458 
RESPIRATION AND EXCRETION 
confused with the indigestible part of the food which is not 
taken up by the blood and which passes out through the large 
intestine as faeces. 
The kidneys are two bean-shaped organs located in the 
abdominal cavity, one on each side of the “ small ” of the 
back. Each is about four inches long, two and a half inches 
wide, and half an inch thick. The color is a dark red. The 
kidney is made up of two layers, the outside or cortical, and 
the inside or medullary. Each layer is composed of many 
Artery 
uriniferous 
tubule, 
Figure 408 . — Diagram. 
Showing relation of artery and 
vein to portion of minute kidney 
tube (uriniferous tubule). 
small tubes (tubules) which 
open into an area called the 
pelvis , 1 the space within the 
kidney. The pelvis continues 
into a duct (ureter), and from 
each kidney the ureter passes 
into the bladder. A* small 
duct (urethra) (u-re'thra) con¬ 
nects the bladder with the 
exterior of the body. 
Each tubule in the kidney 
is in close relation with the 
blood capillaries. At the 
place where this close relation is found, glomerulus (glo-mer'u- 
lus), the walls of the capillary and the walls of the kidney are 
very thin. Through these thin walls a large amount of water 
filters out of the blood into the tubes. At the same time 
waste material which contains nitrogen, salts, and other 
organic wastes is removed. If these wastes are not removed, 
they create toxins which poison the body. 
Skin. — The skin covers and protects the voluntary 
muscles, regulates the body temperature, gives off waste 
matter, and acts as a general sense organ. Thus we see that 
it is incorrect to think of the skin simply as an organ of 
1 The word pelvis is also used in referring to the hip bones, and it is better 
to call the latter structure the bony pelvis. 
