CHAPTER VIII 



ADDITIONAL TOPICS IN HUMAN BIOLOGY 



I. The Skin 



198. Characteristics of the skin. — The whole outer surface of 

 our bodies is incased in a flexible, elastic skin of varying thickness 

 and texture. In regions like the pahn of the hand and the sole of 

 the foot, for instance, the skin is thick and tough ; the covering of 

 the lips, on the other hand, is extremely thin. At the ends of the 

 fingers and toes are the nails. All other parts of the body, with 

 ■ the exception of the palms of the hands and the soles of the feet, 

 have a covering of hair. Both the hair and the nails are modified 

 parts of the skin. 



199. Uses of the skin. — The most obvious use of the skin is 

 the protection that it affords to the muscles and other organs that 

 lie beneath. In the second place, it has a countless number of sense 

 organs which receive messages from the outside of the body. These 

 are carried along nerve fibers to £he spinal cord and brain, and then 

 we get impressions of temperature, of pressure, and of pain. In the 

 third place, by means of the perspiratory action of the skin, the body 

 throws off a great deal of water and small quantities of other waste 

 matters. And, finally, as a result of the evaporation of this water 

 from its outer surface, the body loses its surplus of heat, and so 

 keeps an even temperature of 98.6° F. 



As we might infer from all these uses, the skin is a complex organ 

 composed of several tissues. "We shall now study its structure and 

 see how it is adapted to perform the four functions that we have 

 just enumerated. 



200. Layers of the skin. — The skin everywhere consists of two 

 different layers : an outer, called the ep-i-der'mis (Greek epi = upon 



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