THE KIDNEYS. 



989 



structure. This portion of the tube does not present a uniform calibre, but 

 becomes narrower as it ascends and irregular in outline. As a narrow tube it enters 

 the cortex and ascends for a short distance, when it again becomes dilated, irregular, 

 and angular. This section is termed the irregular tubule ; it terminates in a 

 convoluted tube which exactly resembles the proximal convoluted tubule ; and is 

 called the distal convoluted tubule. This again terminates in a narrow curved tube, 

 which enters the straight or collecting tube. 



Each straight, otherwise called a collecting or receiving, tube commences by a 

 small orifice on the summit of one of the papillae, thus opening and discharging 

 its contents into the interior of one of the calices. Traced into the substance of 

 the pyramid, these tubes are found to run from apex to base, dividing dichotomously 

 in their course and slightly diverging from each other. Thus dividing and sub- 

 dividing, they reach the base of the pyramid, and enter the cortical structure 

 greatly increased in number. Upon entering the cortical portion they continue a 

 straight course for a variable distance, and are arranged in groups called medullary 

 rays, several of these groups corresponding to a single pyramid. The tubes in the 

 centre of the group are the longest, and reach almost to the surface of the kidney, 

 while the external ones are shorter, and advance only a short distance into the 



Descending limb \ 



of Henle's loop, j 



FIG 547. Uriniferous tube. For the sake of clearness the epithelial cells have been represented more 

 highly magnified than the tubes in which they are contained. 



cortex. In consequence of this arrangement the cortical portion presents a number 

 of conical masses, the apices of which reach the periphery of the organ, and the 

 bases are applied to the medullary portion. These are termed the pyramids of 

 Ferrein. As they run through the cortical portion the straight tubes receive on 

 either side the curved extremity of the convoluted tubes, which, as stated above, 

 commence at the Malpighian bodies. 



It will be seen from the above description that there is a continuous seric 

 of tubes from their commencement in the Malpighian bodies to their termina- 



