358 



COMPARATIVE ANATOMY 



FIG. 284. DIAGRAMMATIC LONGI- 

 TUDINAL SECTION THROUGH THE 

 KIDNEY OF A MAMMAL. 



Mammals. The definitive kidneys (metanephros) of Mammals 

 are proportionately small, and lie on the quadratus lumborum 

 muscle and ribs. They usually possess a convex outer, and a 

 concave inner border; the latter is called the hilum, and at this 

 point the ureters arise and the blood-vessels enter. The expanded 

 proximal portion of the ureter is divided up to form one 

 or more calyces (Fig. 284), into which small papilliform processes 



of the pyramids (see below) project; 

 on the summits of these the urinary 

 tubules open in varying number. 

 The calyces are continuous with a 

 large cavity in the widened portion 

 of the ureter called the pelvis, and 

 from this the ureter (metanephric 

 duct) passes freely backwards for 

 some distance to open into the 

 bladder (except in Monotremes, Fig. 

 296) on its dorsal side, sometimes- 

 nearer the apex, sometimes towards 

 the fundus. The bladder communi- 

 cates with the urinogenital canal or 

 urethra. 



The kidney is greatly lobulated 

 in the embryo; this condition may 

 remain throughout life, or the lobes 

 may become more or less completely 

 united (Fig. 285). In the latter case 

 the original division into lobes may 



still be recognised more or less plainly internally. A section of 

 the kidney shows an inner layer, the medullary substance, arranged 

 in the form of wedges the urinary pyramids, and an outer layer, 

 or cortical substance, extending as the columns of Bertini between 

 the pyramids (Fig. 284). The pyramids correspond roughly to the 

 embryonic lobes of the kidney, though several lobes may fuse 

 together in one pyramid. 



The glomeruli as well as the coiled portions of the tubules, sur- 

 rounded by a network of blood-capillaries, lie in the cortical 

 substance, while the straight portions of the tubules extend through 

 the pyramids, where they gradually anastomose to form larger 

 collecting tubes. 



The greater part of the urinary bladder does not corre- 

 spond with the proximal end of the allantois, but to a special 

 differentiation of the cloaca, which becomes divided into a 

 dorsal and a ventral portion by the formation of a horizontal 

 septum. The ventral portion gives rise to the bladder, which 

 is continuous distally with the stalk of the allantois (urachus, 

 see p. 340), from which the median ligament of the bladder 

 is formed. In Monotremes and nearly all Marsupials (see 



R, H, cortical substance ; M, M, 

 medullary substance arranged in 

 pyramids (Pr) ; between the 

 latter the cortical substance ex- 

 tends in the form of the columns 

 of Bertini (B, B) ; Ca, calyces ; 

 Pe, pelvis ; Ur, ureter. 



