli contorti, which form the greater part of the 

 cortical substance, and, together with vessels and connecting stroma, the 

 whole of its outermost portion, vary considerably in diameter, but they 

 maintain commonly the same average width as the straight tubes, namely 

 g^th of an inch. The epithelium in the convoluted tubules may be termed 

 cubical ; it does not present any marked contrasts in thickness, but in some 

 of the smaller tubules it is clear, while in the majority it is turbid, and 

 with its cells ill-defined. 



Besides these tubes, long well-known to anatomists, attention has more 

 recently been called by Henle to the presence in the Malpighian pyramids 

 of a number of tubes, which may be roughly estimated as having only a 

 third or a fourth of the diameter of the others, and which, after descending 

 between the larger tubes a variable distance towards the papillae, then 

 turn abruptly and reascend. The tubes in question have been designated 

 looped tubes of Henle. According to this author, the small differ from the 

 large tubes not only in size but in the greater thickness of their walls. By 

 the action of dilute hydrochloric acid, the epithelium of the large tubes is 

 destroyed and that of the looped tubes brought into view. The epithelium 

 of the looped tubes, Henle also states, is clear and squamous towards the 

 papillae, but towards the bases of the pyramids it becomes turbid, like that 

 of the convoluted tubules. 



Chrzonszczewsky, while he both figures and describes looped uriniferous tubes, 

 considers that the merit of having discovered them rests with Ferrein, and that those 

 described by Henle as having squamous epithelium are really blood-vessels. Although, 

 however, it is admitted that loops are formed by blood-vessels very similar to the 

 looped tubes of Henle, it must be regarded as certain that loops of the uriniferous 

 tubules are much more numerous than Chrzonszczewsky is willing to admit, and for 

 a knowledge of them as constant and regularly disposed elements of the renal struc- 

 ture science is indebted to Henle. 



Imbedded among the convoluted tubules are the Malpighian corpuscles, 

 the structure and connections of which must be taken into consideration, 

 before the disputed course of the uriniferous tubes can be discussed. 



The Malpighian corpuscles are small bodies of a rounded or slightly ob- 

 long shape, which have an average diameter of ^th of an inch, but 



