PHYSIOLOGICAL ANATOMY OF THE KIDNEYS. 361 



divide at very acute angles, generally dichotomously, until a bundle of tubes 

 arises, as it were, from each opening. These bundles constitute the pyra- 

 mids of Ferrein. In their course the tubes are slightly wavy and are nearly 

 parallel to each other. These are called the straight tubes of the kidney, 

 or the tubes of Bellini. They extend from the apices of the pyramids to 

 their bases and pass then into the cortical substance. The pyramids contain, 

 in addition to the straight tubes, a delicate, fibrous matrix and blood-vessels, 

 which latter generally pass beyond the pyramids, to be finally distributed in 

 the cortical substance. Small tubes, continuous with the convoluted tubes 

 of the cortical substance, dip down into the pyramids, returning to the corti- 

 cal substance in the form of loops. This arrangement will be fully described 

 in connection with the cortical substance. 



The tubes of the pyramidal substance are composed of a strong, struct- 

 ureless basement-membrane, lined with granular, nucleated cells. Accord- 

 ing to Bowman, the tubes measure ^-j-g- to -gfa of an inch (85 to 127 /*), in 

 diameter at the apices, and near the bases of the pyramids their diameter is 

 about -g^- of an inch (42 /*). 



The cells lining the straight tubes exist in a single layer applied to the 

 basement-membrane. They are thick and irregularly polygonal in shape, with 

 abundant albuminoid granules. They present one, and occasionally, though 

 rarely, two granular nuclei, with one or two nucleoli. They readily undergo 

 alteration and are seen in their normal condition only in a perfectly fresh, 

 healthy kidney. Their diameter is about J-^Q of an inch (17 /A). The cali- 

 ber of the tubes is reduced by the thickness of their lining epithelium to ?fa 

 or -5^0 of an inch (28 or 30 ft). 



Cortical Substance. In the cortical portion of the kidney, are found 

 tubes, differing somewhat from the tubes of the pyramidal portion in their 

 size and in the character of their epithelial lining, but presenting the most 

 marked difference in their direction. These tubes are rather larger than the 

 tubes of the pyramidal substance, and are very much convoluted, interlacing 

 with each other in every direction. Scattered pretty uniformly throughout 

 this portion of the kidney, are rounded or ovoid bodies, about four times the 

 diameter of the convoluted tubes, known as the Malphigian bodies. These 

 are simply flask-like, terminal dilatations of the tubes themselves. 



The cortical portion of the kidney presents a delicate, fibrous matrix, 

 which forms a support for the secreting portion and its blood-vessels. The 

 tubes of the cortical substance present considerable variations in size, and 

 three well defined varieties can be distinguished : 



1. The ordinary convoluted tubes, directly connected with the Malpig- 

 hian bodies. 2. Small tubes, continuous with the convoluted tubes, dipping 

 down into the pyramids and returning to the cortical portion in the form of 

 loops. 3. Communicating tubes, forming a plexus connecting the different 

 varieties of tubes with each other and finally with the straight tubes of the 

 pyramidal portion. 



In tracing out the course of the tubes, it will be found most convenient 

 to begin with a description of the Malpighian bodies and to follow the tubes 



