604 SPECIAL HISTOLOGY. 



the arrangement presented in the bipolar retia mirabilia^ as they are 

 termed; the vas afferens, immediately after its entrance into the coil, 

 dividing into 5-8 branches and each of these into a bundle of capillaries, 

 which are much convoluted and interlaced, without anastomosing, and 

 ultimately, in the same way as that in which they were formed, reunite 

 into a single trunk. Usually the two main vessels enter and quit the 

 glomerulus near together, opposite the origin of the urinifero us duct, 

 and its finest vessels of 0-003-0.004 of a line, the peripheral loops as it 

 were, are invariably situated exactly at the commencement of the duct. 

 In Birds, Amphibia, and Fishes, each glomerulus consists of a single con- 

 voluted vessel. 



The vasa efferentia, although composed of capillaries, are not veins, 

 but in their nature, and to some extent in their structure, minute arte- 

 ries, a character that they retain until, in their further course, they are 

 merged in the capillary plexus of the kidney. This plexus exists in the 

 cortical substance and in the pyramids, presenting somewhat different 

 characters in these two situations. In the former (Fig. 245, d) the 

 vasa efferentia 0-004-0-008 of a line in size, after a short course, ter- 

 minate in a very rich plexus of capillaries 0-0020-004-0-006 of a line 

 in diameter, the rounded-angular meshes of which, 0-0050*015 of a 

 line wide, encompass the convoluted tubuli on all sides arid must be re- 

 garded as continuous throughout the whole cortical substance. The 

 efferent vessels of the glomeruli in. nearest contiguity to the Malpighian 

 pyramids alone present an exception to this condition, inasmuch as, 

 characterized by their more considerable size (0-01-0-016 of a line), 

 they are distributed not in the cortical substance, but in the pyramids, 

 and are distinguished by their long, straight course, and upon the whole, 

 scanty ramifications. These vessels (Fig. 245 g\ which, with Arnold, 

 I would term arteriolce reccet, penetrate around the entire circumference 

 of the pyramids, in a straight course, between the ducts of Bellini, and 

 descend, repeatedly dividing at acute angles, and gradually attenuated 

 to the diameter of 0-004-0-01 of a line, towards the papittce, in which, as 

 in the interior of the medullary substance (in the latter situation, either 

 by their extremities or by twigs given off at right angles), they are con- 

 tinuous with the capillaries in those regions, measuring from 0-002 to 

 0-004 of a line. These capillaries are very importantly distinguished 

 from those of the cortical substance by their less number and the elon- 

 gated figure of the meshes of the plexus which they form, although at 

 the boundary of the pyramids the two sets are continuous with each 

 other. The renal veins commence in two situations, viz., at the sur- 

 face of the organ and at the apices of the papillae. In that situation, 

 minute venous radicles collect together from the outermost portions of 

 the capillary plexus of the cortical substance,' which either regularly 

 surround each cortical lobule, and between them unite in a stellate 



