350 THE KIDNEYS AND THEIR SECRETION. 



injury to the subjacent cortical portion of the organ. At the 

 hilus or notch of the kidney, it becomes continuous with the 

 external coat of the upper and dilated part of the ureter. 



On making a section lengthwise through the kidney (Fig. 

 120) the main part of its substance is seen to be composed of 

 two chief portions, called respectively the cortical and the 

 medullary portion, the latter being also sometimes called the 

 pyramidal portion, from the fact of its being composed of about 

 a dozen conical bundles of urine-tubes, each bundle being called 

 a pyramid. The upper part of the duct of the organ, or the 

 ureter, is dilated into what is called the pelvis of the kidney ; 

 and this, again, after separating into two or three principal 

 divisions, is finally subdivided into still smaller portions, vary- 

 ing in number from about 8 to 12, or even more, and called 

 calyces. Each of these little calyces or cups, again receives the 

 pointed extremity or papilla of a pyramid. Sometimes, how- 

 ever, more than one papilla is received by a calyx. 



The kidney is a gland of the class called tubular, and both 

 its cortical and medullary portions are composed essentially of 

 secreting tubes, the tubuli uriniferi, which by one extremity, 

 in the cortical portion, end commonly in little saccules con- 

 taining bloodvessels, called Malpighian bodies, and by the 

 other open through the papillae into the pelvis of the kidney, 

 and thus discharge the urine which flows through them. 



In the pyramids they are chiefly straight dividing and 

 diverging as they ascend through these into the cortical por- 

 tion ; while in the latter region they spread out more irregu- 

 larly, and become much branched and convoluted. 



The tubuli uriniferi (Fig. 121) are composed of a nearly 

 homogeneous membrane, lined internally by spheroidal epithe- 

 lium, and for the greater part of their extent are about g j^ of 

 an inch in diameter, becoming somewhat larger than this 

 immediately before they open through the papillae. On trac- 

 ing these tubules upwards from the papillse, they are found to 

 divide dichotomously as they ascend through the pyramids, 

 and on reaching the bases of the latter, they begin to branch 

 and diverge more widely, and to form by their branches and 

 convolutions the essential part of the cortical portion of the 

 organ. At their extremities they become dilated into the 

 Malpighian capsules. Until recently, it was believed that the 

 straight tubules in the pyramids branch out and become con- 

 voluted immediately on reaching the bases of the pyramids ; 

 but between the straight tubes in the pyramids and the convo- 

 luted tubes in the cortical portion, there has been shown to be 

 a system of tubules of smaller diameter than either, which 

 form intercommunications between the two varieties formerly 



