THE KIDNEYS. 



791 



covers the pyramids, and sends numerous prolongations inwards, towards the 

 sinus between the pyramids. 



The cortical substance is soft, reddish, granular, easily lacerated, and contains 

 numerous small, red bodies scattered through it in every part, excepting towards 

 the free surface. These are the Malpighian bodies. The cortical substance is 

 composed of a mass of convoluted tubuli uri- 



niferi, bloodvessels, lymphatics, and nerves, Fig. 434. Vertical Section of Kidney, 

 connected together by a firm, transparent, 

 granular substance, which contains small 

 granular cells. 



The medullary substance consists of pale, 

 reddish-colored, conical masses, the pyramids 

 of Malpighi; they vary in number from eight 

 to eighteen ; their bases are directed towards 

 the circumference of the organ; whilst their 

 apices, which are free from the cortical sub- 

 stance, converge towards the sinus, and termi- 

 nate in smooth, rounded extremities, called 

 the papillae (mammilla) of the kidney. Some- 

 times, two of the masses are joined, and have 

 between them only one papilla. The kidney 

 is thus seen to consist of a number of conical- 

 shaped masses, each inclosed, excepting at the 

 apex, by an investment of the cortical substance: 

 these represent the separate lobules of which 

 the human kidney in the foetus consists, a con- 

 dition which is permanent in the kidneys of 

 many of the lower animals. As the human kid- 

 ney becomes developed, the adjacent lobules 

 coalesce, so as to form a single gland, the surface 

 of which, even in the adult, occasionally pre- 

 sents faint traces of a lobular subdivision. 



The medullary substance is denser in structure than the cortical, darker in 

 color, and presents a striated appearance, from being composed of a number of 

 minute converging tubes (tubuU uriniferi). If traced backwards the tubuli 

 uriniferi are found to commence at the apices of the cones by small orifices, 

 which vary from 5^ to J- fi of an inch; as they pass up 

 in the medullary substance, towards the periphery, they 

 pursue a diverging course, dividing and subdividing at 

 very acute angles, until they reach the cortical sub- 

 stance, where they become convoluted, anastomose freely 

 with each other, and retain the same diameter. The 

 number of orifices on the entire surface of a single pa- 

 pilla is, according to Huschke, about a thousand; from 

 four to five hundred large, and as many smaller ones. 

 The tubuli uriniferi are formed of a transparent homo- 

 geneous basement membrane, lined by spheroidal epi- 

 thelium, which occupies about two-thirds of the diameter 

 of the tube. The tubes are separated from one another, 

 in the medullary cones, by capillary vessels, which form 

 oblong meshes parallel with the tubuli, and by an inter- 

 mediate parenchymatous substance composed of cells. 



As soon as the tubuli uriniferi enter the cortical sub- 

 stance (Fig. 435), they become convoluted, arid anastomose freely with each 

 other; they are sometimes called the tubes of Ferrein. At the bases of the 

 pyramids, the straight tubes are described as being collected into small conical 

 bundles, the tortuous tubuli corresponding to which are prolonged upwards 



Fig. 435. Minute Struc- 

 ture of Kidney. 



