208 



AJSTATOMY FOR NURSES. [Chap. XVIL 



blood, with some dissolved salts, takes place through the walls 

 of the blood-vessels and of the capsule into the tubule. 



(2) After leaving the capsule, the efferent vessel communi- 

 cates with other similar vessels which together form a mesh- 

 work of capillaries closely surrounding the tubules, so that the 



blood is again brought into close 

 communication with the interior 

 of the tubules. The tubules are 

 lined with secreting cells, and 

 these cells appear to have the 

 power of selecting from the blood 

 the more solid waste matters 

 (especially the urea) which fail 

 to filter through the flat cells 

 forming the wall of the capsule. 

 Thus the elimination of urine 

 is a double process, being par- 

 tially accomplished by transuda- 

 tion, and partially by the selective 

 action of the secreting cells lining 

 the tubules. 



Excretion of urine. — The uri- 

 niferous tubules commence in a 

 dilated extremity, the capsule, and, 

 after a very devious course, ter- 

 minate in the collecting tubules 

 which open on the pointed projec- 

 tions or papillae of the pyramids. 

 The fluid they contain passes into 

 Fig. 126. — Diagram of the the pelvis of the kidney, whence it 



Course of Two Ukixiferous Tu- • ^„ ..,• j „i +i,„ „4-„,„ -.^j.^ 



BULEs. M, Maipighiau capsule, or ^^ Carried along the ureters into 



dilated extremity ; c, convoluted por- the bladder, partly by pressure and 



tionoftube; //, loop, consisting of a •, n ^11,1 



descending and ascending limb; D, gravity, and partly by the peri- 

 collecting tubui© staltic contractions of the muscular 

 walls of the ureters. In the bladder the urine collects, its re- 

 turn into the ureters being prevented by the oblique entrance 

 of these tubes into the walls of the bladder. 



Micturition is normally caused by the accumulation of urine 

 within the bladder. The accumulation stimulates the muscular 

 walls to contract, the resistance of the sphincter at the neck of 



