412 FOODS CIRCULATED AND USED IN THE BODY 



Remove the kidney from this capsule. Notice its color and shape. 

 The depression called the hilum is deeper than the corresponding region 

 in a kidney bean. The hollow tube passing out from this region is 

 called the ureter. Blood vessels also enter and leave the kidney at 

 the hilum. 



Cut the kidney lengthwise into halves. Try to find the following 

 regions : (1) the outer or cortical region (note its color) ; (2) the inner 

 or medullary layer (this layer is provided with little projections, which 

 are the pyramids of Malpighi, so called after their discoverer, Marcello 

 Malpighi, a celebrated Italian physiologist) ; (3) the cavity or pelvis 

 of the kidney. At the summit of each pyramid is a small opening 

 through which escapes into the pelvis the secretion formed in the little 



tubules, which make up the pyra- 

 mids, and in which the real work 

 of excretion is performed. 



Where is the waste taken from 

 the blood in the kidney ? Where 

 does this waste pass out of the 

 body? 



Organs of excretion. All the 



life processes which take place 

 in a living thing result ulti- 

 mately, not only in the giving 

 off of carbon dioxide, but also 

 in the formation of organic 

 wastes within the body. The 

 retention of the wastes which 

 contain nitrogen is harmful to 

 animals. In man, the skin and 

 kidneys remove these wastes 

 from the body, hence they are 

 called the organs of excretion. 



The human kidneys, like 

 those of the sheep, are com- 

 posed of masses of excretory 

 tubules. The outer end of 



The organs of excretion. Note the blood supply h f thege tubules ultimately 



to the kidneys. Of what use is the bladder? u 



opens into the pelvis, the space 

 within the* kidney ; the inner end forms a tiny closed sac. In 

 each sac, the outer wall of the tube has grown inward and carried 

 with it a very tiny artery. This artery breaks up into a mass of 



urethra 



