KIDNEY 315 



cles" and surface markings. The base of the pyramid is toward the 

 periphery of the kidney, and may be lobular as in the figure. From two 

 to nine embryonic or primary pyramids are said to fuse to form a pyramid 

 of the adult kidney. In favorable specimens the pyramid is seen to be 

 divided into an inner and an outer zone, and the latter is composed of two 

 concentric bands. The significance of these markings will be considered 

 later. The pyramids collectively constitute the medulla of the kidney, 

 a term more fittingly applied to the kidneys of many animals which have 

 but a single pyramid. The base of each pyramid is surrounded by a 

 lighter zone, the cortex, which shows radial striations. With low magnifi- 

 cation the striations are seen to taper outward. They constitute the 

 processes or pyramids of Ferrein and are known collectively as the radiate 

 part of the cortex (pars radiata). They consist of straight radial tubules 

 which are continuous with those in the medulla. Consequently they are 

 often called "medullary rays," but being in the cortex they may more 

 properly be designated "cortical rays." Between these rays is the con- 

 voluted part of the cortex (pars cowuoluta] ; it may be recognized by the 

 presence of many renal corpuscles (Malpighian corpuscles), which are 

 bodies consisting of a glomerulus and its surrounding capsule. They are 

 barely visible without magnification. 



Over the outer surface of the kidney, there is a fibrous capsule (tunica 

 frbrosa) which may be readily stripped off when normal; and outside of 

 this there is a fatty layer (capsula adiposa] . The fat surrounds the pelvis 

 and extends into a hollow of the kidney known as the renal sinus; this is 

 the excavation which contains the pelvis and its calyces. In this fatty 

 tissue the large blood vessels enter the kidney, passing chiefly over the 

 anterior or ventral surface of the pelvis; having reached the boundary 

 zone between cortex and medulla they enter it, and pursue an arched 

 course, giving off both cortical and medullary branches. In certain places, 

 the cortex dips down to the renal sinus; this occurs between the Mal- 

 pighian pyramids, and constitutes the renal columns (of Bertini); one of 

 them is shown in Fig. 311. 



The arrangement of the renal tubules in relation to the cortex and 

 medulla is as follows. The convoluted part of the cortex contains the 

 capsules, and both proximal and distal convoluted tubules. The rays 

 contain the collecting tubules, together with the outer portions of Henle's 

 loops. The medulla contains the larger collecting tubules and the deeper 

 portions of Henle's loops; since these are all straight tubules, the medulla 

 resembles the radiate part of the cortex. Tubules which are connected 

 with capsules deep in the cortex,- near the boundary zone, send their 

 Henle's loops much further into the medulla than those from the outer 

 capsules; and in the deeply placed tubules the thin segment of Henle's 

 loop is not limited to the descending limb but extends well up into the 



