

THE KIDNEYS. 1265 



Extremities of the Kidney. The kidney, fixed and hardened in situ, is usually 

 more pointed at its inferior than at its superior end. The latter is wider from 

 side to side, and often somewhat flattened from before backwards. The superior 

 end of the kidney is bent somewhat forwards and rests upon the diaphragm, which 

 separates it from the inferior part of the pleural cavity. 



Sinus Renalis. The sinus of the kidney (Fig. 985), into which the hilum opens, 

 is a narrow space, having its long axis corresponding to that of the kidney. The 

 thick walls of the sinus cavity are formed by the substance of the kidney, and are lined 

 by a part of the fibrous kidney capsule which enters the sinus over the lips of the 

 hilum. The floor of the sinus is not even, but presents a series of small projecting 

 conical elevations called papillae renales, which vary from six to fifteen in number. 

 Eadiating from each papilla are a number of somewhat raised bars, or ridges, of 

 kidney substance, separated by depressed areas. The blood-vessels and nerves enter 

 and leave the kidney by piercing the wall of the sinus where it is formed by these 

 little depressed areas (Fig. 985). The rounded summit of each renal papilla is 

 pierced by a number of minute openings called foramina papillaria, which are the 

 terminal apertures of the secreting tubules of which the kidney is mainly com- 

 posed. These openings all lie close together, and give rise to the so-called area 

 cribrosa upon the apex of the renal papilla. The urine secreted by the kidney 

 escapes through the foramina papillaria into the subdivisions (or calyces) of the 

 ureter or kidney duct. 



Kidney in Section. Sections through the kidney (Fig. 985) show that it 

 is composed to a large extent of a number of conical masses, known as pyramides 

 renales or pyramids. These together constitute the substantia medullaris or medulla 

 of the kidney, and are arranged with their bases directed towards the surface, and 

 their apices projecting into the renal sinus, where they form the papillae renales 

 already mentioned. The pyramids are more numerous than the papillae, two or 

 three usually ending in each papilla in the middle part of the kidney, and some- 

 times as many as six or more in each papilla near the superior and inferior ends 

 of the organ. The bases of the pyramids do not reach the surface of the kidney, 

 but are separated from it by a thin layer of kidney substance called the cortex, 

 or substantia corticalis of the kidney. The cortical substance not only covers over 

 the bases of the pyramids, but also sends in prolongations, called columnae renales 

 or renal columns, between the pyramids, towards the sinus. The medullary part 

 of the kidney exhibits in section a striated appearance, while the cortical part is 

 more granular and usually different in colour. The outer part of each pyramid is 

 called the basis pyramidis, and appears in section to be composed of alternate 

 dark and light streaks, while the inner, or papillary part, is often of a lighter colour, 

 and more uniformly and faintly striated. 



In sections of the kidney the larger blood-vessels are seen, after they have 

 entered the kidney substance, to lie between the pyramids ; and some of their 

 main branches are visible passing across the bases of the pyramids. 



In the foetus and young child, and sometimes, though much less distinctly, in the adult, 

 the surface of the kidney is marked by a number of grooves dividing it into polygonal 

 areas. These represent the lobes, lobi renales or reniculi, of which the kidney is 

 originally composed, and each corresponds to one papilla with its pyramids and surrounding 

 cortical substance. 



An examination, with an ordinary pocket lens, of a section through the kidney 

 shows that the lighter striae of the bases of the pyramids are continued into the 

 cortex. As they pass through the cortex towards the surface of the kidney the 

 striae become less distinct, and appear, when cut longitudinally, as separate ray- 

 like prolongations carried outward from the bases of the pyramids. These parts 

 ' of the cortex, which seem, in this way, to be continuations of the medulla, 

 are called " medullary rays " and constitute the pars radiata ; the portions which 

 intervene between them form what is known as the pars convoluta or " labyrinth." 

 The appearance presented by the cortex of the kidney in section varies much 

 according to the plane in which the section has been taken. If the section 

 passes through and lies parallel to the axis of a pyramid, the radiate part met 



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