MEDULLARY AND CORTICAL SUBSTANCES. 929 



circular muscle round the papilla where the wall of the calyx is attached 

 to it. 



The substance of the kidney consists of two parts, the medullary and 

 cortical, differing in colour, consistence, and structure. 



The internal or medullary substance does not form a continuous struc- 

 ture, but is collected into a series of conical masses called the pyramids of 

 Malpighi, the bases of which, directed towards the surface of the kidney, 

 are imbedded in the cortical substance, whilst their apices are turned to- 

 wards the sinu.*, and, projecting into the calyces, form the papillm already 

 mentioned. There are generally more than twelve pyramids, but their 

 number is inconstant, varying from eight to eighteen. The medullary 

 portion of the kidney is more dense than the cortical, and is distinctly 

 striated, owing to its consisting of small diverging uriniferous tubes, and to 

 its blood-vessels being arranged in a similar manner. Towards the papillae 

 the pyramids are of lighter colour than the cortical substance, but at their 

 base they are usually purplish and darker. 



The external or cortical substance is situated immediately within the 

 fibrous capsule, and forms the superficial part of the organ throughout its 

 whole extent to the depth of about two lines, and moreover sends prolonga- 

 tions inwards (septula renum, or columns Bertini) between the pyramids. 

 It is of a nearly uniform light crimson brown appearance, and is soft and 

 easily lacerated in directions vertical to the surface. When so lacerated, 

 its torn surface exhibits a columnar appearance, coarser than that of torn 

 medullary substance, and more rough and irregular ; the columnar appear- 

 ance arising from the alternation of groups of straight and convoluted 

 tubules, and the roughness being caused by the convoluted tubules and the 

 interspersion of small round bodies of a deeper colour, the Malpighian 

 corpuscles. The groups of straight tubules in the cortical substance are con- 

 tinued from those of the medullary substance, and are surrounded by tha 

 convoluted tubes into which they pass, not only on their sides but likewise 

 at their outer extremities, so that no straight tubules reach the surface 

 of the organ : they are termed pyramids of Ferrein. The Malpighian cor- 

 puscles are imbedded among the convoluted tubes, and appear disposed in 

 double rows between the pyramids of Ferrein, and likewise more superficially, 

 but nowhere reach quite to the surface. 



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

 ration of this gland into lobules in the earlier stages of its growth (fig. GG1). 

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

 investment of cortical substance, and is analogous 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 single mass, and the contiguous parts of 

 the originally separate cortical investments, being blended together, form 

 the partitions between the pyramids already described. Moreover, upon 

 the surface of the kidney even in the adult, after the removal of the fibrous 

 capsule, faintly marked furrows may be traced on the cortical substance, 

 opposite the intervals in the interior between the several papillie with their 

 calyces ; and not unfrequently instances occur in which a deeper separation 

 of the original lobules by grooves remains apparent in the adult kidney. 



Tubuli uriniferi. On examining the summit of one of the papillae care- 

 fully, especially with the aid of a lens, a number of small orifices may be 

 seen, varying in diameter from 3-^0 tn to 20 o tn of an inch > they are fre " 

 quently collected in large numbers at the bottom of a slight depression or 



