172 ANIMAL PHYSIOLOGY. 



or a lobule with one Malpighian pyramid; several lobules 

 may become blended in one Malpighian pyramid and larger 

 lobule; and, lastly, in such a kidney as the human, the lobules 

 are completely fused in a higher unity. 



The great majority of the uriniferous tubules are about 

 ^.i_ inch in diameter; near the summit of the Malpighian 

 pyramids, however, trunks of twice or thrice that size are 

 formed by the union of others; and there are also loops of 

 much smaller diameter, each of which is intercalated in the 

 course of a convoluted tubule, and descends into the medul- 

 lary substance. 



Where the change of arrangement from the convoluted to 

 the straight condition takes place, there is likewise a change 

 of structure; for the columnar or rather cubical epithelial 

 corpuscles of the convoluted tubes are turbid masses without 

 distinct outline, while those of the straight tubes have the 

 appearance of cell walls round them, and clear contents; a 



difference which suggests that 

 the convoluted tubes are more 

 active in secretion, or take at 

 least a different part. 



130. Each convoluted tubule 

 begins in a little spherical body, 

 named after the same old ob- 

 server as the pyramids, Mal- 

 pighian corpuscles. Each of 

 these consists of a capsule, 

 which is the dilated commence- 

 ment of the tubule, and a 

 bunch of blood-vessels called a 

 glomerulus ; and, when the 

 minute blood-vessels are filled 

 with colouring - matter, the 

 Fig. 93. Relations of MAL- glomeruli are distinguishable 

 PIGHIAN CORPUSCLE to tubule with the n ^ ked scattered 



and blood-vessels. 1 odd and ,1.1 T (1 , -, i 



Bowman. ai * through the cortical sub- 



stance. Each glomerulus con- 

 sists of a bunch of loops of blood-vessels taking origin from a 

 single little afferent artery which enters the capsule at the 

 part opposite to that which is continued into the tubule, 



