CHAP, XVI.] 



ELIMINATION. 



185 



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, arid, 

 after a very devious course, termi- 

 nate in the collecting tubules, 

 which open on the pointed projec- 

 tions or papillae of the pyramids. 

 The fluid they contain passes into 

 the pelvis of the kidney, whence it 

 is carried along the ureters into 

 the bladder, partly by pressure and 

 gravity, and partly by the peri- 

 staltic contractions of the muscular 

 walls of the ureters. In the blad- 

 der the urine collects, its return 

 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 accu- r FlG " .- DIAGRAM OF THE 



COURSE OF TWO URINIFEROUS TUB- 



mulation Stimulates the muscular ULES. M, malpighian capsule, or 



walk tn onntrapt thp rpsktannp dilated extremity ; (7, convoluted por- 



Ct > 1 J tion of tube ; H, loop, consisting of a 



of the Sphincter at the neck of the descending and ascending limb; D, 



bladder is overcome, and the urine c 



is ejected through the urethra. Involuntary micturition may 

 occur as a result of spinal injury, involving the nerve centres 

 which send nerves to the bladder. It may be due to a want 

 of " tone " in the muscular walls, or it may result from some 

 abnormal irritation. 



General characters of the urine. Normal urine may be de- 



