352 THE ANIMALS AND MAN 



The cells which line the capsules and part of the tubules 

 are secreting cells (somewhat similar to those of the salivary 

 glands and of the pancreas). These cells secrete water, in- 

 organic salts and organic wastes (urea, etc.) from the blood. 

 Thus the blood gets rid of its load of waste products. These 

 accumulate in the tubules drop by drop, and finally exude 

 into the calices and into the pelvis of the kidney and are then 

 carried away to the bladder. 



The amount of urine formed depends chiefly upon the 

 quantity of blood sent to the liver and upon the healthful 

 action of the kidneys. The wastes of the tissues, constant- 

 ly accumulating in the blood, are as constantly being re- 

 moved by the kidneys. 



Cool temperature, the drinking of large quantities of 

 water and the use of proteid food favor the healthy action of 

 the kidneys. 



THE SKIN 



The skin is a complex organ of the body that performs 

 several functions: 



1. It protects the underlying structures as a covering, 



2. It regulates the temperature of the body. 



3. It receives the stimuli from without, by means of 

 which the body is guided in its behavior. 



4. It serves as an excretory organ. 



Structure of the skin.- -The skin consists of two layers 

 (fig. 178), the epidermis (a), an outer or superficial layer, 

 and the dermis (d), a deep layer or true skin. 



The epidermis is composed of epithelial cells. The 

 outer layer consists of flattened or dead cells that are con- 

 stantly peeling off. This is sometimes called the cuticle. 

 The inner cells of the epidermis are live growing cells, 

 constantly dividing and forming new cells to take the place 

 of those lost from the surface. The inner cells of the epi- 

 dermis contain pigment granules which give the skin its 



