640 THE KIDNEYS. 



Each capsule is perforated by an artery called vas inferens, 

 which goes to form this vascular ball, and from the inte- 

 rior of each there passes out through the capsule a small 

 vein called vas ejferens, which, with other veins of like 

 kind, goes to the venus plexus surrounding the convoluted 

 urinary tubes. Two systems of vessels are here described, 

 first the arterial capillary, composing the acini of Malpighi 

 within the urinary capsule, and second, the venous capillary 

 on the outside of the capsule surrounding the urinary 

 tubes, and constituting a plexus which is compared to the 

 portal plexus of the liver. This plexus is believed by Mr. 

 Bowman to secrete the urea, lithic acid, and other solid 

 portions, while he assigns to the arterial plexus or acini, 

 the separation of the water and soluble elements of the 

 urine. 



The internal or tubular portion, (Fig. 206,) called also 

 pyramides Malpighi, consists of fine tubes collected in fasci- 

 culi, forming cones, of which there are about fifteen. These 

 cones are of a dense structure and pale color, with their 

 bases at the circumference, and their apices at the central 

 cavity of the kidney. They are separated and surrounded 

 by the cortical substance, constituting the several conoidal 

 fasciculi so many distinct lobes, or miniature kidneys, 

 which are distinctly marked upon the surface of the foetal 

 kidney. The apex of each cone is free, and forms a projec- 

 tion into the central cavity termed papilla or mammary 

 process. 



The papillae are not so numerous as the cones, as two or 

 more of the latter converge into one of the former. The 

 papillae are arranged into an anterior, middle, and poste- 

 rior row ; and each one presents several foramina, through 

 which the urine passes from the tubuli uriniferi. Each 

 cone or pyramid consists of a collection of these tubuli, 

 which are more numerous at the base than at the apex, in 

 consequence of their coming together as they approach the 

 latter. They are compared to fine hairs, and are proven 

 to be conductors of the urine, by making a section of them 

 and squeezing the cortical portion, when drops of urine 



