CHAPTER VI 



THE EXCRETORY ORGANS 



When protein substances undergo decomposition or combus- 

 tion they ordinarily yield ammonia together with sulphides or 

 sulphates. But ammonia being a poisonous substance is not 

 permitted to circulate freely in the body in any considerable 

 quantity. The ammonia, therefore, which is produced as a 

 result of the oxidation of proteins is converted into other sub- 

 stances which are innocuous. The princij)al of these is urea 

 which is one of the principal constituents of normal urine. It is 

 probably produced in the organism from ammonium carbonate 

 which is itself formed from ammonia, carbon dioxide and water : 



2NH3 + CO2 + H2O = (NH4)2C03 

 and (NH4)2C03 = 2H2O + C0(NH2).,. 



Urine, the composition of which is described below, is excreted 

 by the kidneys. These organs are of the nature of compound 

 tubular glands and are situated dorsally on each side of the 

 lumbar region of the vertebral column, the right kidney being 

 l^laced in front or headwards of the left. The ureter or kidney 

 duct issues from about the centre of the inner or concave surface 

 of the kidney and conveys the urine to the bladder into the 

 posterior side of which the two ducts open side by side not far 

 from the opening of the bladder into the common urogenital 

 passage. The latter communicates with the exterior in the male 

 by the penis and in the female by the common vaginal opening. 



To the naked eye the kidney (as seen in longitudinal section) 

 appears to be formed of a cortical portion on the outside, and a 

 medullary portion on the inside. The medullary portion sur- 

 rounds the cavity or pelvis which is the dilated end of the ureter. 

 Into the pelvis one or more conical processes project. These are 

 called the papillae, and they contain the openings of excretory 

 tubules into the pelvis.' Each papilla really represents the 

 apical part of a pyramid, for the medulla of the kidney in most 



