THE FRAMEWORK OF THE BODY 49 



Everything that must be or can most efficiently be performed by the 

 external body surface is a function of the integumentary system. We may 

 list these things as follows: 



1. Protection of the body against: 



a. Mechanical injury or abrasion. 



b. Loss of water. 



c. Harmful light rays. 



d. Disease-producing organisms (parasites). 



2. Heat regulation and heat elimination. 



The skin forms an efficient, thermostatically controlled radiator. 



3. Reception of stimuli. 



The skin houses a tremendously numerous and varied set of sense 

 receptors, capable of receiving detailed information about the 

 general and special conditions of the body's environment. 



4. Production of skin appendages. 



The skin produces the hair, nails, and teeth that form more or less 

 useful parts of the body. 



The structure of the skin. The skin is composed of two closely knit 

 layers — a comparatively thin, bloodless epidermis and a much thicker 

 dermis, crowded with blood vessels and rich in nerve endings. An exam- 

 ination of Fig. 3.13 will show the general arrangement of parts and make 

 the following description much clearer. 



The inner portion of the epidermis is a compact layer of columnar 

 epithelial cells (malpighian layer) that is everywhere in close, inseparable 

 contact with the upper surface of the dermis. It receives a rich supply 

 of nourishment from the blood vessels of the dermis. All through the 

 life of the individual, cells of this inner layer of the epidermis are multi- 

 plying by cell division, and the excess cells are being pushed outward 

 toward the surface of the skin; the. many-cell-thick epidermis is thus 

 composed of cells that are in process of being shoved to the surface, 

 where they are constantly being worn away. As each cell progresses 

 from the inner layer toward the surface, it becomes more and more com- 

 pressed and changed in composition, until at the surface the formerly 

 columnar cell has now become flat, thin, and lifeless and is ready to be 

 shed from the body. 



The dermis, which is many times thicker than the epidermis, is com- 

 posed chiefly of connective tissues, through which ramify numerous small 

 blood and lymph vessels and in which are imbedded a huge number of 

 nerve endings, dermal sense organs, and smooth muscle cells. It is the 

 comparatively thick, tough, outer portion of the dermis that, when 

 tanned, is known as leather. The inner, less tightly compacted region 



