406 THE HUMAN BODY. 



it is closed. Usually the bladder, which, has a thick coat 

 of unstriped muscular tissue lined by a mucous membrane, 

 is relaxed, and the urine flows readily into it from the 

 ureters. The commencement of the urethra being kept 

 closed by elastic tissue around it (which can voluntarily be 

 reinforced by muscles which compress the tube) the urine 

 accumulates in the bladder. When this latter contracts and 

 presses on its contents, the ureters are closed in the way 

 above indicated, the elastic fibres closing the urethra! exit 

 from the bladder are overcome, and the liquid forced 

 out. 



Naked Eye Structure of the Kidneys. These organs 

 have externally a red-brown color, which can be seen 

 through the transparent capsule of peritoneum which en- 

 velops them. When a section is carried through a kidney 

 from its outer to its inner border (Fig. 115) it is seen that 

 a deep fissure, the hilus, leads into the latter. In the Mlus 

 the ureter widens out to form the pelvis, which breaks up 

 again into a number of smaller divisions, the cups or cahces. 

 The cut surface of the kidney proper is seen to consist of 

 two distinct parts; an outer or cortical portion, and an 

 inner or medullary. The medullary portion is less red and 

 more glistening to the eye, is finely striated in a radial 

 direction, and does not consist of one continuous mass but 

 of a number of conical portions, the pyramids of Malpiglii, 

 %', each of which is separated from its neighbors by an in- 

 ward prolongation,*, of the cortical substance. This, how- 

 ever, does not reach to the inner end of the pyramid, 

 which projects, as the papilla, into a calyx of the ureter. 

 At its outer end each pyramid separates into smaller por- 

 tions, the pyramids of Ferrein, %", separated by thin layers 

 of cortex and gradually spreading everywhere into the lat- 

 ter. The cortical substance is redder and more granulal 

 looking and less shiny than the medullary, and forms every- 

 where the outer layer of the organ next its capsule, besides 

 dipping in between the pyramids in the way described. 



The renal artery divides in the hilus into branches (5) 

 which run into the kidney between the pyramids, giving off 

 a few twigs to the latter and ending finally in a much 



