ANATOMY OF THE KIDNEYS. 157 



mid. It is so arranged that the interstices are just large 

 enough to admit the collections of tubes that form the so- 

 called pyramids of Ferrein. 



From this arcade of vessels, branches are given off in two 

 opposite directions. From its concavity, numerous small 

 branches, measuring at first from y^-g- to -^j- of an inch in 

 diameter, pass downward toward the papillae, giving off 

 small ramifications at very acute angles, and becoming re- 

 duced in size to about -^-^ of an inch. These vessels 

 called sometimes the arteriolae rectae surround the straight 

 tubes and pass into capillaries in the substance of the pyra- 

 mids and at their apices. 



From the convex surface of the arterial arcade, numerous 

 branches are given off at nearly right angles. These pass 

 into the cortical substance, breaking up into a large number 

 of little arterial twigs, from y-L- to -g-^- of an inch in diame- 

 ter, which penetrate the MalpigoiaH bodies at a point oppo- 

 site to the origin of the convoluted tubes. Once within the 

 capsule, the arteriole breaks up into from five to eight 

 branches, which then divide dichotomatously into vessels 

 measuring from d ^ QQ to 1 g 1 00 of an inch in diameter, ar- 

 ranged in the form of coils and loops, constituting a dense, 

 rounded mass (the Malpighian coil), filling up the capsule. 

 These vessels break up into capillaries without anastomoses. 

 Their coats are amorphous and provided with numerous 

 nuclei rather shorter than those found in the general capil- 

 lary system. 



The blood is collected from the vessels of the Malpighian 

 bodies by veins, sometimes one, and frequently three or four, 

 which pass out of the capsule and form a second capillary 

 plexus surrounding the convoluted tubes. "When there is but 

 one vein, it emerges near the point of penetration of the 

 arteriole. The walls of the vein are much more fragile than 

 those of the arteriole, and consequently, in ordinary micro- 

 scopical preparations of the cortical substance, the arteriole 

 is left attached, while the veins are torn off. 



