URINIFEEOUS TUBULES. 



405 



Fig. 287. 



between these two, a double layer of muscular fibres, longitudinal and 

 circular. The lon)2:itudinal fibres are lost near the extremity of the calix, 

 but the circular fibres, according to Henle, form a continuous circular 

 muscle round the jDapilla where the wall of the calix is attached to it. 



The pyramidal masses found in the adult kidney indicate the original sepa- 

 ration of this gland into lobiiles in the earlier stages of its growth (fig. 299). 

 Each of these primitive lobules is in fact a pyramid surrounded by a proper in- 

 vestment of cortical substance, and is analagous to one of the lobules of the 

 divided kidneys, seen in many of 

 the lower animals. As the human 

 kidney continues to be- developed, 

 the adjacent surfaces of the lobules 

 coalesce and the gland becomes a 

 smgle mass ; the contiguous parts 

 of the originally separate cortical 

 investments, being blended together, 

 fonn the partitions between the 

 pjTamids ah-eady described. More- 

 over, upon the surface of the kidney 

 even in the adult, after the removal 

 of the fibrous capsiile, faintly marked 

 furrows may be traced on the corti- 

 cal substance, opposite the intei-vals 

 in the interior between the several 

 Slalpighian pyramids ; and not un- 

 fiequently instances occur in which 

 a deeper separation of the original 

 lobules by grooves remains apparent 

 in the adult kidney. 



Tubuli uriniferi. — On ex- 

 amining the summit of one ,of 

 the papilljB carefully, especially 

 with the aid of a lens, a num- 

 ber of small orifices may be seen 

 varying in diameter from ^^th 

 to g-^th of an inch. They are 

 frequently collected in large 

 numbers at the bottom of a 

 slight depression or foveola 

 found near the summit of the 

 papilla, but most commonly the 

 surface is pitted over with about 

 a score of small depressions of 

 this sort. On tracing these 

 minute openings into the sub- 

 stance of the pyramids, they are 

 discovered to be the mouths of 

 small tubes or ducts, the urini- 

 fcrous tubes before mentioned, 

 which thus open upon the sur- 

 face of the several papillte into 

 the interior of the calices. 



As these tubuli pass up into 

 the pyramidal substance, they bifurcate again and again at very acute 

 angles, their successive branches running close together in straio-ht and 



Fig. 287. — Diagram of the course and ar- 

 rangement OF THE UrINIFEROUS TuBES 

 (from Ludwig). 



J5, corresponds to the apical part, and (/, to 

 the base of a pyramid of Malpighi ; r, cortical 

 part ; ix, excretory tube ; viii, vii, vi, straight 

 or collecting tube with its branches ; v, junc- 

 tional tube ; IV, ascending limb of Henle's 

 loop (h) ; III, descending limb ; ii, commencing 

 convoluted tube; i, Malpighian capsule with 

 glomerulus. 



